1756-IG16 — ControlLogix 16-Point TTL Input Module

The 1756-IG16 is a 16-point TTL (5V DC) digital input module for the ControlLogix 1756 platform. Each input reads logic-level signals in the 0–5V range — the 1 and 0 thresholds follow standard TTL convention, with anything above 2.0V reading as ON and anything below 0.8V reading as OFF. The module draws from the backplane's 5V supply; no separate field supply is needed for the input circuitry itself.

TTL-level I/O occupies a narrower niche than 24V DC modules, but it's the right tool for specific situations — particularly when the field devices are logic-level sensors, signal conditioners, or encoders with TTL outputs that can't be stepped up to 24V without additional circuitry. Using the 1756-IG16 in those cases eliminates a layer of signal conversion hardware and the reliability risk that comes with it.

Specifications

Parameter	Value
Part Number	1756-IG16
Platform	ControlLogix 1756
Input Points	16
Input Voltage Range	0–5V DC (TTL)
ON-State Voltage Min	2.0V DC
OFF-State Voltage Max	0.8V DC
Input Current (at 5V)	3.2 mA typical
Commons	2 groups of 8, isolated from backplane
Input Filter	Software selectable: 1 ms, 2 ms, 9 ms
Backplane Current (5V)	175 mA
RTB Connector	1756-TBNH or 1756-TBNF (36-pin)
Operating Temperature	0°C to 60°C
Standards	UL 508, CE, C-Tick

TTL in Industrial Control — Where It Still Makes Sense

Most modern industrial sensors output at 24V DC, so TTL modules are less common than they once were. But a few application areas still rely on 5V logic-level signals:

Legacy Test Equipment Integration

Older automated test systems — particularly in electronics manufacturing and semiconductor production — use TTL-level handshaking between instruments. Integrating this equipment into a ControlLogix system without a signal conversion layer is simpler with the 1756-IG16 than converting every test point to 24V.

Custom Sensor Electronics

Photodetectors, optical encoders, and custom signal conditioning boards built around 5V logic ICs produce TTL-compatible outputs. Running them directly into the 1756-IG16 avoids the noise, delay, and cost of a buffer stage between the sensor and the PLC.

High-Density Data Acquisition

In applications where a large number of binary status bits need to be sampled simultaneously — end-of-travel on multi-axis positioning systems, pass/fail bits from inspection cameras — the 16-point TTL module lets those signals connect directly if the source devices are 5V logic.

Wiring Considerations

TTL signals are inherently more noise-susceptible than 24V DC signals — the smaller voltage swing means less noise margin. A few wiring practices are worth following consistently on 1756-IG16 installations:

Keep field wiring short. TTL signals over long cable runs pick up capacitive coupling and ground noise more readily than 24V circuits. Where possible, keep TTL wiring under 10 m and use shielded cable for longer runs.
Ground the cable shield at one end only — at the panel end. Grounding at both ends creates a shield loop that can introduce more noise than it suppresses.
Route TTL wiring separately from power wiring, contactors, and drive output cables. The signal level makes TTL more vulnerable to inductive coupling from high-current circuits in the same duct.
Verify the field device's output drive capability. The 1756-IG16 draws about 3.2 mA per input at 5V. Most TTL and CMOS outputs can drive this comfortably, but very old or marginal-spec driver ICs may need a buffer.

The 1756-IG16 is not compatible with 24V DC field signals. Applying 24V to a TTL input will damage the module. Always verify field wiring voltage before connecting to this module, particularly during panel retrofits where 24V wiring may be present from a previous module.

FAQ

Q: Can the 1756-IG16 read 3.3V CMOS logic outputs?

Marginal — the ON-state threshold is 2.0V, so a 3.3V output clears it with about 1.3V of margin. In practice this usually works, but it is outside the module's specified input range. For reliable operation with 3.3V sources, verify the output drive strength and noise conditions in your specific installation.

Q: Does each input channel have independent filtering, or is it group-wide?

Filter time is configured per module in Studio 5000, applied uniformly across all 16 inputs. Individual per-channel filter settings are not available on this module.

Q: What RTB (removable terminal block) is required?

The 1756-IG16 uses a 36-pin RTB. Compatible options are the 1756-TBNH (standard wire) and 1756-TBNF (fine wire). The RTB is purchased separately from the module.

Q: Can this module be used in a remote chassis over EtherNet/IP or ControlNet?

Yes. The 1756-IG16 is a standard ControlLogix I/O module and works in any 1756 chassis, whether local to the controller or in a remote chassis connected via 1756-EN2T (EtherNet/IP) or 1756-CN2 (ControlNet) communication modules.

Q: Are there diagnostic functions available for this module?

The 1756-IG16 provides basic module status diagnostics (module fault, comm fault) accessible through Studio 5000. It does not provide per-channel diagnostics such as wire-break detection — those functions are available on higher-specification diagnostic input modules.

1756-IG16 New Sealed Digital Input Module 1756IG16