When TTL Signals Meet the ControlLogix Chassis
Plant floors changed fast over the last three decades. Relays gave way to solid-state logic, and discrete 24V devices started sharing cabinets with low-voltage TTL electronics. Engineers needed a way to bring those 5V DC signals directly into a modern control platform without adding conversion hardware. The Allen-Bradley 1756-IG16 answered that call. It is a ControlLogix digital DC input module built specifically for 16 TTL-level inputs, and it remains a practical choice for OEM panels, test rigs, and legacy system support.
What the 1756-IG16 Actually Does
The 1756-IG16 is a 16-point digital DC input module from the Allen-Bradley ControlLogix series, manufactured by Rockwell Automation. It reads 5V DC TTL signals and reports them to the ControlLogix processor. The module uses a low-true format, meaning a low voltage at the terminal represents a true, or ON, state. That inverted logic is common in TTL environments and is worth understanding before you wire anything.
Each group shares a common, and the module uses a source configuration. Inputs are organized with 9 points per common, so you get flexible grouping across the 16 channels. The module communicates through the ControlLogix backplane, drawing 110 milliamps at 5 volts. Maximum power dissipation is 1.4 Watts at 60 degrees Celsius.
Technical Specifications
| Specification | Value |
|---|---|
| Module Type | Digital DC Input Module |
| Inputs | 16-points (9 per common) |
| Input Voltage | 5 Volts DC with TTL Source |
| Operating Voltage Range | 4.5-5.5 Volts DC |
| Input Delay Time | 0.25 milliseconds plus selected filter time |
| Max Input Current, Off-State | 4.1 milliamps |
| Power Dissipation (Max) | 1.4 Watts at 60 degrees Celsius |
| Backplane Current at 5 Volts | 110 milliamps |
| Removable Terminal Block(s) | 1756-TBNH, 1756-TBSH |
Wiring and Terminal Block Selection
You cannot connect the 1756-IG16 directly to a chassis without a removable terminal block. The module accepts either the 1756-TBNH or the 1756-TBSH RTB. Choose the housing style that matches your panel layout and wiring practice. Because TTL inputs are inverted, verify your signal source before commissioning. A low voltage between -0.2 and 0.8 volts registers as true. Voltages between 2.0 and 5.5 volts register as off. Anything between 0.8 and 2.0 volts sits in an undefined region, so avoid operating there.
The module supports user-selectable filter times to reject noise on the input lines. Off-to-on transitions carry a nominal hardware delay of 270 microseconds, with a maximum of 450 microseconds. On-to-off transitions run 390 microseconds nominal, up to 700 microseconds maximum. Add your chosen filter time on top of those figures when calculating scan response.
Where This Module Fits
Test benches and semiconductor tools often output TTL-level signals directly. The 1756-IG16 brings those signals into a ControlLogix chassis without external level shifters. OEM panel builders also use it when retrofitting older equipment that already speaks 5V logic. And because it lives in the same chassis as analog, motion, and communication modules, it simplifies architecture for mixed-signal machines.
Integration and Compatibility
The 1756-IG16 slots into any ControlLogix 1756 chassis alongside other modules. It communicates over the backplane, so no external network configuration is needed for basic input reading. If your application requires fieldbus integration, confirm gateway and cabling requirements with your supplier before ordering.
Before You Order
Confirm your signal levels, terminal block choice, and chassis slot availability. Oil AutoTech offers hands-on technical selection help and fast-dispatch spares for oil, gas, and industrial automation applications. Contact Oil AutoTech today for pricing and availability.
