Datasheet ACS102A-TQ, ACS102A-PL Datasheet (Semtech Corporation)

Advanced Communications

* Full duplex serial transmission over single/twin fiber.
* Link lengths up to 25km.
* Supports asynchronous data rates from DC to 162kbps.
* Full diagnostic modes - Remote and Local loopback.
* Ultra low power consumption, typically 2-3mA, which could be

* Uses a single Ping Pong LED or Laser Duplex Device for single fiber

* Additional operating mode to support PIN with integrated TIA.
* Supports 3 additional low frequency asynchronous channels or

* Digital and differential voltage input modes, plus modes for non

* Bit Error Rate (BER) < 10
* Available in 44 pin TQFP (part no: ACS102A-TQ) and 44 pin

ACS102A Fiber Modem

ACS102A Data Sheet

Features

extracted from the RS-232 port for self powered applications.

applications, low cost LED/PIN or Laser/PIN combinations for twin
fiber applications.

the RS-232 handshake signals.

fiber applications - RF

-9

ACS102A

PLCC (part no: ACS102A-PL) packages.

2

TxD

RxD

Digital

Filter

Digital

Filter

DCDB CTS DSR RTS DTR DR(3:1) DM(3:1) DP

Data

Compress

Data

Decompress

RS-232 Interface Control Logic

Equivalent Block Diagram of ACS102A

FIFO Time

Compress

FIFO Time

Decompress

3B4B

Encoder

3B4B

Decoder

TRC

LED/Laser

Driver

LED/PIN

Receiver

PORBRIO RII HD(2:1) HBT ERL

PPLED
LDD
LED/PIN
Laser/PIN
combinations

Description

The ACS102A is a complete controller, driver and receiver IC, supporting full­duplex asynchronous transmission from DC to 162kbps over a single serial link.
Although primarily designed for single optical fibre applications, any other simple
serial media may be used. The ACS102A is optimised for very low power
consumption, consuming only 2 - 3mA at RS-232 data rates including power
provided to the LED and 'heartbeat' monitor. In applications where the power is
extracted from the RS232 data lines, this leaves a generous amount of power left
for any power extraction and RS-232 level shifting circuitry.

The ACS102A employs data compression and time compression techniques,
affording high launch power in short bursts, leading to a low average power
consumption. The advantage of this approach is that high link budgets can be
achieved with inexpensive optical components.

For example, the recommended set-up for RS-232 applications (19.2kbps +
handshake signals) assumes that the LED is driven with a peak current of

ACS102A Revision 1.6 September 2000

100 SERIES

approximately 15.4mA for 6 % of the time. The machine cycle is short enough to
facilitate power supply smoothing with a small external capacitor in the region of
100µF.

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Advanced Communications

Single/Dual Fiber Modem for Asynchronous Data Rates from DC to 160kbps

ACS102A Data Sheet

2

Transmitter and Receiver Functions

This device offers one high speed and three low speed full duplex
channels to the user in a completely transparent way, appearing as
4 full duplex channels even though the medium connecting the
devices may only be a single fiber link.

Data from the TxD and low frequency channels is time
compressed in an internal FIFO and sent over the fiber link in a
burst within a predefined window. The device at each end of the
link automatically synchronise with each other such that the
transmit and receive windows are interleaved. The TxD input data
of the transmitting modem is also data compressed. The 3B4B
encoding method is used for communication between ACS102As,
thus ensuring that there is no DC component in the signal. The
encoding and decoding is transparent to the user.

In the receiving modem, 3B4B encoding ensures easy extraction of
the bit-clock. The received data is filtered, decoded, and then
stored in the output memory. The memory provides time expansion,
de-jittering and frequency compensation functions. The data is then
decompressed and directed to the RxD output pin, appearing after a
minimal delay, in the same format as that presented at the TxD pin
at the far end.

Operational Modes

The ACS102A is compatible with the ACS102 but offers over twice
the max data rate and incorporates the laser interface modes
previously associate with the ACS402. The ACS102A is a pin and
functional compatible replacement for both the ACS102 and
ACS402. The following sections detail the operating modes for all
configurations of LED, LASER and LED/PIN or LASER/PIN
combinations. Additional modes are also described for new ways of
interfacing the device with external PIN / amplifier modules.

LED Interface Modes
Mode 1 - Single Fiber LED mode

Setup : DP5=0, DP4=0, DP3=0, DP2=1, DP1=0
This is the operational mode for single fiber transmission with a

PPLED. The LED is used for both transmission and reception of
data over the fiber. An example circuit diagram showing the
necessary connections is shown in figure 4. This also shows an
example circuit for interfacing to the RS232 voltage levels of a PC
serial port.

Mode 2 - Dual Fiber LED/PIN mode

Setup : DP5=0, DP4=0, DP3=0, DP2=1, DP1=1
This is a twin-fiber mode where the LED is used for transmission

and a separate PIN Diode is used for reception. This allows the use
of less expensive standard LEDs and PINs rather than bi-directional
PPLEDs or Duplex devices. An example circuit diagram showing
the necessary connections is shown in figure 5.

LASER Interface Modes
Mode 3 - Dual Fiber LASER/PIN mode

Setup : DP5=0, DP4=1, DP3=0, DP2=1, DP1=0
This is a twin-fiber mode where the LASER is used for transmission

and a separate PIN Diode is used for reception. An example circuit
diagram showing the necessary connections is shown in figure 6.
Differential reception from the PIN diode is used to maximise
sensitivity. Since PINP is also used for LASER current control via
monitoring of the monitor diode current, the LAP and LAN pins are
automatically floated during data reception.

Either 3 or 4 pin LASERs may be used in this mode. For 4 pin
LASERS the extra pin of the monitor diode cathode is connected
to the LASER anode, the same as it is shown in figure 6 with the
internal connection of a typical 3-pin LASER.

