ON Semiconductor MC68194 Technical data
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MC68194
Carrier Band Modem (CBM)
The bipolar LSI MC68194 Carrier Band Modem (CBM) when
combined with the MC68824 T oken Bus Controller provides an IEEE
802.4 single channel, phase–coherent carrier band Local Area
Network (LAN) connection. The CBM performs the Physical Layer
function including symbol encoding/decoding, signal transmission
and reception, and physical management. Features include:
• Implements IEEE 802.4 single channel, phase–coherent Frequency
Shift Keying (FSK) physical layer including receiver blanking.
• Provides physical layer management including local loopback mode,
transmitter enable, and reset.
• Supports data rates from 1 to 10 Mbps. IEEE 802.4 standard uses 5
or 10 Mbps.
• Interfaces via standard serial interface to MC68824 T oken Bus
Controller.
• Crystal controlled transmit clock.
• Recovery of clocked data through phase–locked loop.
• RC controlled Jabber Inhibit Timer.
• Single +5.0 volt power supply.
• A vailable in 52–lead Cerquad package.
PIN ASSIGNMENTS AND DEVICE MARKING
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CERQUAD
FJ SUFFIX
CASE 778B
ORDERING INFORMATION
Device Package Shipping
MC68194FJ CERQUAD 20 Units / Rail
765432152 51 50 49 48 47
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MC68194FJ
AWLYYWW
A = Assembly Location
WL = Wafer Lot
YY = Year
WW = Work Week
MC68194FJR2 CERQUAD 450 Units / Reel
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Semiconductor Components Industries, LLC, 1999
February , 2000 – Rev. 6
1 Publication Order Number:
MC68194/D
MC68194
TABLE OF CONTENTS
PAGE
SECTION 1 — GENERAL DESCRIPTION
1.1 TOKEN BUS LAN CARRIER BAND NODE OVERVIEW 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 CARRIER BAND MODULATION TECHNIQUE 3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 MESSAGE (FRAME) FORMAT 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4 SYSTEM CONFIGURATION 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 2 — SIGNAL DESCRIPTION TABLE 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 3 — TRANSMITTER
3.1 OVERVIEW 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 TRANSMIT BUFFER 8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 JABBER INHIBIT 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 CLOCK GENERATOR 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 Parallel–Resonant, Fundamental Mode Crystal 9. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.2 Parallel–Resonant, Overtone Mode Crystal 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.3 External Clock Source 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 4 — RECEIVER AMPLIFIER/LIMITER WITH CARRIER DETECT
4.1 OVERVIEW 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 AMPLIFIER 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 CARRIER DETECT 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 5 — CLOCK RECOVERY
5.1 OVERVIEW 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 ONE–SHOT 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 PHASE–LOCKED LOOP (PLL) COMPONENTS 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.1 Phase Detector (PD) 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.2 Voltage Controlled Multivibrator (VCM) 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.3 Loop Filter 13. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3.4 Loop Characteristics 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 6 — DATA RECOVERY
6.1 OVERVIEW 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 RECEIVER END–OF–TRANSMISSION BLANKING 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 7 — SERIAL INTERF ACE
7.1 OVERVIEW 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 PHYSICAL DA TA REQUEST CHANNEL 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.1 TXCLK — Transmit Clock 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.2 SMREQ* — Station Management Request 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.3 TXSYM0, TXSYM1, and TXSYM2 — Transmit Symbols 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3 PHYSICAL DATA INDICATION CHANNEL 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1 RXCLK — Receive Clock 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.2 SMIND* — Station Management Indication 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.3 RXSYM0, RXSYM1, and RXSYM2 — Receive Symbols 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 8 — PHYSICAL MANAGEMENT
8.1 OVERVIEW 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.2 RESET 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3 INTERNAL LOOPBACK 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.4 ST ANDARD OPERATION 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.5 IDLE 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.6 COMMAND RESPONSE TIMING 17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
SECTION 9 — ELECTRICAL SPECIFICATIONS TABLES 20. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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MC68194
SECTION 1
GENERAL DESCRIPTION
1.1 TOKEN BUS LAN CARRIER BAND NODE OVERVIEW
The MC68194 Carrier Band Modem (CBM) is part of ON
Semiconductor’s solution for an IEEE 802.4 token bus
carrier band Local Area Network (LAN) node. The CBM
integrates the function of the single–channel,
phase–coherent Frequency Shift Keying (FSK) physical
layer. Figure 1–1 illustrates the architecture of a token bus
LAN node as commonly used in Manufacturing Automation
Protocol (MAP) industrial communications. Based on the
ISO–OSI model, the LLC Sublayer and additional upper
layers are typically supported by a local MPU subsystem,
while the IEEE 802.4 token bus MAC Sublayer and Physical
Layer are implemented by the MC68824 Token Bus
Controller (TBC) and MC68194 CBM respectively .
