Troubleshooting PLC Digital Input Modules, Part 1

By definition, digital inputs are either on or off. Thus, verifying the absence or presence of voltage is a critical part of troubleshooting digital input modules. But what voltage should you look for? That depends on the power source for the input loop(s) in question.

The following are some basic facts about input loop power sources:

• The input module usually isn't a loop power source.
• Although 120V is common for digital loops, other voltages are also used.
• There are two types of digital input, differentiated by their power source: isolated and non-isolated.
• Non-isolated inputs are powered by a common source. For example, the wiring diagram shows a non-isolated input module with L1 (common power source) feeding an array of input devices displayed in ladder fashion.
• An isolated input is on its own channel and may be powered by its own source. If your system had four such inputs, then the wiring diagram would again show an array of input devices displayed in ladder fashion. But this time, they wouldn’t share L1. Instead, each would be supplied by L1a, L1b, L1c, and L1d.

Your first step in troubleshooting digital input modules is to verify the power sources:

1. Identify which inputs are isolated and non-isolated.
2. Verify the correct voltage is present at each L1 (a blown input fuse is a common reason it is not present).