Mode 4 - Single Fiber 3-pin LASER/PIN mode

Setup : DP5=0, DP4=0, DP3=1, DP2=0, DP1=1
This is a single-fiber mode where the LASER is used for transmis-

sion and the monitor PIN diode within the LASER is used for
reception. Differential reception from the PIN diode is used to
maximise sensitivity. Connections are shown below :

LASER

LAP

PINP
LAN

Fiber

3-pin LASER single fiber mode

Mode 5 - Single Fiber 4-pin LASER/PIN mode

Setup : DP5=0, DP4=0, DP3=1, DP2=0, DP1=0
This is a single-fiber mode where the LASER is used for transmis-

sion and the monitor PIN diode within the LASER is used for
reception. Differential reception from the PIN diode is used to
maximise sensitivity. Connections are shown below :

LASER
LAP
PINN

PINP
LAN

Fiber

4-pin LASER single fiber mode

LASER Duplex Device Use

The Laser duplex device is composed of a 3 or 4 pin Laser for
transmission and a PIN diode for reception in a single housing.
Mode 3, as detailed previously is used for interfacing to these
devices. The Duplex devices are driven by the ACS102A in a half­duplex manner, even though to the user it appears as a full duplex
link. As a consequence potential cross-talk between the transmit­ter and receiver is ignored, allowing excellent performance from

low cost components.

Additional Alternative Modes

The previous modes detail the most common setups for most
typical LED, LED/PIN or LASER/PIN combinations. Many other
possible operating modes are possible via the DP1-5 pins setups.

Some of the other less common connection combinations are
shown below. These include modes for using a LASER as a
receiver as well as a transmitter in a single fiber link, where the
LASER device supports this, receiving from both the LASER and
monitor PIN, and modes for digital interfacing to external PIN/
transimpedance amplifier (TIA) modules. Only use those setups
on DP1-5 indicated in this specification, other pin combinations
may activate unpublicised functional or test modes which may lead
to damage of the LASER, where this is used.

Mode 6 - Single Fiber 4-pin LASER/PIN mode (Las & mon recv)

Setup : DP5=0, DP4=0, DP3=0, DP2=0, DP1=0
This is a single-fiber mode where the LASER is used for transmis-

sion and the LASER and the monitor PIN diode within the LASER is
used for reception. Connections are as in mode 5.

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Advanced Communications

ACS102A Data Sheet

Mode 7 - Single Fiber 3-pin LASER/PIN mode (Laser recv)

Setup : DP5=0, DP4=0, DP3=0, DP2=0, DP1=0
This is a single-fiber mode where the LASER is used for transmis-

sion and only the LASER is used for reception. Connections are as
in mode 4.

Preamp Interface modes
Mode 8 - Preamp Voltage Input & LED Drive

Setup : DP5=1, DP4=0, DP3=1, DP2=0, DP1=0, NSB=0
This is a mode for use with external amplifier and PIN modules. An

LED is used for transmission and connected as normal with its
anode to LAP and cathode to LAN. The differential voltage from an
external PIN/TIA module is connected to PINN and PINP via
100pF capacitors to provide DC isolation. The signals should be
connected such that PINP is connected to the TIA output that goes
high when light is received. A single input can also be applied from
a single ended PIN/TIA by feeding the input to PINP only, PINN is
left floating. This mode uses the new NSB pin, in all other modes
this pin should be left disconnected or connected to VA+.

Mode 9 - Preamp Voltage Input & LASER Drive

Setup : DP5=1, DP4=0, DP3=1, DP2=0, DP1=1, NSB=0
This is a mode for use with external amplifier and PIN modules. A

LASER is used for transmission and connected as normal as
described under mode 3. The differential voltage from an external
PIN/TIA module is connected to PINN and PINP via 100pF
capacitors to provide DC isolation. The signals should be
connected such that PINP is connected to the TIA output that goes
high when light is received. A single input can also be applied from
a single ended PIN/TIA by feeding the input to PINP only. With a
LASER drive the PINN and PINP inputs are also connected to the
LASER monitor diode. This may induce extra noise but should not
interfere with the operation. This mode uses the new NSB pin, in
all other modes this pin should be left disconnected or connected
to VA+.

Digital interface modes
Mode 10 - Digital Data Input & LASER Drive

Setup : DP5=0, DP4=1, DP3=1, DP2=0, DP1=0
This is a mode for use with external amplifier and PIN modules that

provide fully digital output levels. A LASER is used for transmission
and connected as normal as described under mode 3. The output
from an external PIN/TIA module is connected to CNT. The
polarity of the input should be such that CNT that goes high when
light is received.

Mode 11 - Digital Data Input & LED Drive

Setup : DP5=1, DP4=1, DP3=0, DP2=1, DP1=0
This is a mode for use with external amplifier and PIN modules that

provide fully digital output levels. An LED is used for transmission
and connected as normal as shown in figure 5. The output from an
external PIN/TIA module is connected to CNT. The polarity of the
input should be such that CNT that goes high when light is
received.

Transmit Current Control

LED current control

The LED transmit current is not critical though it is important not to
exceed the LED manufacturer's recommendation for maximum
current. The current is controlled by a resistance Rtrc connected
between TRC and GND. The lower the value of Rtrc the greater
the current. The lower limit for Rtrc is 800Ω while a practical
maximum is 40kΩ.

The LED current is inversely proportional to Rtrc while Rtrc > 800Ω.

LED current = (100 / Rtrc) +/- 25 %

LASER current control

The LASER output current must be set for each individual device
in accordance with the manufacturer’s recommendations. The
output current to the LASER is controlled by a variable resistor
(Rtrc) between TRC and ground. The lower the value of Rtrc the
greater the current. The minimum value of Rtrc is 800Ω. The
ACS102A derives and controls the average optical power being
produced by measuring the current in the LASER's monitor PIN
diode and integrating this measurement using the capacitor on the
CTX pin, which is typically 10nF. A control loop is established
which works to maintain the average optical power at a constant
level whilst parameters such as voltage, temperature and LASER
efficiencies may vary. The average optical power is always one
half of the peak power since the LASER is driven between full on
and full off, with an average mark-space ratio of 50%. An example
circuit arrangement is shown in figure 6.

