DS276
OPERATION
Designed for the unique requirements of battery-backed systems, the DS276 provides a low-power full-or
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 DS276
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 DS276 will not consume
system power in this condition. Sys-tem 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 +3 or +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 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 can use the RXIN line for
swinging to negative levels. RXIN can be connected via external circuitry to VDRV- to allow stored charge
to supply this voltage during marking (or idle) states. 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 3 to 12 volts, and in
many situations it can be tied directly to the 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 9V battery in order to provide a higher voltage level
when TXOUT is in a space state. When VCC is shut off to the DS276 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
TXOUT is loaded during such a condition, VDRV+ will draw current only if RXIN is not in a negative state.
During normal operation (VCC = 3 or 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 circuitry prevents damage, even if continuously shorted.
RS-232 COMPATIBILITY
The intent of the DS276 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
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DALLAS [ DALLAS SEMICONDUCTOR ]
