Dallas Semiconductor DS275S-T-R, DS275S, DS275E-T-R, DS275E-N, DS275E Datasheet
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FEATURES
Low-power serial transmitter/receiver for
battery-backed systems
Transmitter steals power from receive signal
line to save power
Ultra-low static current, even when connected
to RS-232-E port
Variable transmitter level from +5 to +12
volts
Compatible with RS-232-E signals
Available in 8-pin, 150 mil wide SOIC
package (DS275S)
Low-power CMOS
ORDERING INFORMATION
DS275 8-pin DIP
DS275S 8-pin SOIC
DS275E 14-pin TSSOP
PIN ASSIGNMENT
PIN DESCRIPTION
RX
OUT
- RS-232 Receiver Output
V
DRV
- Transmit driver +V
TXIN - RS-232 Driver Input
GND - System Ground (0V)
TX
OUT
- RS-232 Driver Output
NC - No Connection
RXIN - RS-232 Receive Input
V
CC
-System Logic Supply (+5V)
DESCRIPTION
The DS275 Line-Powered RS-232 Transceiver Chip is a CMOS device that provides a low-cost, very
low-power interface to RS-232 serial ports. The receiver input translates RS-232 signal levels to common
CMOS/TTL levels. The transmitter employs a unique circuit which steals current from the receive RS232 signal when that signal is in a negative state (marking). Since most serial communication ports
remain in a negative state statically, using the receive signal for negative power greatly reduces the
DS275’s static power consumption. This feature is especially important for battery-powered systems such
as laptop computers, remote sensors, and portable medical instruments. During an actual communication
session, the DS275’s transmitter will use system power (5-12 volts) for positive transitions while still
employing the receive signal for negative transitions.
DS275
Line-Powered RS-232 Transceiver Chip
www.dalsemi.com
DS275 8-Pin DIP (300-mil)
DS275 8-Pin SOIC (150-mil)
7
TX
IN
V
CC
NC
1
2
3
4
8
6
5
V
DRV
GND
RX
OUT
RX
IN
TX
OUT
DS275E 14-Pin TSSOP
13
V
DRV
TX
IN
GND
V
CC
RX
IN
NC
NC
TX
OUT
1
2
3
4
5
6
7
14
12
11
10
9
8
NC
NC
NC
RX
OUT
NC
NC
DS275
2 of 8
DS275 BLOCK DIAGRAM Figure 1
OPERATION
Designed for the unique requirements of battery-backed s ystems, the DS275 provides a low-power h alfduplex interface to an RS-232 serial port. Typically, a designer must use an RS-232 device which uses
system power during both negative and positive transitions of the transmit signal to the RS-232 port. If
the connector to the RS-232 port is left connected for an appreciable time after the communication
session has ended, power will statically flow into that port, draining the battery capacity. The DS275
eliminates this static current drain by stealing current from the receive line (RXIN) of the RS-232 port
when that line is at a negative level (marking). Since most asynchronous communication over an RS-232
connection typically remains in a marking state when data is not being sent, the DS275 will not consume
system power in this condition. System power would only be used when positive-going transitions are
needed on the transmit RS-232 output (TXOUT) when data is sent. However, since synchronous
communication sessions typically exhibit a very low duty-cycle, overall system power consumption
remains low.
RECEIVER SECTION
The RXIN pin is the receive input for an RS-232 signal whose levels can r ange from ±±±±3 to ±±±±15 volts. A
negative data signal is called a mark while a positive data signal is called a space. These signals are
inverted and then level
-shifted to normal +5-volt CMOS/TTL logic levels. The logic output associated
with RX
IN is RXOUT which swings from +VCC to ground. Therefore, a mark on RXIN prod uces a logi c 1 at
RXOUT; a space produces a logic 0.
The input threshold of RX
IN is typically around 1.8 volts with 500 millivolts of hysteresis to improve
noise rejection. Therefore, an input positive-going signal must exceed 1.8 volts to cause RXOUT to switch
states. A negative-going signal must now be lower than 1.3 volts (typically) to cause RX
OUT to switch
again. An open on RXIN is interpreted as a mark, producing a logic 1 at RXOUT.