Adjustment Procedure

Select the appropriate LASER drive mode using the pins DP1-5
(see section headed Operational Modes). The LASER drive
current and hence transmitted optical power is set by adjusting
Rtrc until the required output power is obtained, taking account of
the maximum allowed drive current set by the LASER manufac­turer.

There are two ways of measuring the output power and drive
current, either dynamically in the normal operating mode or
statically by setting the pin SETB low.

If measuring power dynamically during the normal mode, the
output from the laser can be measured using an optical-power
meter that is capable of detecting peak optical-power. If an
averaging optical power meter is employed then a correction factor
of 16 must be used to obtain the peak value :

LASER(peak power) = Laser(average power) * 16.
To measure power statically, the SETB pin must be pulled low to

ground. This forces the device to constantly transmit through the
LASER at a fixed level. This fixed level will be equivalent to half of
the peak level, since the normal control loop within the device
works to control the average power level through integrating out
the alternating data pulses.

LASER(peak power) = Laser power(with SETB=0) * 2.
Since all currents are static in this mode, a simple optical power

meter can be used and the drive current in the laser can be easily
measured by connecting an ammeter between pin LMN and VA+.
LMN provides a convenient means of monitoring the LASER drive
current through the relationship :

LASER(current) = 100 x LMN(current) +/- 8%.
Dynamic measurement of the LMN current is also possible by

connecting a resistor to LMN and measuring the voltage pulses.

Data-Rate Selection

The ACS102A benefits from data compression circuitry which
reduces power consumption and improves the BER (Bit Error
Rate). The compression technique employed, demands a
minimum TxD data-bit time of 10 sample-clocks. This defines the
maximum data rate:

Maximum data rate = sample-clock/10
However, an allowance must be made for any variation in the TxD

data-bit period to accommodate frequency variation and jitter.
Hence the maximum data rates specified in the following are
decreased by 10% to include a sufficient safety margin.

The ACS102A includes an input pulse shaper which ensures that
the system is very tolerant to jitter, and helps achieve a maximum
data-rate close to the theoretical maximum of sample-clock/10
(bps). The pulse shaper will expand data pulses of less than 10
clock-samples to meet the compression criteria. This is performed
on up to three consecutive data-bits which fail to meet the
minimum pulse width criteria.

2

3

Advanced Communications

ACS102A Data Sheet

2

DR3 DR2 DR1 XTAL Sample Max TxD

0 1 1 10MHz XTAL/160 5.6kbps
1 0 0 10MHz XTAL/80 11kbps
1 0 1 10MHz XTAL/40 22kbps
1 1 0 10MHz XTAL/20 45kbps
1 1 1 10MHz XTAL/15 60kbps
1 1 1 20MHz XTAL/15 120kbps

1 1 1 27MHz XTAL/15 162kbps

Table 1. TxD Data-Rate Selection

Table 1. shows the maximum TxD data rate, which includes a 10%
tolerance margin, when using various frequency crystals, other
sample-clock frequencies may be generated by using the
appropriate value XTAL in combination with the divide constant
selected by DR(1:3) namely 15,20,40,80 or 160.

The advantage of using a slower crystal and a lower sample clock
is the reduced power consumption of the device.

RS-232 Handshake Signals / Low Frequency Data Channels

Three additional low frequency data channels are provided on the
ACS102A which are often used for the RS-232 handshake signals.
The RS-232 handshake signals comprise the set RTS, CTS, DTR
and DSR. These are treated as pass through data channels rather
than using local handshaking. Hence the status of inputs RTS and
DTR appear at the far-end outputs CTS and DSR respectively. An
extra data channel has also been provided, which may be used for
sending the RS232 Ring Indicator signal, for example. The input
and output lines are RII and RIO respectively.

The transmission method employed on the ACS102 has been
designed to give low skew (1 - 2 data-bits) on the main RTS, CTS,
DTR and DSR handshake signals relative to the main TxD/RxD
data channel, while maintaining low power consumption.

The handshake signals are updated by two stimuli:
i. an internal interval timer at a frequency proportional to the

XTAL; at 10.0MHz this is approximately 1.6ms.
ii. changes detected on RTS and DTR.
The maximum bandwidth for the handshake signals may be

programmed using pins HD(1:2) in accordance with the Table 2.

HD2 HD1 Sampling Skew

0 0* 600 Hz 10ms.
0 1 10 kHz 1 - 2 data bits
1 0 5 kHz 1 - 2 data bits
1 1 2.5 kHz 1- 2 data bits

Table 2. Handshake signal bandwidth allocation

* When HD2 = HD1 = 0 super-compress mode is selected. See
section headed Super-Compress mode.

Handshake data rates which exceed the allocated bandwidth will
be delayed, and consequently result in additional skew between
handshake signals and data.

The HD pins enable the user to allocate a maximum bandwidth to
the handshake signals and thus limit the power consumption of the
device. The power consumption is, however, dependent on the
actual bandwidth used and not the bandwidth selected. For
example; if the handshake signals were toggled at 1kHz the power
consumption would be the same for an allocated bandwidth of

2.5kHz as it would for an allocation of 10kHz. See section headed
Current and Power Consumption for more details.

Super-Compress mode

This mode is selected when HD2 = HD1 = 0. Super-compress
mode performs a second stage of data compression, thus further
reducing the power consumption of the modem. Normally, data is
compressed in a manner which is independent of the data type. In
super-compress mode, an additional stage of compression further
reduces the data by a factor of 1 to 3 depending on the data itself.

Example: The super-compress stage will compress DC data by an
additional Compression Factor (CF) of 3, whilst data close to the

Clock Clock Data Rate

Frequency w.r.t. RxD

maximum frequency will not be compressed beyond the standard
CF of 1.

Super-compress mode provides benefits where the user is
interested in low average power consumption (e.g. battery life)
rather than peak power. If the intended system is idle for most of
the time with periodic bursts of activity, the additional data
compression afforded will approach a CF of 3.

