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RS-485/422 Cabling

To monitor RS-485 or RS-422 circuits, the FTS cables are not used because they're specific to RS-232.  Since RS-485 and RS-422 have no standard pin out, you'll need to construct your own cables.  Basically, you just need to tap onto connect each transmit line on your circuit and connect it to one of the receive pins on the serial port(s) you're using to monitor.

      T+  ----------------------------   R+

      T-  -----------------------------  R-

  GND  -----------------------------  GND

     R+  -----------------------------  T+

     R-   -----------------------------  T-

To monitor the circuit above, you'd tap off of the T+ > R+ line to R+ on your serial port, the T- > R- line to R- on that port, then the R+ > T+ to R+ on your other port and R- > T- to R- on that port.  Then connect the ground to ground on each port.

If you're using a Quatech PCI or PCMCIA RS-485 serial card, please download the attached instructions.  These instructions may also be valuable if you're not using a Quatech card, just substitute the pin assignments for your serial port.

An excellent discussion of RS-485 and RS-422 theory may be found at the B&B Electronics web site, http://www.bb-elec.com/bb-elec/literature/tech/485appnote.pdf.

RS 422-485 Cabling.pdf