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3. Key Technical Specifications
| Parameter | Value |
|---|---|
| Controller Family | Logix5555 (ControlLogix) |
| Data and Logic Memory | 3.5 MB |
| I/O Memory | 208 KB |
| Nonvolatile Memory | Yes (integrated) |
| Max Digital I/O | 128,000 |
| Max Analog I/O | 4,000 input / 2,000 output |
| Backplane Current @ 5.1 V DC | 1.25 A |
| Backplane Current @ 24 V DC | 0.014 A (14 mA) |
| Power Dissipation | 5.7 W |
| Thermal Dissipation | 19.4 BTU/hr |
| Built-in Port | RS-232 serial |
| Battery | 1756-BA1 or 1756-BATM |
| Operating Temperature | 0 °C to +60 °C |
| Weight (approx.) | 0.36 kg (12.8 oz) |
4. Product Introduction
The Allen-Bradley 1756-L55M24 is a Logix5555 ControlLogix controller equipped with 3.5 MB of data and logic memory plus integrated nonvolatile memory. It installs in any slot of a 1756 ControlLogix chassis and supports standard Logix programming languages (ladder, function block, structured text, and SFC).
The controller provides a built-in RS-232 serial port and works with the full range of ControlLogix communication modules (EtherNet/IP, ControlNet, DeviceNet, DH+, etc.). This model belongs to the earlier L55 family and has been discontinued; current replacements are the L7x and L8x series controllers. Remaining units are used primarily for maintaining existing installations.
5. Troubleshooting Quick Reference
| Symptom | Possible Cause | Relevance to this Part | Quick Check Method | Recommendation |
|---|---|---|---|---|
| Controller will not go to Run | Major fault, missing battery, or memory issue | ✅ High | Check fault log and battery status LED | Clear faults; replace 1756-BA1 battery if needed |
| Keyswitch or mode problems | Keyswitch failure or remote mode conflict | ✅ High | Verify keyswitch position and remote/local settings | Cycle keyswitch; check GSV/SSV mode bits |
| Loss of program on power cycle | Battery dead or nonvolatile memory fault | ✅ High | Test battery voltage; check nonvolatile memory status | Replace battery; re-download program if necessary |
| High backplane current / overheating | Overloaded chassis or poor ventilation | ❌ Medium | Measure total chassis current draw | Redistribute modules or improve cooling |
| Serial port communication failure | Incorrect cable or driver settings | ✅ Medium | Test with 1756-CP3 / 1747-CP3 cable | Verify baud rate and RSLinx driver |
| I/O not updating | Connection or produced/consumed tag issue | ❌ Low | Monitor connection status in Studio 5000 / RSLogix 5000 | Re-establish I/O connections |
If diagnostics remain unclear, capture the fault log, keyswitch position, and battery status before contacting support.
1756-L55M24
1756-L55M24
6. Frequently Asked Questions (FAQ)
Q: Is the 1756-L55M24 still available new from Rockwell? No. The L55 series has been discontinued for many years. Available units are New Surplus or refurbished stock from secondary markets.
Q: How much user memory does the 1756-L55M24 have? It provides 3.5 MB of data and logic memory and 208 KB of I/O memory, plus integrated nonvolatile memory for program storage.
Q: Does it require a battery? A battery (1756-BA1 or 1756-BATM) is recommended to retain the real-time clock and volatile data during power loss. The integrated nonvolatile memory stores the user program.
Q: What is the recommended replacement? Modern replacements are the ControlLogix 5570 (L7x) or 5580 (L8x) series controllers. Migration typically requires project conversion in Studio 5000.
Q: Can multiple L55 controllers be placed in one chassis? Yes. ControlLogix supports multiple controllers in the same chassis, subject to power supply and backplane capacity limits.
Q: What programming software is required? RSLogix 5000 (older versions compatible with L55 firmware). Later Studio 5000 versions may require specific firmware or conversion steps.
Q: Why is New Surplus priced lower than original list? These are genuine Allen-Bradley modules from excess inventory or cancelled projects. They are unused and typically supplied with anti-static protection. They are not refurbished unless specifically stated.