Locking

To achieve low power consumption the ACS102A is active for a
small percentage of the frame (machine-cycle) known as the
'transmit' window and the 'receive' window, collectively these
windows are known as the 'active time'. Outside the 'active time'
the device is largely dormant accept for the maintenance of the
oscillator and basic 'house-keeping' functions.

Communicating modems attain a stable state known as 'locked',
where the 'transmit' window of one modem coincides with the
'receive' window of the other, allowing for the delay through the
optical link. Adjustments to machine cycles are made
automatically during operation, to compensate for differences in
XTAL frequencies which cause loss of synchronisation.

The ACS102A locking algorithm is statistical, and consequently
the locking time will differ on each attempt to lock.

Diagnostic and Locking Modes

The diagnostic and operational modes, shown in Table 3, are
selected using the DM pins.

DM3 DM2 DM1 Mode Lock

0 0 0 Full-duplex Drift
0 0 1 Full-duplex Active
0 1 0 Full-duplex Memory
1 0 1 Local loopback Random
1 1 0 Remote loopback Random
1 1 1 Full-duplex Random

Table 3. Diagnostic and operational modes

Local Loopback

In local loopback mode TxD data is looped back inside the near-end
modem and appears at its own RxD output. RTS, DTR and RII are
also looped back appearing at their own CTS, DSR and RIO outputs
respectively. The data is also sent to the far-end modem and
synchronisation between the modems is maintained.

In local loopback mode data received from the far-end device is
ignored, except to maintain lock. If concurrent requests occur for
local and remote loopback, local loopback is selected.

The local loopback diagnostic mode is used to test data flow up to,
and back from, the local ACS102A and does not test the integrity of
the link itself, i.e. local loopback operates independently of
synchronisation with a second modem.

Remote Loopback

In remote loopback mode, the near-end modem sends a request to
the far-end modem to loopback its received data, thus returning the
data so that it appears at the RxD of the initiating modem. RTS,
DTR and RII follows the same path, returning data back to CTS,
DSR and RIO respectively of the initiating modem. Data also
appears at the far-end modem outputs RxD, CTS, DSR and RIO. In
the process both modems are exercised completely, as well as the
LED/PINs and the fiber optic link. The remote loopback test is
normally used to check the integrity of the entire link from the near­end (initiating) modem. Whilst a device is responding to a request
for remote loopback from the initiating modem (far-end), requests to
initiate remote loopback will be ignored.

Drift lock

Communicating modems attain a stable state where the 'transmit'
window of one modem coincides with the 'receive' window of the
other, allowing for delay through the optical link. Adjustments to
machine cycles are made automatically during operation to
compensate for differences in XTAL frequencies which would
otherwise cause loss of synchronisation.

4

Advanced Communications

ACS102A Data Sheet

Using drift lock, synchronisation described above depends on a
difference in the XTAL frequencies at each end of the link, and the
greater the difference the faster the locking. Therefore, if the
difference between XTAL frequencies is very small (a few ppm),
automatic locking may take tens of seconds or even minutes.

Drift lock will not operate if the two communicating devices are driven by
a clock derived from a single source (i.e. tolerance of 0ppm).

Active Lock Mode

Active lock mode may be used to accelerate synchronisation of a
pair of communicating modems. This mode synchronises the
modems in less than 3 seconds by adjusting the machine cycles of
the modems. Active lock reduces the machine cycle of the device
by 0.5 % ensuring rapid lock. After synchronisation the machine
cycle reverts automatically to normal.

Only one device may be configured in active lock mode at any one
time. Active lock mode is usually invoked temporarily on power-up.
This can be achieved on the ACS102A by connecting DM1 to an RC
arrangement, i.e. with the capacitor to 5V and the resistor to GND, to
create a 5V à 0V ramp on power-up. The RC time constant should
be Ca. 5 seconds. Active lock will succeed even when
communicating devices are driven from clocks derived from a single
source (0ppm).

Random Lock

This mode achieves moderate locking times (typically 5 seconds,
worst case 10 seconds) with the advantage that the ACS102’s are
configured as peers. Communicating modems may be permanently
configured in this mode by hard wiring the DM pins.

Random lock will succeed even when communicating devices are
driven from clocks derived from a single source (0ppm). Random
lock mode is compatible with drift lock and active lock.

Memory Lock

Following the assertion of a reset (PORB = 0) communicating
devices will initiate an arbitration process where within 10 seconds
the communicating modems will achieve synchronisation with one
establishing itself as an active lock modem and the other
establishing itself as a drift lock modem. On subsequent attempts to
lock, synchronisation will be achieved within 3 seconds. It is only
necessary to apply reset to one device in the communicating pair to
initiate an arbitration process.

Since memory lock uses on-chip storage, loss of power to the
modem will require a new reset (PORB=0). Furthermore, should
there be a need to synchronise with a third modem a reset will again
be required.

Mixing Lock modes

It is possible to mix all combinations of locking modes once the
modems are locked, however, prior to synchronisation two modems
configured in active lock will not operate. The effect of mixing
locking modes on locking speed is given in Table 4 :

Device A Device B Locking Speed
Mode Mode

Drift Drift Drift
Drift Active Active
Drift Random Random
Drift Memory Random
Active Active Not allowed
Active Random Random
Active Memory Random
Random Random Random
Random Memory Random
Memory Memory Active

(Random on first synchronisation)

Table 4. Mixing lock modes

PORB

The Power-On Reset or PORB resets the device if forced Low for
100ms or more. This pin should be connected as figure 4.

Crystal Clock

Normally, a parallel resonant crystal will be connected between the
pins XLI and XLO with the appropriate padding capacitors. The

crystal oscillator will operate with padding capacitors of value 0 -50pF,
and the designer should endeavour to use padding capacitors of low
value since this will ensure the lowest power consumption. The
ACS102A has been designed to operate with a crystal tolerance of +/

- 250ppm giving a relative tolerance between communicating modem
pairs of 500ppm. This wide tolerance will support the use of low value
padding capacitors.

