Multiplexer Enables Pseudomultidrop RS-232 Transmission

Maxim MAX399 MAX3221

Dan Christman, Maxim Integrated Products


RS-232 communications with one µC and more than one remote system can be problematic, because most µCs contain only one UART, which provides an interface between synchronous and asynchronous ports. The multiplexer in Figure 1, IC2, allows multiple channels (four, in this case) to share a single UART. The dual four-to-one multiplexer allows transceiver IC1 to form a network with the four remote transceivers IC3 to IC6. Table 1 defines the channel-selection codes. Selecting Channel 1, for instance, enables IC1 to communicate with IC3 without being loaded by IC4 to IC6. Pulldown resistors inside the remote transceivers force the outputs of unselected receivers to a known state.

Table 1. Channel selection
A1 A0 EN Selected channel
X X 0 All channels disconnected
0 0 1 Channel 1 (IC3)
0 1 1 Channel 2 (IC4)
1 0 1 Channel 3 (IC5)
1 1 1 Channel 4 (IC6)
One UART and one multiplexer enable one RS-232 transceiver to communicate with four others.
Figure 1. One UART and one multiplexer enable one RS-232 transceiver to communicate
with four others.

The circuit's supply-voltage range (3 to 5.5 V) makes it compatible with 3 and 5 V logic. IC2 receives its power directly from the V+ and V– terminals of IC1, whose ±5.5 V 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 0.47 µF. Note that pulling too much current from the charge-pump terminals of IC1, V+ and V–, can cause these rails to droop and may pull the IC's RS-232 transmission levels out of specification.

Materials on the topic

  1. Datasheet Maxim Integrated MAX399
  2. Datasheet Maxim Integrated MAX3221


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