Can I connect two OPTO-4 cables together to get bi-directional isolation?
This is possible when connecting the wire ends together, but not when connecting the DB9 connectors together. Connecting the DB9 9 pin connectors together will remove the isolation, connecting the ground of both devices together.
Connecting the wire ends together will preserve the isolation. Both OPTO-4 cables will require power on the 9 pin RS232 connector, typically provided by the RS2323 port handshaking lines.
This configuration can also be used to extend an RS232 cable from the standard 20 metres potentially up to 200 metres as was demonstrated by one customer.