Alternatively, XLI may be driven directly by an external clock. The
clock frequency for the purpose of this specification is referred to as
the XTAL frequency. The operational range for the XTAL frequency is
5 - 27MHz, though communicating devices must use the same
nominal value.

DCDB

The Data Carrier Detect (DCDB) signal goes Low when the modems
are synchronised ('locked') and ready for data transmission. Prior to
lock (DCDB = High), the data channel output RxD will be forced Low
and the handshake outputs CTS and DSR will be forced High.

The status of DCDB is also given by the HBT pin. See section headed
HBT Status pin.

CNT Capacitor

The CNT value is inversely proportional to the XTAL frequency. The
capacitor is connected between pins CNT and GND. A 20 %
tolerance on CNT is sufficient. For a XTAL frequency range of
5 to 27MHz the recommended value of the capacitor on CNT is from
47nF at 5MHz, 22nF at 10MHz down to 10nF at 27MHz . A ceramic
type is required to ensure low leakage. The CNT capacitor value has
an effect on the initial locking time and the receiver sensitivity limit.
Higher values giving improved sensitivity and lower values giving
faster locking.

ERL (Error Detector)

This signal can be used to give an indication of the quality of the
optical link. Even when a DC signal is applied to the data and
handshake inputs, the ACS102A modem transmits up to 200kbps
over the link in each direction. This control data is used to maintain
the timing and the relative positioning of 'transmit' and 'receive'
windows.

The transmit and control data is constantly monitored to make sure it
is compatible with the 3B4B format. If a coding error is detected, ERL
will go High and will remain High until reset. ERL may be reset by
asserting PORB, or by removing the fiber-optic cable from one side of
the link thereby forcing the device temporarily out of lock.

Please note that ERL detects coding errors and not data errors,
nevertheless because of the complexity of the coding rules on the
ACS102A the absence of detected errors on this pin will give a good
indication of a high quality link.

HBT Status pin ('Heartbeat' Indicator LED)

The ACS102A HBT pin affords a method of driving a display LED in a
manner which is sympathetic to low power consumption. The HBT pin
is pulsed to indicate 'locked' status (DCDB = 0) and 'out of lock' status
(DCDB =1). The frequency of pulses is 8 times greater for 'out of lock'
than for 'lock'. The LED 'on' indicates power-up whilst the frequency
of pulsing denotes locking status.

Since the display LED is on for (at most) 3.2 % of the total time, the
HBT requires little power which may be further reduced by employing
high efficiency LEDs.

Powered-up, but not locked

Frequency (Hz): XTAL / 3.89 * 10
Duration (s): 61,440 / XTAL
On time (%): 3.2 % of time.
With 10MHz XTAL : Frequency: 2.5Hz (approx.)

Duration: 6.1ms (approx.)

6

Powered-up and locked

Frequency (Hz): XTAL / 15.36 * 10
Duration (s): 61,440 / XTAL
On time (%): 0.4 % of time.
With 10MHz XTAL : Frequency: 0.65Hz (approx.)

Duration: 6.1 ms (approx.)

6

2

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Advanced Communications

ACS102A Data Sheet

2

The HBT pin is active High and can supply up to 16 mA at a voltage of
> VDD - 0.5 Volts. The display LED should be placed between the
HBT pin and GND with a series resistor. The resistor value is a
function of the efficiency of the display LED, and the power budget.

Example: Calculating the HBT resistor value

LED on voltage: 2.0V
VDD (ACS102A): 5.0V
Resistor voltage: 3.0V
Current to LED: 2mA (high efficiency LED)
Resistor value: 3/2*10
Average current: 64µA
Average power: 0.32mW

-3

= 1500Ω

Note: The LED referred to in this section is of the inexpensive display
type and should not be confused with the LED that interfaces with the
fiber optic cable itself.

Power consumption considerations

The power consumption of the ACS102A is a function of the
following:

i. The sample-clock DR(1:3)
ii. The transmit current setting (TRC)
iii. Handshake signals frequency
iv. XTAL frequency
v. Supply voltage

The sample-clock

The sample-clock selected by DR(1:3), see section headed Data­Rate Selection, determines the quantity of data transmitted over the

fiber link. The 'transmit' window opens once each frame and closes
when the time compress FIFO is empty. The 'receive' window is
aligned with the 'transmit' window of the far-end modem, and tracks
the 'transmit' window such that it closes on detection of the last data
bit. Clearly, the lower the sample-clock the smaller the active time
and the lower the power consumption.

The transmit current setting

The formula given in section headed LED current control, relates to
the peak current delivered to the LED. The average current however
is very much lower. The DC balanced nature of data encoding means
the LED consumes current for approximately 50 % of the 'transmit'
window time. The average current delivered to the LED is therefore a
function of both the peak current and the duration of the 'transmit'
window.

Handshake signals frequency

Handshake data which is interleaved with the main data channel is
generated and written to the time compress FIFO each time a change
is detected on either RTS or DTR. The power consumption is lower
when the signals change at low frequency or are held at a DC level. It
is possible to limit the power consumption dedicated to the
handshake signals by limiting the frequency of operation using
HD(1:2) input pins. See section headed RS-232 Handshake Signals.

XTAL frequency

The ACS102A uses CMOS technology and therefore the power
consumption is proportional to the frequency of switching.
Consequently, the effect of reducing the value of the XTAL alone will
result in lower power consumption. However, the current component
delivered to the LED and sourced from outputs such as RxD and HBT
are static and as such are independent of the XTAL frequency.

It is worth noting that a modem pair configured with an XTAL of
10MHz and a sample-clock of XTAL/40 will yield the same
performance as a modem pair configured with an XTAL of 5MHz and
a sample-clock of XTAL/20. However, the modem pair with the lower
value XTAL is likely to consume the higher power with a higher data
delay (see section headed Data delay and skew). This is because,
although the dynamic power has reduced, the higher sample-clock
leads to a much longer active time, a factor which dominates the
overall power calculation.

Current and Power Consumption

The average current consumption may be split into two components;
the dynamic component and the static component. The dynamic

component is dependent on the XTAL frequency while the static
component is dependent on static current loads. (See Calculating
average current and power consumption for details).

