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Dallas Semiconducotr

DS275 Datasheet Preview

DS275 Datasheet

Line-Powered RS-232 Transceiver Chip

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www.dalsemi.com
DS275
Line-Powered RS-232 Transceiver Chip
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
RXOUT
VDRV
TXIN
1
2
3
8 VCC
7 RXIN
6 NC
GND 4
5 TXOUT
DS275 8-Pin DIP (300-mil)
DS275 8-Pin SOIC (150-mil)
RXOUT
NC
VDRV
NC
TXIN
NC
GND
1
2
3
4
5
6
7
14 VCC
13 NC
12 NC
11 RXIN
10 NC
9 NC
8 TXOUT
DS275E 14-Pin TSSOP
PIN DESCRIPTION
RXOUT
VDRV
TXIN
GND
- RS-232 Receiver Output
- Transmit driver +V
- RS-232 Driver Input
- System Ground (0V)
TXOUT
NC
- RS-232 Driver Output
- No Connection
RXIN
VCC
- RS-232 Receive Input
-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 RS-
232 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.
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Dallas Semiconducotr

DS275 Datasheet Preview

DS275 Datasheet

Line-Powered RS-232 Transceiver Chip

No Preview Available !

DS275 BLOCK DIAGRAM Figure 1
DS275
OPERATION
Designed for the unique requirements of battery-backed systems, the DS275 provides a low-power half-
duplex 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 range 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 RXIN is RXOUT which swings from +VCC to ground. Therefore, a mark on RXIN produces a logic 1 at
RXOUT; a space produces a logic 0.
The input threshold of RXIN 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 RXOUT 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 signal) at TXOUT while a logic 0 produces a space (positive data signal).
As mentioned earlier, the transmitter section employs a unique driver design that uses the RXIN line for
swinging to negative levels. The RXIN line must be in a marking or idle state to take advantage of this
design; if RXIN is in a spacing state, TXOUT will only swing to ground. When TXOUT needs to transition to
a positive level, it uses the VDRV power pin for this level. VDRV can be a voltage supply between 5 to 12
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Part Number DS275
Description Line-Powered RS-232 Transceiver Chip
Maker Dallas Semiconducotr
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DS275 Datasheet PDF






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