Current loop and RS-485 converters troubleshooting

This chapter discusses how to troubleshoot problems related to current loop and RS-485 interface converters. The troubleshooting of interface converters is similar to that of the RS-232 and RS-485standards. Care should be taken in using non-isolated converters where there are any possibilities of electrical surges and transients. In addition, loop-powered RS-232/485 converters should also be avoided because they can be unreliable. Using one of the handshaking pins of the RS-232 port may sound clever and avoid the necessity of providing a separate power supply, but RS-232 was never intended to be a power supply. If there is no power to the converter or the power supply on the converter is damaged, the LED on the converter will not light up. In addition, there is provision on some converters for an isolated power LED, which should also be illuminated solid green. With all the connections made, the converter should have the power light on. This LED will be lit up to indicate that the RS-485 converter is active and driving the network. In a multi-drop network, only one unit should be driving the line at one time; otherwise, there will be contention with no resultant transmission at all. Although the configuration jumpers are often set internally, sometimes the user can also set them. This jumper determines what happens if the RS-232 host is disconnected from the converter. Sometimes, noise rejection performance can be improved by grounding the digital logic.