I am connecting a RS422 device to an NI AT485/2 (ISA bus) board in an IBM-compatible PC, and would appreciate help with pin connections and where to put terminating resistors, if needed.

[Alan McLeod]

The device (eventually four NBS BIOFLO III Fermenters) has a 25 pin connector, for which I have the pin designations (it has GND, IRXD+, IRXD-, ITXD+, ITXD-). It can be configured for RS232 or RS422 for multi-drop configuration. I have tried some "sensible" two and four-wiring configurations without success. I am not sure about terminating resistors and where they would go, or if they are even necessary. I am attempting to communicate with the device using a program written in VB6, using the MS Comm control. Any help would be appreciated!

[Ronaldo_Schwarz]

The best to way to connect is in the RS-422 form. In this way you don´t need terminating resistors. Here is the pin configuration for your cable

Device AT485/2
GND GND (PIN 1)
IRXD+ TXD+ (PIN 😎
IRXD- TXD- (PIN 9)
ITXD+ RXD+ (PIN 4)
ITXD- RXD- (PIN 5)

[Alan McLeod]

Thanks Ronaldo, I will try these connections (again). I will set the driver to four wire connection. What should handshaking be, or does it matter?

[Ronaldo_Schwarz]

I have never used handshaking with RS-422 and it always worked for me. By your description I think you will not have problems.