DS275
Line-Powered RS-232 Transceiver Chip
www.dalsemi.com
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
PIN ASSIGNMENT
RX
OUT
V
DRV
TX
IN
GND
1
2
3
4
8
7
6
5
V
CC
RX
IN
NC
TX
OUT
DS275 8-Pin DIP (300-mil)
DS275 8-Pin SOIC (150-mil)
RX
OUT
NC
V
DRV
NC
TX
IN
NC
GND
1
2
3
4
5
6
7
14
13
12
11
10
9
8
V
CC
NC
NC
RX
IN
NC
NC
TX
OUT
ORDERING INFORMATION
DS275
DS275S
DS275E
8-pin DIP
8-pin SOIC
14-pin TSSOP
RX
OUT
V
DRV
TX
IN
GND
TX
OUT
NC
RX
IN
V
CC
DS275E 14-Pin TSSOP
PIN DESCRIPTION
- RS-232 Receiver Output
- Transmit driver +V
- RS-232 Driver Input
- System Ground (0V)
- RS-232 Driver Output
- No Connection
- 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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DS275
DS275 BLOCK DIAGRAM
Figure 1
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 (RX
IN
) 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 (TX
OUT
) when data is sent. However, since synchronous
communication sessions typically exhibit a very low duty-cycle, overall system power consumption
remains low.
The RX
IN
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 RX
IN
is RX
OUT
which swings from +V
CC
to ground. Therefore, a mark on RX
IN
produces a logic 1 at
RX
OUT
; 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 RX
OUT
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 RX
IN
is interpreted as a mark, producing a logic 1 at RX
OUT
.
RECEIVER SECTION
TRANSMITTER SECTION
TX
IN
is the CMOS/TTL-compatible input for digital data from the user system. A logic 1 at TX
IN
produces a mark (negative data signal) at TX
OUT
while a logic 0 produces a space (positive data signal).
As mentioned earlier, the transmitter section employs a unique driver design that uses the RX
IN
line for
swinging to negative levels. The RX
IN
line must be in a marking or idle state to take advantage of this
design; if RX
IN
is in a spacing state, TX
OUT
will only swing to ground. When TX
OUT
needs to transition to
a positive level, it uses the V
DRV
power pin for this level. V
DRV
can be a voltage supply between 5 to 12
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DS275
volts, and in many situations it can be tied directly to the +5 volt V
CC
supply.
It is important to note that
V
DRV
must be greater than or equal to V
CC
at all times.
The voltage range on V
DRV
permits the use of a 9-volt battery in order to provide a higher voltage level
when TX
OUT
is in a space state. When V
CC
is shut off to the DS275 and V
DRV
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, V
DRV
will draw current only if RX
IN
is not in a negative state.
During normal operation (V
CC
=5 volts), V
DRV
will draw less than 2 uA when TX
OUT
is marking. Of
course, when TX
OUT
is spacing, V
DRV
will draw substantially more current
=
about 3 mA, depending
upon its voltage and the impedance that TX
OUT
sees.
The TX
OUT
output is slew rate-limited to less than 30 volts/us in accordance with RS-232 specifications.
In the event TX
OUT
should be inadvertently shorted to ground, internal current-limiting circuitry 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 kΩ
=
load and V
DRV
= +5 volts. Typically a voltage of 4
Ω
volts will be present at TX
OUT
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 TX
OUT
output, the DS275 only draws significant V
DRV
current when TX
OUT
transitions positively (spacing). This current flows primarily into the RS-232 receiver’s 3-7 kΩ
=
load at the
Ω
other end of the attaching cable. When TX
OUT
is marking (a negative data signal), the V
DRV
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 V
DRV
current when connected to an RS-232 port, always maintain TX
IN
at a logic 1 when data is
not being transmitted (idle state). This will force TX
OUT
into the marking state, minimizing V
DRV
current.
Second, V
DRV
current will drop to less than 100 nA when V
CC
is grounded. Therefore, if V
DRV
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, RX
IN
should remain idle in the
marking state when transmitting data out TX
OUT
and visa versa. 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 TX
OUT
attempts to
swing negative while RX
IN
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 TX
OUT
may have voltage jumps
from some negative value (corresponding to RX
IN
marking) to approximately ground. One possible
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DS275
problem that may occur in this case is if the receiver at the other end requires a negative voltage for
recognizing a mark. In this situation, the full-duplex circuit shown in Figure 3 can be used as
analternative. The 22
µ
F capacitor forms a negative-charge reservoir; consequently, when the TXD line is
spacing (positive), TX
OUT
still has a negative source available for a time period determined by the
capacitor and the load resistance at the other end (3-7 kΩ). This circuit was tested from 150-19,200 bps
Ω
with error-free operation using a SN75154 Quad Line Receiver as the receiver for the TX
OUT
signal. Note
that the SN75154 can have a marking input threshold below ground; hence there is the need for TX
OUT
to
swing both positive and negative in full-duplex operation with this device.
HANDHELD RS-232-C APPLICATION USING A STEREO MINI-JACK
Figure 2
FULL-DUPLEX CIRCUIT USING NEGATIVE-CHARGE STORAGE
Figure 3
NOTE:
The capacitor stores negative charge whenever the TXD signal from the PC serial port is in a marking
data state (a negative voltage that is typically -10 volts). The top DS275’s TX
OUT
uses this negative
charge reservoir when it is in a marking state. The capacitor will discharge to 0 volts when the TXD line
is spacing (and TX
OUT
is still marking) at a time constant determined by its value and the value of the load
resistance reflected back to TX
OUT
. However, when TXD is marking the capacitor will quickly charge
back to -10 volts. Note that TXD remains in a marking state when idle, which improves the performance
of this circuit.
ABSOLUTE MAXIMUM RATINGS*
V
CC
V
DRV
-0.3 to +7.0 volts
-0.3 to +13.0 volts
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DS275
RX
IN
TX
IN
TX
OUT
RX
OUT
Storage Temperature
Operating Temperature
*
±15 volts
-0.3 to V
CC
+ 0.3 volts
±15 volts
-0.3 to V
CC
+ 0.3 volts
-55°C to +125°C
0°C to 70°C
This is a stress rating only and functional operation of the device at these or any other conditions
above those indicated in the operation sections of this specification is not implied. Exposure to
absolute maximum rating conditions for extended periods of time may affect reliability.
RECOMMENDED DC OPERATING CONDITIONS
PARAMETER
Logic Supply
Transmit Driver Supply
Logic 1 Input
Logic 0 Input
RS-232 Input Range (RX
IN
)
Dynamic Supply Current
TX
IN
= V
CC
TX
IN
= GND
Static Supply Current
TX
IN
= V
CC
TX
IN
= GND
Driver Leakage
Current (V
CC
=0V)
SYMBOL
V
CC
V
DRV
V
IH
V
IL
V
RS
I
DRV1
I
CC1
I
DRV1
I
CC1
I
DRV2
I
CC2
I
DRV2
I
CC2
I
DRV3
MIN
4.5
4.5
2.0
-0.3
-15
TYP
5.0
5-12
MAX
5.5
13.0
V
CC
+0.3
+0.8
+15
800
100
5.0
100
10.0
15.0
5.0
20.0
1.0
UNITS
V
V
V
V
V
µA
µA
µA
µA
µA
µA
mA
µA
µA
NOTES
1
1
2
400
40
3.8
40
1.5
10.0
3.8
10.0
0.05
3
4
5
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