Multiplexer Enables Pseudomultidrop RS-232 Transmission
One UART and one multiplexer enable one RS-232 transceiver to communicate with four others.
Channel Selection Codes
|All channels disconnected||X||X||0|
|Channel 1 (IC3)||0||0||1|
|Channel 2 (IC4)||0||1||1|
|Channel 3 (IC5)||1||0||1|
|Channel 4 (IC6)||1||1||1|
The circuit's supply-voltage range (3V to 5.5V) makes it compatible with 3V and 5V logic. IC2 receives its power directly from the V+ and V- terminals of IC1, whose ±5.5V outputs come from an internal charge pump. The multiplexer handles rail-to-rail signals, so obtaining its power from IC1 ensures that RS-232 signals pass directly through, regardless of amplitude. Each transceiver's charge pump requires four small capacitors (not shown), whose values depend on the VDD range but do not exceed 0.47µF. Note that pulling too much current from the charge-pump terminals of IC1, V+ and V-, can cause these rails to droop and can pull the IC's RS-232 transmission levels out of specification.
A similar version of this article appeared in the September 28, 2000 issue of EDN.