Why the 1756-L55M13 Still Earns Its Rack Space
Downtime on a ControlLogix line rarely waits for a purchase order. When a processor faults, every downstream station stops with it. The Allen-Bradley 1756-L55M13 is the Logix5555 controller that keeps older ControlLogix installations running without a full platform migration.
This Series A processor carries 1.5 MB of RAM for data and logic, plus 208 KB of I/O memory. It supports up to 128,000 I/O points, including 4,000 analog inputs and 2,000 analog outputs. For plants running legacy ControlLogix racks, that headroom covers most process and discrete applications.
Technical Specifications
| Specification | Value |
|---|---|
| Series | ControlLogix |
| Module Type | Processor Module |
| RAM | 1.5 MB |
| I/O Memory | 208 KB |
| Maximum I/O | 128,000 |
| Maximum Analog I/O | 4,000 Input / 2,000 Output |
| Program Scan Time | 0.08 ms |
| Backplane Current (5V DC) | 1.23 A |
| Backplane Current (24V DC) | 14 mA |
| Power Dissipation | 5.6 W |
| Thermal Dissipation | 19.1 BTU/hr |
| Weight | 0.78 lb (0.35 kg) |
| Operating Temperature | 0-60 °C |
| Storage Temperature | -40 to 85 °C |
| Relative Humidity | 5-95% noncondensing |
| Vibration | 10-500 Hz |
What These Numbers Mean on the Floor
The 0.08 ms program scan time matters most on high-speed discrete lines. Fast scan cycles keep sequential logic tight, so cams, solenoids, and reject gates fire when the product is still in position.
Backplane current draw is modest: 1.23 A at 5V DC and 14 mA at 24V DC. That leaves power budget in the chassis for additional I/O modules, which is useful when a line is expanded rather than replaced.
The 1.5 MB RAM ceiling is the practical limit to watch. Large recipe libraries, extensive data logging, or heavy string handling can consume it quickly. Engineers should audit memory usage before adding new routines.
Communication and Integration
This controller supports DeviceNet, ControlNet, EtherNet/IP, Universal Remote I/O serial, Data Highway Plus, Modbus via ladder routine, SynchLink, and DH-485. That range lets it bridge older networks with newer Ethernet-based architectures.
For redundant configurations, the 1756-L55M13 must sit on the same ControlNet network as its partner. Programming requires either the 1756-CP3 or 1747-CP3 cable. The battery is replaceable and uses the 1756-BA1 or 1756-BATM.
Service Notes for Maintenance Teams
Battery health is the first thing to check on any scheduled inspection. A depleted battery risks losing memory when power cycles. Keep a 1756-BA1 on the shelf and replace only when needed.
Because this is an open-style module with no enclosure rating, cabinet cleanliness and airflow matter. The operating range is 0-60 °C, so verify panel cooling is working before summer load peaks. Vibration tolerance spans 10-500 Hz, which suits most industrial mounting, but loose chassis screws still cause intermittent faults.
Oil AutoTech supplies ready-to-ship ControlLogix spares with hands-on technical selection help. Contact Oil AutoTech today for pricing and availability.