Since the peak current can be very much greater than the average
current, it is important to use a substantial smoothing capacitor on
VA+ and VD+. The recommended values are at least 47µF* for
VD+ and 100µF* for VA+. The configuration can be seen in Figure 1.
(* Capacitor tolerance +/- 20 %)

Data delay and skew

The Full Duplex Delay (FDD) through the system, which applies to
TxD à RxD, RTS à CTS and DTR à DSR, is shown in Table 5.

DR3 DR2 DR1 FDD

0 1 1 6.5ms
1 0 0 3.8ms
1 0 1 2.8ms
1 1 0 2.3ms
1 1 1 2.0ms

Table 5. FDD with XTAL = 10MHz

The FDD is inversely proportional to the XTAL frequency and may
be calculated for other XTALs using the formula below:

= (10

7

/ XTAL) * FDD

10MHz

FDD

XTAL

The skew between the main TxD data channel and handshake
signals is 1 - 2 data bits as long as the maximum handshake data­rate of 2kbps is respected. For handshake frequencies above
2kbps, the skew will be proportional to the handshake signal
frequency.

LED considerations & Suppliers

Since LEDs from different suppliers may emit different
wavelengths, it is recommended that the LEDs in a communicating
pair of modems are obtained from the same supplier. The
ACS102A can support any wavelength LED or LASER.
Furthermore, the emission spectrum is a function of temperature,
so a temperature difference between the ends of a link reduces the
responsivity of the receiving LED, resulting in a reduction in the link
budget. Information is given in the suppliers’ data sheets. The
following manufacturers have components that will be tested with
the ACS102A and Acapella will be glad to assist with contact
names and addresses on request:

MITEL (e.g. 1A-212ST, 1A-212SMA)
Acapella (e.g. A-ST, A-SMA)
GCA (e.g. 1A-212-ST-05, 1A-212-SM-02)
Honeywell (e.g. HFE4214-013, HFE4404-013)

Power Supply Decoupling

The ACS102A contains a highly sensitive amplifier, capable of
responding to extremely low current levels. To exploit this sensitivity
it is important to reduce external noise to a low level compared to the
input signal from the LED or PIN. The modem should have an
independent power trace to the point where power enters the board.

Figures 4 to 6 all show the recommended power supply decoupling.
The LED/PIN/LASER should be sited very close to the PINP, PINN,
LAN and LAP pins. A generous ground plane should be provided,
especially around the sensitive PINP, PINN, LAN and LAP pins. The
modem should be protected from EMI/RFI sources in the standard
ways.

Link Budgets

The link budget is the difference between the power coupled to the
fiber via the transmit LED and the power required to realise the
minimum input-amplifier current via the receive LED/PIN. The link
budget is normally specified in dB or dBm, and represents the
maximum attenuation allowed between communicating LEDs. The
budget is utilised in terms of the cable length, cable connectors and
splices. It usually includes an operating margin to allow for
degradation in LED performance. The power coupled to the cable, is
a function of the efficiency of the LED, the current applied to the LED
and the type of the fiber optic cable employed. The conversion
current produced by the reverse biased LED is a function of the LED
efficiency and the fiber type.

6

Advanced Communications

ACS102A Data Sheet

PIN DESCRIPTION

PLCC-

TQFP-

44 Pin

16 DP1I

2 7 GND - Ground Po wer S upply

3 8 DCDB O

49RTSI

510RIOO

611DSRO

712LMNOLaser monitor

8 13 RxD O Received Data Received data

914DR3I

10111516XLI

12 17 GND - Ground Power Supply ground.

13 18 DP5 I

14 19 VD + -

15 20 TxD I Transmit Data Transmitted data

16 21 ERL O Error Detector

17 22 DTR I

18 23 HBT O

19 24 H D 1 I

Sym bol IO Nam e Descrip tion

44 Pin

Selects operating mode for use
with PPLED, LED/PIN or
LA SE R/PIN.

Modem control signal - LOW
when modems locked

Modem control signal or
additional low frequency data
channel input

An alternative data channel
which may be for the
propagation of the RS232 Ring
indicator signal.

Modem control signal or
additional low frequency data
channel output

A pull down current equal to
1/100 th of the Laser current.

The DR(1:3) input s select the
Data Rates, see p2.

Co nnect funda ment al p arallel
resonance crystal with padding
capacitors to GND

Selects operating mode for use
with PPLED, LED/PIN or
LA SE R/PIN.

Power Supply, 3.3-5.25 Volts

Indicates quality of line. If a
co ding infringement is detected,
ERD goes High. Reset by
PORB to Lo w

Modem contrrol signal or
additional low frequency data
channel input

Indicates power up and modem
lock, pulses slowly when
locke d, fast unlocke d.

Sets the Handshake bandwidth,
see p4

XLOIO

Mode
Selection

Data C arrier
Detect

Request To
Send & Data
Channel 2 i/p

Ring indicato r
outpu t

Data S et
Ready & Dat a
Channel 3 o/p

Data Rate
Select

Oscil l ator
Crystal

Mode
Selection

+ve power
suppl y

Data Terminal
Ready/Da ta /
Channel 3 i/p

'Heart beat'
Lock & power
up indicator

Handshake
Delay

PLCC-

TQFP-

44 Pin

Sym IO Nam e Description

44 Pin

Force low to put LASER in

27 32 SETB I

Setup for
Laser testing

constant t ransmit mod e for
power adjustment. Leave
disconnected when using LEDs.

29 34 NSB I New Slice Bar Connect to GND.

30 35 GND - Ground Ground Supply

31 - NC -

Not con nected

Not connected

Integrating capacitor is placed

32 36 CNT IO

Capacitor
Integration

between CNT and GND of
value 10nF-47nF with an XTAL
of 27-5Mhz

- 37 GND - Ground Ground Supply

33343839PINN

PINPII

35364041LAN

LAPIOIO

PIN Cathode
PIN Anode

LED Cathode
LED Anode

Connections to a PIN diode or
LASER monitor diode.

