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Phone: +86 16626708626Description
Key Technical Specifications
- Product Type: Modbus Master/Slave Communication Module
- Compatible Platform: Allen-Bradley 1771 I/O rack — PLC-5, PLC-2, PLC-3
- Command List Entries: 100, shared across the module’s application ports
- Word Reads/Writes: 125 per command
- Max Registers: 4,000
- Command Types: Broadcast, event-driven bit and register writes
- Processor Data Transfer: Asynchronous
- Serial Interfaces: RS-232 / RS-422 / RS-485 — confirm exact port count and pin assignment against the nameplate or OEM datasheet before wiring
- Max Baud Rate: 38,400 bps (per ProSoft’s own comparison documentation against the later MVI71-MCM, which runs up to 115,200 bps)
- Optical Isolation: 500 V
- Backplane Current Load: 0.65 A
- Weight: approx. 1.05 kg (2.31 lb) — minor variance seen across listings depending on packaging
- UPC: 10611320405498
Product Introduction
The 3100-MCM is a Modbus master/slave communication module built by ProSoft Technology for the Allen-Bradley 1771 I/O platform, running in PLC-5, PLC-2, and PLC-3 racks. It handles up to 4,000 registers through 100 command list entries, moving data between the processor and Modbus RTU field devices over RS-232, RS-422, or RS-485.
Plants still running PLC-5 systems keep this module in service because Rockwell no longer manufactures new units for this platform. A tested surplus 3100-MCM extends the life of an existing rack without forcing a controller migration, provided the firmware and DIP switch configuration match what the original module was running.
Troubleshooting Quick Reference
| Symptom | Possible Cause | Relevance to this Part | Quick Check Method | Recommendation |
|---|---|---|---|---|
| No LEDs on power-up | Backplane power or slot fault | ❌ Low | Measure 5V backplane supply at an adjacent slot | Check rack power supply and backplane connector seating before replacing |
| Module LED shows persistent fault/error state | Internal microcontroller or DUART fault | ✅ High | Cycle power, check LED pattern against the ProSoft manual’s fault codes | Usually requires module replacement if the pattern repeats after a clean power cycle |
| Modbus devices show no response, but module LEDs look normal | Wiring, baud rate, or DIP switch mismatch | ❌ Low | Verify RS-485 termination, baud rate setting, and node address against the field device | Reconfigure DIP switches and wiring first — this is rarely a module failure |
| Registers transfer intermittently or drop out under load | Backplane current overload or loose card seating | ❌ Low | Reseat the module, check total rack backplane current draw against supply rating | Correct rack loading before assuming the module is bad |
| Module accepted by rack but PLC logic shows stale data | Command list configuration error, not hardware | ❌ Low | Review the command list entries in the module’s configuration | Reprogram the command list; this is a configuration issue, not a hardware fault |
If none of these checks isolate the problem, send us the fault LED pattern, a photo of the wiring terminals, and any diagnostic buffer output you can pull — we’ll help narrow it down before you commit to a replacement.
3100-MCM
3100-MCM
Frequently Asked Questions (FAQ)
Is the 3100-MCM still made by Rockwell or ProSoft?
No. The PLC-5 platform is discontinued, and this module hasn’t been in regular production for years. What you’re buying is pulled from decommissioned racks or dealer surplus stock, tested, and resold. That’s normal for this platform — there isn’t a “new in box from the factory” option anymore.
Why is this cheaper than what I’d pay for a comparable new module?
Because it isn’t new — it’s surplus or refurbished, and the platform itself is legacy. You’re paying for a tested, working unit, not a fresh production run. If a seller’s listing doesn’t clearly state New Surplus versus Refurbished, ask before you buy.
Can I hot-swap this module while the rack is powered?
Don’t. Pulling or inserting the module with power applied risks damaging the backplane connector and the module itself. Kill rack power first, swap the module, then power back up.
Will it keep my existing configuration if I swap out a failed unit?
No — the command list, node address, and port settings live in the module’s onboard configuration, not the PLC-5 processor. Document your current DIP switch positions and command list entries before pulling the old module, since you’ll need to reproduce them on the replacement.
Can this replace an MVI71-MCM or 3150-MCM directly?
Not automatically. The 3100-MCM runs an older processor with a lower max baud rate (38,400 bps versus 115,200 bps on the MVI71-MCM) and lacks the MVI71-MCM’s debug port and data analyzer. Check your command list size and baud rate requirements against the 3100-MCM’s limits before treating it as a drop-in replacement.
What’s actually tested before this ships?
At minimum: power-up self-test, backplane communication, and a Modbus handshake test against a live master or slave device. Ask the seller for their specific test procedure and whether test logs or photos are available — a legitimate refurbisher should be able to provide them.
What warranty comes with a surplus unit like this?
Warranty terms vary by seller and are typically separate from any original Rockwell/ProSoft warranty, since the product is no longer factory-supported. Confirm the return window and warranty period directly with whoever you’re buying from before you finalize the order.
