PLC expansion modules — why and when you need them
A base CPU has limited I/O — typically 14-32 points. When a project demands more, you add expansion modules: discrete, analog, communication, specialized. This is cheaper than buying a higher-end CPU and lets you configure the system precisely.
We select modules for 10-15 projects monthly. Typical situation: a customer bought a Delta DVP-SS2 with 14 I/O, and a year later needs analog inputs for sensors and RS-485 for VFDs.
Module types
| Type | Function | Delta | Siemens |
|---|---|---|---|
| DI | Buttons, limit switches | DVP-16SP | SM 1221 |
| DO | Contactors, valves | DVP-16SN | SM 1222 |
| AI | Sensors 4-20 mA | DVP-04AD | SM 1231 |
| AO | VFD control | DVP-02DA | SM 1232 |
| RTD | Pt100, thermocouples | DVP-04PT | SM 1231 RTD |
| COM | RS-485, Ethernet | DVP-EN01-SL | CM 1241 |
Selection algorithm
- Count signals beyond base CPU
- Check series compatibility
- Account for max module count per CPU
- Calculate power draw from internal bus
Delta DVP modules
- DVP-04AD: 4 AI, 12-bit. For pressure sensors
- DVP-02DA: 2 AO, 12-bit
- DVP-16SP: 8 DI + 8 DO
- DVP-04PT: 4 Pt100 inputs
- DVP-EN01-SL: Ethernet module
Siemens S7-1200 modules
- SM 1231 AI4: 4 AI, 13-bit
- SM 1221 DI16: 16 DI
- SB 1232 AO1: 1 AO in CPU housing
- CM 1241: RS-485 for Modbus RTU
Mitsubishi FX5U modules
- FX5-4AD: 4 AI, 16-bit
- FX5-4DA: 4 AO, 16-bit
- FX5-C32E: adapter for FX3U modules
Frequently Asked Questions
How many modules can I connect?
Delta DVP-SS2 — up to 8, Siemens S7-1200 — up to 8 SM + 3 CM + 1 SB, Mitsubishi FX5U — up to 16.
Can I use modules from a different manufacturer?
No. Modules only work with their own CPU. Exception — distributed I/O via Modbus TCP or PROFINET.
Cheaper to buy a higher CPU or add modules?
Usually modules are cheaper. One DVP-04AD costs $80-120. But with 3+ modules, a higher CPU may be more economical.
Do modules affect PLC performance?
Yes, slightly. Each module adds 0.1-1 ms to scan cycle time.
How to connect Pt100 to a PLC?
Via a specialized RTD module. A standard analog module will not work.
Summary
Expansion modules let you grow the system without replacing the CPU. Browse: PLC expansion modules, Delta PLCs, Siemens PLCs.
