DS276 datasheet(2/11 Pages) DALLAS

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DS276

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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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