What Makes Up Your Allen-Bradley SLC 500?
SLC stands for Small Logic Controller. That is not to say the control system you are working with is small — rather, it controls small processes and detailed commands. The focus of this article is to help you identify the 6 major parts of your control system and how to find their part numbers.
The purpose of this article is to give you additional information to help you better identify the parts you need to keep your systems running.
The 6 Major ComponentsAlso known as a Programmable Logic Controller (PLC), the processor is the brain of the system. It executes the control program and coordinates all other components.
Sensors that feed real-world data into the processor. Input modules detect conditions in the field and send signals to the PLC to trigger programmed responses.
Devices such as motors, lights, and actuators that receive commands from the processor. Output modules convert the PLC's logic decisions into physical actions on the plant floor.
The rack that physically holds all system components together. The chassis provides the backplane connections that allow the processor, I/O modules, and power supply to communicate and share power.
Provides regulated power to the processor and I/O modules seated in the chassis. Selecting the correct power supply capacity is critical to stable system operation.
Connects the processor to other systems, devices, and networks. Communication modules allow the SLC 500 to exchange data with HMIs, SCADA systems, and other PLCs across your facility.
I/O Modules: Inputs & Outputs
I/O modules allow the PLC to receive signals from sensors (inputs) and send signals to devices like motors or lights (outputs).
All I/O module part numbers start with 1746- and are available in 4, 8, 16, and 32 channel configurations.
Check the inside of the front door first. If you need additional information, it is located on the side label of the module. You will need to remove the module from the chassis to see it.
Turn off your system before removing any module.
Knowing your module type before ordering helps you navigate supplier websites and determine your exact part.
On/Off signals only. The most common module type. Used for discrete devices like push buttons, limit switches, pilot lights, and solenoids.
Continuously variable signals such as current, voltage, and temperature. Used where a range of values must be measured or controlled rather than a simple on/off state.
Mixed inputs and outputs in a single module. Useful when rack space is limited and both input and output points are needed without dedicating two separate module slots.
Application-specific modules such as counter modules, blow molding controllers, and other purpose-built configurations that go beyond standard I/O functions.
When sourcing a replacement I/O module, always try to match the following:
- Voltage
- Channel count
- Series — when possible
"Series" refers to a hardware revision of the module. If the exact series is unavailable, upgrading is acceptable (for example, Series A to Series C). Downgrading is not recommended, though in some cases it will work.
Important exception: Analog modules must be replaced with the same series. Pay close attention to the series designation when sourcing any analog replacement.
Processor (PLC)
The processor is the brain of the SLC 500 system. It executes your control program and coordinates every other component in the chassis. All SLC 500 processors begin with the part number prefix 1747-L (for example, 1747-L531).
There are five processor models available, with the main differences being memory size and communication type.
The Five Processor Models
The SLC 5/05 includes a built-in Ethernet port, making it the most network-capable model. Earlier models use DH-485 or RS-232 connections. Confirm your network type before selecting a replacement processor.
Always start with the exact part number from the unit label. SLC 500 processor part numbers always start with 1747-L and are found on the inside of the door and on the side of the unit.
The series is a hardware revision of the processor. Match it when possible. If the exact series is unavailable, the same model in a newer series is generally acceptable.
The firmware revision is listed on the processor label. Changes to firmware may require updates to your existing program. Confirm the firmware version before swapping a unit.
Important: Changes to part number, series, or firmware may require updates to your existing program. The processor label on the unit itself lists the model number, series, and firmware revision. Always verify all three before ordering a replacement.
Battery & Memory Modules
Keeps the program stored when power is off. Typical lifespan is 2 to 5 years depending on the PLC model and usage.
A dead battery can mean a lost program. Stock a spare and replace it proactively before it fails.
External modules that plug into a socket on the processor, providing an additional layer of program backup.
- 1747-M1 — 1K memory
- 1747-M2 — 4K memory
- 1747-M13 — 64K memory
PLC model and series matter when selecting the correct EPROM. Confirm compatibility before ordering.
Chassis & Power Supplies
Holds the processor, I/O modules, and power supply together. Each chassis requires its own dedicated power supply.
- 1746-A4 — 4-slot
- 1746-A7 — 7-slot
- 1746-A10 — 10-slot
- 1746-A13 — 13-slot
Confirm slot count to fit all current modules plus any planned expansion before ordering.
Provides power to the entire SLC 500 system. Both AC and DC input versions are available.
Later series power supplies offer improved performance. Always confirm the input voltage of the existing unit before selecting a replacement.
Input voltage and series are printed on the unit label. Check before ordering.
Communication Modules
Communication modules allow the PLC to connect to other devices or networks. Most communication module part numbers start with 1747-, while bridge modules may start with 1761-.
Ethernet, DH+, or Serial. Network type determines module compatibility.
SLC 5/01 through 5/05 each have different built-in communication capabilities.
Know the device on the other end to ensure the correct cable and module pairing.
Finding Parts for Your SLC 500 System
Now that you know how to identify your components, here is a quick reference for searching by part type.
- I/O Modules: 1746-XXXX
- Communication Modules: 1747-XXXX or 1761-XXXX
- Processors: 1747-LXXX
- Chassis: 1746-AXXX
- Power Supplies: 1746-PXXX
When calling or submitting a parts request, having the following information ready will help ensure you get the right part quickly.
- Part number
- Series
- Firmware version (if applicable, for processors)
- Chassis size
- Power supply model
- I/O module types and voltage
- Communication requirements
Additional Services Worth Considering
If you are replacing a processor or sourcing a spare, a few additional services are worth keeping in mind.
Classic Automation offers program transfer services, including standard program transfers to a PLC and program transfers to a memory module. This is especially valuable if your battery has failed or you are working with a replacement unit that needs your existing program loaded.
Memory modules are a low-cost way to back up your program and protect against battery failure or processor replacement scenarios. A simple investment that can save significant downtime.
A replacement battery is a simple, inexpensive item that can prevent a significant headache. If you do not know when the last battery was replaced, it is worth addressing proactively.
Repair service is available for many SLC 500 components. Having a repaired spare on hand means you are not waiting on shipping the next time something fails.
Classic Automation stocks a wide range of Allen-Bradley SLC 500 parts, including processors, I/O modules, chassis, power supplies, communication modules, and batteries. Search your part number at classicautomation.com or contact our team if you need help identifying what you have.