The MC68194 provides the three basic functions of the
physical layer including data transmission to the coax cable,
data reception from the cable, and management of the
physical layer. For standard data mode (also called MAC
mode), the carrier band modem receives a serial transmit
data stream from the MC68824 TBC (called symbols or
atomic symbols), encodes, modulates the carrier, and
transmits the signal to the coaxial cable. Also in the data
mode, the CBM receives a signal from the cable,
demodulates the signal, recovers the data, and sends the
received data symbols to the TBC. Communication between
the TBC and CBM is through a standardized serial interface
inconsistent with the IEEE 802.4 DTE–DCE serial
interface.
MC68000
SYSTEM BUS
INTERFACE
MODULATOR
TRANSMITTER
/
PROCESSOR
TOKEN BUS
CONTROLLER
S
T
DEMODULATOR
A
T
M
I
G
O
M
N
T
MEMORY
/
RECEIVER
TOKEN BUS COAX
LLC
&
UPPER
LAYERS
MAC SUB–LAYER
SERIAL INTERFACE
PHYSICAL
LAYER
MEDIA LAYER
Figure 1–1. IEEE 802.4 T oken Bus Carrier Band Node
The physical layer management provides the ability to
reset the CBM, control the transmitter, and do loopback
testing. Also, an onboard RC timer provides a “jabber”
inhibit function to turn off the transmitter and report an error
condition if the transmitter has been continuously on for too
long. Similar to the data mode, the CBM management mode
makes use of the TBC serial interface.
1.2 CARRIER BAND MODULATION TECHNIQUE
The CBM uses phase–coherent frequency shift keying
(FSK) modulation on a single channel system. In this
modulation technique, the two signaling frequencies are
integrally related to the data rate, and transitions between the
two signaling frequencies are made at zero crossings of the
carrier waveform. Figure 1–2 shows the data rate and
signaling frequencies. An {L} is represented as one half
cycle of a signal, starting and ending with a nominal zero
amplitude, whose period is equal to the period of the data
rate, with the phase of one half cycle changing at each
successive {L}. An {H} is represented as one full cycle of
a signal, starting and ending with a nominal zero amplitude
whose period is equal to half the period of the data rate. In
a 5 Mbps implementation, the frequency of {L} is 5.0 MHz
and for {H} is 10 MHz. For a 10 Mbps implementation, the
frequency of {L} is 10 MHz and for {H} is 20 MHz. The
other possible physical symbol is when no signal occurs for
a period equal to one half of the period of the data rate. This
condition is represented by {off}.
Data Rate
MBPS
5
10
Figure 1–2. Data Rate versus Signaling Frequencies
Frequency of Lower
Tone MHz {L}
5.0
10
Frequency of Higher
Tone MHz {H}
10
20
The specified physical symbols ({L}, {H} and {off}) are
combined into pairs which are called MAC–symbols. The
MAC–symbols are transferred across the serial link. The
encodings for the five MAC–symbols are shown in Figure
1–3. Figure 1–4 shows the phase coherent FSK modulation
scheme for ONE, ZERO, and NON–DA T A. The IEEE 802.4
document does not specify the polarity used to transmit data
on the physical cable. The receiver must operate without
respect to polarity.