Connections to LED or LASER

37 42 VA+ - +ve Supply Power supply, 3.3-5.25 Volts

A 10 nF capacitor is connected
between this pin and ground for

38 43 CT X IO

Capacitor for
Laser transmit

LASER applications. It is used
in monitoring of the average
transmit power. Can be left
disconnected when using LEDs.

Defines transmit current to the
LED. Minimum and maximum

39 44 TR C I

Transmit
Curren t

values are set by connecting
TRC to GND via a resistor,
value R defined by equation on
page 2.

An alternative data channel

40 1 RII I

Ring indicator
input

which may be for the
propagation of the RS232 Ring
indicator signal.

41 2 DP4 I

42 3 DP 3 I

Mode
Selecti on

Mode
Selecti on

Selects operating mode for use
with PPLED, LED/PIN or
LA SE R /PIN .

Selects operating mode for use
with PPLED, LED/PIN or
LA SE R /PIN .

43 4 VD+ - +ve Supply Power Supply, 3.3-5.5 Volts

44 5 DP 2 I

Mode
Selecti on

Selects operating mode for use
with PPLED, LED/PIN or
LA SE R /PIN .

2

20 25 CTS O

21 26 HD2 I

22 27 PORB I

23242829DR1

25
26
28

DR2

DM3

30

DM2

31

DM1

33

Clear To Send
& Data
Channel 2 o/p

Handshake
Delay

Power-on ­Reset

Data Rate

I

Select

Diagnostic

I

Modes

Modem control signal or
additional low frequency data
channel output

Sets the Handshake bandwidth,
see p4

Will reset the device when
PORB = 0. Connect to an RC
circuit as in figure 4, so a reset
performed on power-up.

The DR(1:3) input s select the
Data Rates, see p3

DM( 1:3 ) input select for
Diagnostic Modes such as local
loopback and remote loopback

7

2

Advanced Communications

ACS102A Data Sheet

Single Fiber link

Link Budget Example (Rtrc set so LED launch current = 50mA peak)

Fiber type Plastic Glass Glass
Fiber size 1000 micron 62.5micron 50 micron

Minimum Transmit Couple power to fiber (µW) 1000 60 40
Minimum LED responsivity (A/W) 0.01 0.16 0.16
Minimum ACS102A sensitivity (nA) 500 500 500
Minimum input power to ACS102A amplifier (µW) 50 3.1 3.1
Link Budget (dB) 10 13 1 1
Average current consumption TxD = 19.2kbps (mA) 3.8 3.8 3.8
Average current consumption TxD = 64kbps (mA) 7.2 7.2 7.2

Dual Fiber link optimised for performance

Link Budget Example (Rtrc set so LED launch current = 100mA peak)

Fiber type Plastic Glass Glass

Fiber size 1000 micron 62.5 micron 50 micron

Minimum Transmit Couple power to fiber (µW) 1000 120 80
Minimum PIN responsivity (A/W) 0.1 0.6 0.6
Minimum ACS102A sensitivity (nA) 500 500 500
Minimum input power to ACS102A amplifier (µW) 5 0.83 0.83
Link Budget (dB) 23 21 19.8
Average current consumption TxD = 19.2kbps (mA) 7 7 7
Average current consumption TxD = 64kbps (mA) 14 14 7 .6

Dual Fiber link optimised for low power & low cost optical components

Link Budget Example (Rtrc set so LED launch current = 12.5mA peak)

Fiber type Plastic Glass Glass

Fiber size 1000 micron 62.5micron 50 micron

Minimum Transmit Couple power to fiber (µW) 125 13 6.5
Minimum PIN responsivity (A/W) 0.1 0.6 0.6
Minimum ACS102A sensitivity (nA) 500 500 500
Minimum input power to ACS102A amplifier (µW) 5 0.83 0.83
Link Budget (dB) 13.9 12 9
Average current consumption TxD = 19.2kbps (mA) 2.2 2.2 2.2
Average current consumption TxD = 64kbps (mA) 3.4 3.4 3.4

Calculating average current and power consumption

Average current

Iav (mA) = XTAL* 10

Power

P (mW) = Iav (mA) * V

Terms used in current/power calculation:

XTAL = Crystal Oscillator Frequency, Hz I
H = Handshake on from digital outputs such

H=1 for handshakes active as (RxD,CTS,DSR,DCD)
H=0 for handshakes at DC level I

U = Handshake constant from HBT pin.

U = 0.001 when HD 2/1 = 0/0 (see section HBT Status pin)
U = 0.028 when HD 2/1 = 0/1
U = 0.014 when HD 2/1 = 1/0 I
U = 0.007 when HD 2/1 = 1/1 set by TRC pin.

A = Active window constant

A = 0.022 when DR 3/2/1 = 0/1/1 V = Voltage supply to the ACS102 V
A = 0.03 when DR 3/2/1 = 1/0/0 Power formula is only accurate
A = 0.045 when DR 3/2/1 = 1/0/1 for voltage supply = 5 Volts
A = 0.08 when DR 3/2/1 = 1/1/0
A = 0.11 when DR 3/2/1 = 1/1/1

-7

(1.3 + 3*(A + U *H ) ) + I

(A + U * H) + I

trc

out

hbt

trc

+ I

out

hbt

= Average current sourced mA

= Average current sourced mA

= Peak Transmit current mA

Note : An application note on power extraction from the RS232 lines is available from Acapella. This shows a typical example

circuit diagram for powering the ACS102A, the optics and all related circuitry from the RS232 data lines.

8

Advanced Communications

ACS102A Data Sheet

ELECTRICAL SPECIFICATION

Important Note: The "Absolute Maximum Ratings" are stress ratings only, and functional operation of the device at conditions other

than those indicated in the "Operating Conditions" sections of this specification are not implied. Exposure to the absolute maximum
ratings for an extended period may reduce the reliability or useful lifetime of the product.

