With this dataselector, only one RS-232 port is used to connect two RS-232 devices (i.e., printer, plotter, etc.) with a mini-or microcomputer. The operation is very simple. Power on will reset FFI (QFFT ~ Low), which enables gates N1, N5, and N7. Now communication between computer and device B is possible. Detection of the switch command, i.e., Control B character ~ CHR$(2), selectable with wirewrap pins, on the parallel outputs of the UART (IM 6402 or equivalent) will set: QFFT ~ High.
Automatic-rs-232-dataselector - schematic

Gates N2, N5, and N7 are open, so device A is connected with the computer until Control B character is detected again. Transistor Q1 converts RS-232 levels to TTL levels while two LEDs indicate whether device A orB is linked. The baud-rate generator provides the 16 x clock needed for the UART. Any baud rate ranging from 50 to 19200 can be selected. Manual control of the selector is available with toggleswitch Sl.

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