Allen-Bradley's 1746 (SLC 500) and 1756 (ControlLogix) I/O families are among the most widely used in industrial automation. While SLC 500 is discontinued, thousands of factories still rely on 1746 modules. This guide compares both families, helping you choose the right modules for maintenance or upgrade projects.
Quick Comparison: 1746 vs 1756
| Feature | 1746 (SLC 500) | 1756 (ControlLogix) |
|---|---|---|
| Chassis | 1746-A4/A7/A10/A13 | 1756-A4/A7/A10/A13/A17 |
| Status | Discontinued (Active Mature) | Active |
| Max I/O Points | 4,096 | 128,000 |
| Backplane | 1.5 Mbps | 100 Mbps ControlBus |
| Programming | RSLogix 500 | Studio 5000 Logix Designer |
| Typical Price | $50–800 (surplus) | $200–3,000+ |
| Best For | Legacy machines, budget maintenance | New installations, high-performance |
Popular 1746 Digital Input Modules
These are the most commonly searched 1746 input modules on the surplus market:
1746-IA8 — 8-Point 120V AC Input
The 1746-IA8 provides 8 discrete AC input points at 120V AC. It's widely used in SLC 500 systems for detecting AC signals from push buttons, limit switches, and proximity sensors. Each input point draws approximately 12 mA at nominal voltage.
- Inputs: 8 points, 120V AC
- Current draw: 12 mA per point @ 120V AC
- Off-state voltage: 30V AC max
- Common terminal: Yes (1 common per 8 points)
- ControlLogix equivalent: 1756-IA8D (8-point 120V AC diagnostic input)
1746-IB32 — 32-Point 24V DC Sinking Input
The 1746-IB32 is a high-density 32-point DC input module. It's ideal when you need many inputs in a compact space. Each input sinks current from a 24V DC field device.
- Inputs: 32 points, 24V DC sinking
- On-state voltage: 10–30V DC
- Off-state voltage: 5V DC max
- Connector: 40-pin header (requires 1746-HT or terminal block)
- Variants: 1746-IB32/C and 1746-IB32/D (newer revisions with improved noise immunity)
1746-OW16 — 16-Point Relay Output
The 1746-OW16 is a versatile 16-point relay output module. Each point is an isolated Form A (N.O.) relay contact, suitable for both AC and DC loads. It's commonly used for controlling solenoids, motor starters, and indicator lights.
- Outputs: 16 relay contacts (N.O.)
- Switching capacity: 2A @ 250V AC / 2A @ 30V DC per point
- Isolation: 1,500V AC between contacts and backplane
- Series C variant: 1746-OW16/C — improved contact life (100k cycles minimum)
Popular 1746 Analog Modules
1746-NO4I — 4-Channel Analog Current Output
The 1746-NO4I outputs 4 channels of 4–20 mA current signals. It's used for controlling analog devices like VFD speed references, valve positioners, and PID loop outputs.
- Channels: 4 output, 4–20 mA
- Resolution: 16-bit
- Update time: 5 ms per channel
- ControlLogix equivalent: 1756-OF4 (4-channel analog output, higher precision)
When to Upgrade: 1746 → 1756 Migration
Consider migrating from 1746 (SLC 500) to 1756 (ControlLogix) when:
- Production expansion: You need more than 4,096 I/O points
- Integration needs: Your system must communicate on EtherNet/IP with modern HMIs
- Safety requirements: ControlLogix offers SIL 3 safety I/O modules (1756-IB16S, 1756-OB8S)
- Spares availability: Finding genuine 1746 modules becomes harder over time
- Performance demands: You need faster scan times and higher data throughput
However, many facilities successfully run SLC 500 systems with surplus 1746 modules for decades. The key is having a reliable supplier for authentic replacement parts.
1746 Series Part Number Reference
| Part Number | Description | Type |
|---|---|---|
| 1746-IA8 | 8-Pt 120V AC Input | Digital In |
| 1746-IB32 | 32-Pt 24V DC Sink Input | Digital In |
| 1746-IB16 | 16-Pt 24V DC Sink Input | Digital In |
| 1746-OW16 | 16-Pt Relay Output | Digital Out |
| 1746-OB32 | 32-Pt 24V DC Source Output | Digital Out |
| 1746-NO4I | 4-Ch Analog Current Output | Analog Out |
| 1746-NI8 | 8-Ch Analog Input | Analog In |
| 1746-HSCE | High-Speed Counter | Specialty |
How to Source 1746 Modules in 2026
Since Rockwell Automation discontinued active production of SLC 500, the main sourcing channels are:
- Authorized surplus dealers — Suppliers like ZZSL who test and warranty used modules
- Equipment liquidators — Factory decommissioning sales
- Online industrial marketplaces — Radwell, DO Supply, eBay industrial
- Direct replacement — Rockwell's official migration program (1756 retrofit kits)
Need 1746 or 1756 I/O modules?
ZZSL Automation stocks a wide range of Allen-Bradley I/O modules. Contact us for pricing and availability, or browse 1746 series products →