RS-232C multiplexer serves as multiport adapter

Multiport RS-232C PC add-on boards are intended for intensive multichannel communication. Therefore, each serial port has its own transmission controller. If your application doesn't need interrupt-driven service, and polling the devices through RS-232C channels one after another is sufficient, then you can use a single controller with switched input and output signals instead of installing a costly multiport adapter.
The ISA bus directly controls the eight-channel multiplexer in Figure 1. The GAL20V8 integrates the address decoder and other glue logic. The latched address and gating signals drive the DG509A analog multiplexers that actually switch the TXD and RXD lines. The circuit automatically selects channel 0 upon system reset (you can download PLD equations from EDN's ftp site). The analog multiplexers feature both overvoltage protection and break-before-make switching so that the circuit operates safely over the ±12V range.

You can easily scale the number of available channels from four to sixteen by using four multiplexers and two decoders with opposite SEL settings. One PLD can accommodate four multiplexers if you decode additional E₂ and E₃ enable signals from Q₂ and Q₃. You can also connect the multiplexers in parallel to switch other signals. (DI #1832)