Mac–Symbol Encoding
Silence
Pad–Idle Pairs
Zero
One
Non–Data
ND1
ND2
{off off}
{L L} {H H}
{H H}
{L L}
{H L}
{L H}
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MC68194
Figure 1–3. MAC Symbol Encoding
ONE
LL
L
LH
ND1
1 BIT TIME =
ZERO
H
H
Figure 1–4. Phase–Coherent Modulation Scheme
1.3 MESSAGE (FRAME) FORMAT
1/ BIT RATE
H
ND2
1 BIT TIME
Although the CBM only uses MAC symbols
one–at–a–time, the MAC or TBC is responsible for
combining the above defined MAC symbols into messages
(more correctly called frames). For the purposes of the
CBM, a simplified frame format can be used consisting of:
SILENCE || PAD–IDLE | START DELIMITER | DATA | END
DELIMITER || SILENCE
where:
PAD–IDLE =
alternating {LL} {HH} pairs which must
occur in octets or groups of eight symbols. Pad–idle provides a training signal
for the receiver and occurs at the beginning of every transmission (and between
frames in a multiple frame transmission).
ST ART
DELIMITER
=
a unique pattern of eight symbols (one octet) that marks the beginning of a frame.
The pattern is:
ND1 ND2 0 ND1 ND2 0 0 0
where ND1 is the first symbol transmitted.
DATA =
octets of ZERO/ONE patterns that are the
actual data or “information” contained
within the frame.
END
DELIMITER
=
a unique pattern of symbols that marks
the end of a frame. The pattern is:
ND1 ND2 1 ND1 ND2 1 {I=0/1} {0/1}
where ND1 is the first symbol transmitted. Note that unlike the Start Delimiter, the last two bits of the End Delimiter
octet are not always the same. The
seventh bit of the octet is called the I Bit
or Intermediate bit which = 1 when there
is more to transmit and = 0 at the end of
a transmission.
A single transmission can consist of one or more frames.
In a multi–frame transmission, Pad–Idle is sent between
consecutive frames to separate them. If an End Delimiter
occurs within a multi–frame transmission its I Bit will = 1,
and the last end delimiter will have its I Bit = 0.
The CBM accepts a stream of MAC symbols from the
TBC and modulates the phase–coherent transmit signal
accordingly. Conversely, the CBM receives a
phase–coherent signal stream from the cable, decodes the
MAC symbols, and reports them. On transmission there is
NON-DATA PAIR
a direct one–to–one correlation between MAC symbols
requested and the modulated signal; however, during
reception exceptions can occur. The CBM is allowed to
report Silence or the actual Zero/One pattern during
preamble which is done to allow the receiver to “train” to the
incoming signal. Also, if noise in the system has corrupted
the data, it may show up as an incorrect MAC symbol or the
CBM can report a BAD SIGNAL symbol if an incorrect
combination of ND symbols is detected (ND2 without an
ND1, ND2 followed by ND2, etc.)
1.4 SYSTEM CONFIGURATION
Figure 1–5 illustrates the CBM and peripheral circuitry
required for an IEEE 802.4 carrierband 5 Mbps or 10 Mbps
data rate phase–coherent FSK physical layer. The CBM
communicates with the MAC or TBC through a TTL
compatible serial interface that is consistent with the IEEE
802.4 exposed DTE–DCE interface. Management and
transmission symbol requests are accepted via the CBM
physical data request channel (TXSYM0–TXSYM2,
SMREQ*, and TXCLK). The physical data indication
channel (RXSYM0–RXSYM2, SMIND*, and RXCLK) is
used to send received symbols and management responses
to the MAC.
The periphery circuitry is primarily associated with
interface to the LAN coaxial cable and data recovery. An
external crystal or clock source is required (20 MHz for 5
Mbps data rate or 40 MHz for 10 Mbps data rate) for onboard
timing and transmit clock. Also, an RC timing network sets
the jabber timeout period.
The coaxial cable interface combines the transmit and
receive signal functions. For transmission, the CBM provides
differential drive signals (TXOUT and TXOUT*) whose
signaling is ECL levels referenced to VCC (logic high
+4.1 V , logic low [ + 3.3 V) and a gate signal called TXDIS.
The IEEE 802.4 standard puts specific requirements on the
signal transmitted to the cable:
Between +63 dB and +66 dB (1.0 mV, 75 Ω) [dBmV]
output voltage level.
Transmitter–off leakage not to exceed –20 dB
(1.0 mV, 75 Ω) [dBmV].
