B&B Electronics 3PCIU4 - Manual (Page 76)

Troubleshooting MIport Cards
70 Chapter 9 Manual Documentation Number 3PCIoUx-1008
Figure 53. RS-232 Loopback Connector
3. To check 2-wire RS-485 RS-422 or 4-wire RS-485 Loopback
Connections, you must either enable the receiver by moving the
receive jumper to RX ENABLE mode, or use one port to transmit to
another 2-wire RS-485 port or converter by cross connecting and
loading ComTest twice, one copy for each port.
Characters typed in one copy of ComTest will appear in the receive
window of the other. Note that software must ignore the RS-232
handshaking lines in RS-422/RS485 mode, the input lines (CTS,
DSR, DCD, RI) are not pulled high.
4. Some manufacturers label their data connections for A and B
reverse of the standard for RS-422 or RS-485, so that the A line is
(+) and B line is inverted (-) following the IC standard. In such a
case, just swap the wires and try it. No damage occurs in the RS-
485 mode, just no devices will respond. The EIA standard for signal
labels defines the B line as positive relative to A during the
“MARK” state.
5. Try another software package for troubleshooting. Some
applications try to bypass the Windows drivers and access the
hardware directly.
6. Check the B&B website for available FAQs or troubleshooting
hints.
7. Contact B&B Electronics Technical Support for troubleshooting
assistance.