Absolute Maximum Ratings

Parameter Symb ol Min Max Units

Powe r s upply VD+ and VA+
(VDD = VD+ or VA+)

Input voltage
(non-supply pins)

Input current
(except
LAN,LAP,PINN,PINP,CNT)

Input current
( LAN, LAP,PINN,PINP,CNT)

Storage temperature Tstor - 50 160 ºC

Operating Conditions

Parameter Symbol Min Typ Max Units

Power supply
(VA+ and VD+)

Ambient temperature range TA -40 - 85 ºC

Static Digital Input Conditions (for specified operating conditions)

For Digital Input pins: TxD, RTS, DTR, PORB, RII, DP, DR(3:1), DM(3:1), HD(2:1).

Parameter Symbol Min Typ Max Units

Vin High Vih 2.0 - - V

Vin L ow Vil - - 0. 8 V
Input current (High) Iin - 0.2 5 µA
Input current (Low) Iin - 8 15 µA

Static Digital Output Conditions (for specified operating conditions)

For Digital Output pins: RxD, DSR, CTS, DCDB, ERRL, RIO, HBT.

Parameter Sym b ol Min Typ Max Units

Vout Low Vol 0 - 0.5 V

VDD -0.3 6.0 V

Vin GND - 0 .3 VDD + 0.3

Iin - 10.0 mA

Iin - 1.0 mA

V+ 3.3 5.0 5.25 V

V

Dynamic Characteristics (for specified operating conditions)

Parameter Sym bol Min Typ Max Units

Crys tal fre que ncy
(XTI, XTO)

External clock (XTI)
High or Low time

RxD and TxD d ata rate
Function of DR(1:3) setting

Digital output - fall time tf - - 100 ns

Digital output - rise time tr - - 100 ns

Powe r c ons umptio n

(Note 2)

Note 2: See section on Calculating average current and power consumption

Matching Characteristics (for specified operating conditions)

Parameter Symbol Min Typ Max Units

Crystal tolerance

use pa ralle l resonat e crystal and
recommended padding capacitors

Amplifier sensitivity input current Irec 500 - - nA

Maximum amplifier input current Imax - - 50 0 µA
Rtrc placed between TRC and GND Rtrc 0.8k - 40k

LED curr ent
Rtr c = 1 k Ohm
Rtrc = 40 kOhms

Bloc k Er ror Rate BER - - 10

Si ngle Fi ber mode Pa rameters

LED capacitance with Vr =0
with Irec = 500 nA
with Irec = 1000 nA

LED leakage with Vr = 1.4 V Lleak - - 150 nA

XTAL 5 27 MHz

fclp 40 - 60 %

fclf DC -

Pc - 20 - mW

Ct -250 0 250 ppm

Iled 75

Cl -

XTAL/150

100

1.8

2.5

---50100

125

3.2

-9

Hz

Ω

mA

pF

2

Vout High Voh

Isource and Isink
(Note 1)

Isource and Isink (HBT) Iout 16 - - mA

Max load capacitance Cl - - 50 pF

Note 1: Output source and sink currents should be kept to a minimum in order to achieve low power

(except HBT)

consumption.

VDD-0.5

Iout 4 - - mA

--V

DTE ACS102A

TxD
RxD

RTS

One single fiber link

CTS

DTR

DSR

RII

RIO

Figure 2. Data and handshake signals transmitted over a single fiber.

LED reverse bias Vr - 0 - V

Dual Fiber mode Parameters

PIN capacitance with Vr = 0 Cl - - 20 pF

PIN leakage current Lleak - - 150 nA

Pin reverse bias Vr - 0 - V

ACS102A DCE

RxD
TxD
CTS
RTS

DSR

DTR

RIO

RII

9

2

Advanced Communications

PACKAGE INFORMATION

ACS102A Data Sheet

TQFP44 D1/E1 A A1 A2 e b L

D imensions i n mm

max

min

10.00

1.60

0.05

0.15

1.35

1.45

0.30

0.80

0.45

0.45

0.75

0°

7°

α

E/D Copl

12.00

0.10

PLCC44 D/E D1/E1 D2/E2 D3/E3 A A1 A2 e b R Copl.

D ime nsions in mm

max

min

17.40

17.65

16.51

16.66

Figure 3. Package Dimensions, PLCC44 & TQFP44

14.99

16.00

12.70

10

4.20

4.57

2.29

3.04

0.51

1.27

0.33

0.53

0.64

1.14 0.10

Advanced Communications

APPLICATION CIRCUITS

ACS102A Data Sheet

Basic RS-232 to Fiber Interface Circuit

ACS102A_PLCC

2

Figure 4. Typical application circuit for linking two PC Serial Ports via a Single Fiber Optic Cable using Ping-Pong LEDs
This diagram shows a PLCC44 package being used. The TQFP44 package option can be used with the same component layout.

ACS102A_PLCC

Figure 5. Basic Circuit for a Twin Fiber Link using LED and PIN.

11

2

Advanced Communications

ACS102A Data Sheet

ACS102A_PLCC

Figure 6. Basic Circuit for a Twin Fiber Link using LASER and PIN.

12

Advanced Communications

ORDERING INFORMATION

Device Code Package Temperature

ACS102A -TQ TQFP44 -40 to 85°C(ambient)
ACS102A -PL PLCC44 -40 to 85°C(ambient)

ACS102A Data Sheet

2

For additional information, contact the following:

Semtech Corporation Advanced Communications Products

E-Mail: [e-mail]
Internet: http://www.semtech.com
USA: 652 Mitchell Road, Newbury Park, CA 91320-2289

Tel: [phone], Fax: [phone]

FAR EAST: 11F, No. 46, Lane 11, Kuang Fu North Road, Taipei, Taiwan, R.O.C.

Tel: [phone], Fax: [phone]

EUROPE: Delta House, Chilworth Research Centre, Southampton, Hants, SO16 7NS, UK

Tel: [phone], Fax: [phone]

ISO9001

CERTIFIED

Semtech reserves the right to change specifications on catalog devices without notice. © Copyright Semtech Corp 2000

13