Signal transition time window (eye pattern)
dependent on data rate.
Because of this, an external amplifier with waveshaping
is required. The CBM TXOUT/TXOUT* outputs provide
complementary signals with virtually no slew, and the
TXDIS is an enable signal helpful for turning the external
amp off “hard” to meet the low level leakage.
[
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MC68194
AMPLIFICATI
On the reception side, the CBM requires a pre–amplifier
to receive the low level signal from the cable. The signal
available at the “F”–connector can range from +10 dB to +66
dB (1.0 mV , 75 Ω) [dBmV]. The signal required at the CBM
is about 12 dB above this (net gain through the transformer,
pre–amp, and any filtering). The receiver can be used in full
differential or single–ended mode.
A second part of the receiver function is the signal detect
or carrier detect function. The IEEE 802.4 requires that the
receiver detect a signal of +10 dBmV or above (i.e., be
turned “on”) and report Silence for a signal of +4.0 dBmV
RESET
SMREQ*
TXSYM2
TXSYM1
TXSYM0
TXCLK
SMIND*
RXSYM2
RXSYM1
RXSYM0
EOTDIS*
RXCLK
XTAL1
XTAL2
INTERFACE
DECODER
BUFFER
GENERATOR
SM MODE
BUFFER
SERIAL
OUTPUT
CLOCK
MUX
DAT A COMMANDS
STATION
MANAGEMENT
COMMANDS
PHYSICAL
MANAGEMENT
RECEIVE
DEMODULATOR
TRANSMIT
MODULATOR
LOOPBACK
RECEIVE
MUX
ONE SHOT
RECOVERY
SYNCHRONIZE
AND SQUELCH
CLOCK
AND
or below (i.e., be turned “off”). Therefore, a 6.0 dB (2:1
voltage ratio) range or window is defined in which the signal
detect must switch. The CBM is optimized for this range
(including the pre–amp gain), although it is trimmed via an
external THRESHOLD.
The remaining external components are associated with
clock recovery. A capacitor and resistor (internal R also
provided) set one–shot timing, and an active filter for a PLL
used in clock and data recovery is required. The active filter
can be implemented via an op amp, or if 5.0 volt operation
is required, an alternate charge pump design can be used.
ON
AND
WAVESHAPING
F–CONNECTOR
RECEIVE
PRE–AMP
BUFFER
JABBER
CONTROL
RECEIVE
AMPLIFIER
CARRIER
DETECT
TXDIS
JAB–RC
FDBK*
RXIN
RXIN*
FDBK
TXOUT
TXOUT*
V
CC
JAB
THRESHOLD
GAIN
CARDET
CPW RPW
+5 V
VCM–C1
SET–PW
Figure 1–5. Functional Block Diagram
The clock recovery and data decoder is a synchronous
design which provides superior performance minimizing
clock jitter.
Although primarily intended for the IEEE 802.4 carrier
band, the CBM is also an excellent device for point–to–point
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D* U* VCX
PLL
FILTER
data links, fiberoptic modems, and proprietary LANs. The
MC68194 can be used over a wide range of frequencies and
interfaces easily into different kinds of media.
5
MC68194
SECTION 2
SIGNAL DESCRIPTION
Symbol T ype Name/Description
TXSYM0–TXSYM2 TTL/I* TRANSMIT SYMBOLS — These TTL inputs are request channel signals used to send
either serial transmission symbols in the MAC mode or commands in station
management mode. They are synchronized to TXCLK and are normally connected to
the TXSYMX outputs of the MC68824. SMREQ* selects the meaning of these signals
as either MAC mode or management mode.
SMREQ* TTL/I* STATION MANAGEMENT REQUEST — A TTL input that selects the mode of the
request channel signals TXSYMX. Synchronized to TXCLK, SMREQ* is equal to one
for MAC mode and equal to zero for management mode. It is normally driven by the
SMREQ* output of the MC68824.
TXCLK TTL/O TRANSMIT CLOCK — A TTL clock output generated from the crystal oscillator (it is 1/4
of the oscillator frequency) used to receive request channel symbols from the MC68824.
TXCLK is equal to the data rate of the application (5.0 MHz or 10 MHz for IEEE 802.4).