TRANSMITTER SECTION
TXIN is the CMOS/TTL-compatible input for digital data from the user system. A logic 1 at TXIN
produces a mark (negative data sign al) at TXOUT while a logic 0 produces a space (positive data si gnal).
As mentioned earlier, the transmitter section employs a unique driver design that uses the RX
IN line for
swinging to negative levels. The RXIN line must be in a marking or idle state to take advantage of this
design; if RX
IN is in a spacing stat e, TXOUT will only swing to ground. When TXOUT needs to transition to
a positive level, it uses the V
DRV power pin for this level. VDRV can be a voltage suppl y between 5 to 12
DS275
3 of 8
volts, and in many situations it can be tied directly to the +5 volt VCC supply. It is important to note that
VDRV must be greater than or equal to VCC at all times.
The voltage range on VDRV permits the use of a 9-volt battery in order to provide a higher voltage level
when TXOUT is in a space state. W hen VCC is shut off to the DS275 and VDRV is still powered (as might
happen in a battery-backed condition), only a small leakage current (about 50-100 nA) will be drawn. If
TX
OUT is loaded during such a condition, VDRV will draw current only if RXIN is not in a negative state.
During normal operation (V
CC=5 volts), VDRV will draw less than 2 uA when TXOUT is marking. Of
course, when TXOUT is spacing, VDRV will draw substantially more current====about 3 mA, depending
upon its voltage and the impedance that TXOUT sees.
The TXOUT output is slew rate-limited to less than 30 volts/us in accordance with RS-232 specifications.
In the event TXOUT should be inadvertently shorted to ground, internal current-limiting circuitr y prevents
damage, even if continuously shorted.
RS-232 COMPATIBILITY
The intent of the DS275 is not so much to meet all the requirements of the RS-232 specification as to
offer a low-power solution that will work with most RS-232 ports with a connector length of less than 10
feet. As a prime example, the DS275 will not meet the RS-232 requirement that the signal levels be at
least ±±±±5 volts minimum when terminated by a 3 kΩΩΩΩ====load and VDRV = +5 volts. Typically a voltage of 4
volts will be present at TXOUT when spacing. However, since most RS-232 receivers will correctly
interpret any voltage over 2 volts as a space, there will be no problem transmitting data.
APPLICATIONS INFORMATION
The DS275 is designed as a low-cost, RS-232-E interface expressly tailored for the unique requirements
of battery-operated handheld products. As shown in the electrical specifications, the DS275 draws
exceptionally low operating and static current. During normal operation when data from the handheld
system is sent from the TXOUT output, the DS275 only draws significant VDRV current when TXOUT
transitions positively (spacing). This current flows primarily into the RS-232 receiver’s 3-7 kΩΩΩΩ====load at the
other end of the attaching cable. When TXOUT is marking (a negative data signal), the VDRV current falls
dramatically since the negative voltage is provided by the transmit signal from the other end of the cable.
This represents a large reduction in overall operating current, since typical RS-232 interface chips use
charge-pump circuits to establish both positive and negative levels at the transmit driver output.
To obtain the lowest power consumption from the DS275, observe the following guidelines. First, to
minimize VDRV current when connected to an RS-232 port, always maintain TXIN at a logic 1 when data is
not being transmitted (idle state). This will force TX
OUT into the marking state, minimizing VDRV current.
Second, VDRV current will drop to less than 100 nA when VCC is grounded. Therefore, if VDRV is tied
directly to the system battery, the logic +5 volts can be turned off to achieve the lowest possible power
state.
FULL-DUPLEX OPERATION
The DS275 is intended primarily for half-duplex operation; that is, RXIN should remain idle in the
marking state when transmitting data out TX
OUT and visa vers a. However, the part can be operated full -
duplex with most RS-232-E serial ports since signals swinging between 0 and +5V will usually be
correctly interpreted by an RS-232-E receiver device. The 5-volt swing occurs when TXOUT att empts to
swing negative while RXIN is at a positive voltage, which turns on an internal weak pulldown to ground
for the TX
OUT driver’s negative reference. So, transmit mark signals at TXOUT may have voltage jumps
from some negative value (corresponding to RX
IN marking) to approximately ground. One possible
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