A PLC (Programmable Logic Controller) is built from modules — a CPU, a power supply, input/output (I/O) modules and communication cards, all sitting on a chassis or backplane. When one fails, you usually replace that single module, and for PLCs the match needs to be precise.
Start with the exact catalog number
Unlike a bearing, a PLC module is rarely cross-referenced to another brand — you replace it with the same manufacturer's part. Read the catalog number printed on the module face:
- Allen-Bradley / Rockwell: ControlLogix 1756-xxxx, CompactLogix 1769-xxxx, MicroLogix 1762/1766, PLC-5 and SLC 500 1746/1747.
- Siemens: SIMATIC S7 starts with 6ES7 — e.g. S7-1200, S7-1500, S7-300, S7-400.
- Also note Schneider (Modicon), Mitsubishi (FX/Q), and Omron (CJ/CP) families.
Match the I/O type and point count
- Direction: input or output module (or combo).
- Signal: digital (discrete) or analog; for analog note voltage/current ranges (0–10V, 4–20mA).
- Voltage: 24V DC, 120V AC, 240V AC for digital points.
- Point count: 8, 16, 32 points — and the terminal/wiring style.
Watch series and firmware compatibility
Modules come in series/revisions (Series A, B, C…) and CPUs run firmware versions. A replacement usually must be the same series or newer, and a CPU's firmware should match what the program expects. Note the series letter and firmware revision before you order — this is the detail that trips people up.
Discontinued PLC parts
Older platforms (SLC 500, PLC-5, S7-300) are being phased out, so many modules are now obsolete — available mainly as new-old-stock or tested refurbished units. Send us the catalog number and series, and we'll find it across the surplus and refurbished market and quote it with condition and warranty stated.