TXSYMX and SMREQ* are synchronized to the positive edge of TXCLK which is
supplied to the MC68824.
RXSYM0–RXSYM2 TTL/O RECEIVE SYMBOLS — These TTL outputs are indication channel signals used to
provide either serial receive symbols in MAC mode or command confirmation/indication
in station management mode. They are synchronized to RXCLK and are normally
connected to the RXSYMX inputs of the MC68824. SMIND* selects the meaning of
these signals as either MAC mode or management mode.
SMIND* TTL/O STA TION MANAGEMENT INDICA TION — A TTL output that indicates the mode of the
CBM and RXSYMX lines. Synchronized to RXCLK, SMIND* is equal to one for MAC
mode and equal to zero for management mode. It is normally connected to the SMIND*
input of the MC68824.
RXCLK TTL/O RECEIVE CLOCK — A TTL clock output used to send indication channel symbols to
the MC68824. Its frequency is nominally equal to the data rate (5.0 MHz or 10 MHz for
IEEE 802.4). RXCLK is generated from a PLL that is locked to the local oscillator during
loopback, station management, or the absence of received data. During frame reception
the PLL is locked to the incoming received data. RXSYMX and SMIND* are
synchronized to negative edge of RXCLK.
EOTDIS* TTL/I* END–OF–TRANSMISSION DISABLE — When low, this TTL input disables the
end–of–transmission receiver blanking required by the IEEE 802.4 Spec, Section
12.7.6.3. When high the blanking works in accordance with the spec requirements.
TXOUT,TXOUT* ECL/O TRANSMIT OUTPUTS — A differential output signal pair (MECL level referenced to
VCC) used to drive the transmitter circuitry . The silence or “off” state is both outputs one
(high). The output data stream is phase–coherent FSK encoded.
TXDIS OC TRANSMIT DISABLE — An open collector output used to disable transmitter circuitry .
This output is high when the transmitter is off (TXOUT and TXOUT* both high).
JAB TTL/O JABBER — A TTL output signal generated from the jabber–inhibit timer. When equal
to one, JAB indicates the timer has timed–out and an error has occurred.
RESET TTL/I* RESET — A TTL input signal that when high asynchronously resets the CBM.
*All TTL inputs include a 15 kΩ pullup resistor to VCC.
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MC68194
Signal Description (Cont.)
Symbol Type Name/Description
RXIN, RXIN* I RECEIVER INPUTS — A differential input signal pair for the receiver amplifier/limiter.
These inputs may be used differentially or single ended.
FDBK, FDBK* DC FEEDBACK BYPASS — These two points are provided to bypass dc feedback
around the receiver amplifier.
THRESHOLD I THRESHOLD ADJUST — The receiver threshold detect is trimmed with this pin.
GAIN O GAIN — This output can be used to monitor the receiver amplifier output signal. Used
only for test purposes.
CARDET O CARRIER DETECT — This output can be used to filter the internal signal that is
sampled to sense carrier detect.
RPW, CPW I PULSE–WIDTH RESISTOR/CAPACITOR — A resistor and capacitor set a one–shot
pulse width used in the clock recovery circuitry.
SET–PW O PULSE WIDTH TEST POINT — Output test point used for adjusting clock recovery
one–shot pulse width.
UP*, DOWN* ECL/O PLL PHASE DETECTOR OUTPUTS — UP* and DOWN* are the pump–up and
pump–down outputs, respectively, of the PLL digital phase detector. They are MECL
levels referenced to +5.0 volts and are used to drive inputs to an active filter or charge
pump for the PLL.
VCX I VCM CONTROL — The control voltage applied to the PLL voltage controlled
multivibrator.
VCM–C1, VCM–C2 I VCM CAPACITOR — VCM capacitor inputs. VCM frequency is 4X RXCLK.
JAB–RC I JABBER–INHIBIT RC — A resistor–capacitor network connected to this pin sets the
jabber–inhibit time constant.
XTAL,1 XTAL2 I CLOCK CRYSTAL — Oscillator circuit inputs may be used with a crystal or an external
clock source. Oscillator frequency is 4X data rate.
VCC–VCM VCM POWER — 5.0 ± 5% volts for VCM.
