1 null, 2 data transmission, 3 data reception, 4 request transmission, 5 clear transmission, 6 data ready, 7 signal grounding, 8 carrier detection, 9-1null, 20 data terminal ready, 2 1 null? The alarm of 22 indicates that 23-25 is empty.

The full name of RS-422 standard is "balanced voltage digital interface circuit", which defines the characteristics of interface circuit. In fact, there is also a signal ground wire, ***5 wire. Because the receiver adopts high input impedance, the transmitter driver has stronger driving ability than RS232, so it is allowed to connect multiple receiving nodes on the same transmission line, up to 256 nodes.

Reference circuit:

Turn 9-pin circular stitch definition