VCC–TXOUT TXOUT POWER — 5.0 ± 5% volts for TXOUT/TXOUT*.
VCC–OSC OSCILLATOR POWER — 5.0 ± 5% volts for oscillator.
VCC–RCV RECEIVER POWER — 5.0 ± 5% volts for receiver amplifier/limiter.
V
CC
VCC–TTL TTL POWER — 5.0 ± 5% volts for TTL output buffers.
GND–TTL, GND–VCM,
GND–LOGIC, GND–OSC,
GND–RCV, GND–SUBS, GND
LOGIC POWER — 5.0 ± 5% volts for remaining logic.
GROUND — Reference voltage for TTL buffers, VCM, internal logic, oscillator, receiver/
limiter, substrate respectively. Two additional grounds are used to isolate signals.
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MC68194
SECTION 3
TRANSMITTER
3.1 OVERVIEW
The transmitter function includes the serial interface
decoder, transmit modulator , transmit buffer , jabber inhibit,
and clock generator. (Although the clock generator is not
used exclusively by the transmit function, the generator will
be discussed here.) The MC68194 receives request channel
symbols on the TXSYMX pins which are synchronized to
TXCLK. As is described in the Serial Interface discussion,
MAC transmit symbols are input serially (CBM in MAC
mode), decoded, and used to modulate an output signal. The
Serial Interface Decoder is used both for MAC mode to
decode data transmit commands (symbols) and
management mode to decode management commands. The
decoded transmit commands or symbols are used by the
Transmit Modulator to generate phase–coherent signaling
as discussed in the CBM General Description. The transmit
buffer receives the modulated signal and drives differential
output signals.
The clock generator provides TXCLK and internal clocks
of 2 times (2X) and 4 times (4X) TXCLK. The 4X clock is
actually the oscillator frequency. These clocks are used to
receive the TX symbols and generate the modulated signal.
3.2 TRANSMIT BUFFER
The modulated transmit data stream drives the TXOUT
and TXOUT* pins of the MC68194. These pins are
complementary outputs with closely matched edge
transitions. This is useful in helping meet the IEEE 802.4
carrierband requirement for a transmit jitter of less than
"
1% of the data rate. TXOUT and TXOUT* are generally
used to drive a differential amplifier which is used to achieve
the necessary output level at the cable and meet the rise/fall
time window (or “eye” pattern) of the IEEE 802.4. A third
output called TXDIS is available to gate the amplifier
circuitry on or off.
The TXTOUT and TXTOUT* have ECL levels
referenced to VCC (Figure 3–1). Levels are typically 4.11 V
for a high and 3.25 for a low. Pulldown resistors are required
with the outputs specified to drive a maximum load of 220
Ω to ground reference.
Operation of the transmit outputs is controlled in the
following manner:
1. Management mode — The TX outputs are always
disabled while the CBM is in management mode. When
leaving management mode the TX outputs remain
disabled if a RESET command has been issued and an
ENABLE TRANSMITTER and DISABLE
LOOPBACK commands have not been issued.
Resetting the CBM enables internal loopback and
disables the transmitter .
2. MAC (data) mode — After leaving management mode,
the CBM can function in internal loopback (for test) with
the transmitter disabled, out of loopback with transmitter
disabled (receive only), or in standard data mode with the
TX outputs controlled by the modulator.
TXDIS
VCC–TXOUT
TXOUT
RP
TX AMP
TXOUT*
RP
Figure 3–1. Transmitter Outputs
3. Jabber inhibit activated — If the jabber inhibit fires, it
forces the CBM into management mode and disables the
TX outputs. This condition can only be cleared by a reset
condition.
The TXDIS output is an open collector switched current
source. TXDIS sinks a nominal 0.5 mA when the
TXOUT/TXOUT* outputs are enabled. TXDIS is off or
high impedance when the transmitter is disabled.
The signaling on the TX outputs and TXDIS is shown in
Figure 3–2. The “off” or silence condition is both TXOUT
outputs high and TXDIS also high. The figure shows an
example of the signal pattern for both leaving and entering
a silence condition.
SILENCE
0
TXDIS
TXOUT
TXOUT*
Figure 3–2. Transmitter Output Signaling
11 ND1 ND21
OFFOFF
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