1769-OF4CI — Compact I/O 4-Channel Isolated Analog Current Output, 4–20 mA

Most analog output modules share a common reference across all channels. That's fine for applications where every output device sits on the same electrical system — but in process plants, that assumption frequently doesn't hold. Valve positioners, variable-speed drives, and transmitters often come from different vendors, sit on different panels, and have different ground references. Running non-isolated outputs to those devices invites ground loops that corrupt setpoint signals in ways that are slow to diagnose.

The 1769-OF4CI takes a different approach: all four 4–20 mA output channels are individually isolated, both from each other and from the backplane. It fits into the 1769 Compact I/O system and is controlled by any compatible CompactLogix controller. Resolution is 13-bit over the 4–20 mA span, accuracy is ±0.1% of full scale at 25°C, and settling time is under 1 ms — performance that covers the majority of continuous process control loops.

Specifications

Parameter	Value
Part Number	1769-OF4CI
Platform	Compact I/O (1769)
Output Channels	4 (individually isolated)
Output Range	4–20 mA (extended hardware range 3.2–21 mA)
Resolution	13-bit
Accuracy	±0.1% of full scale @ 25°C
Max Loop Resistance	870 Ω @ 24V DC external supply
Settling Time	<1 ms to within 1% of final value
Channel Isolation	Channel-to-channel and channel-to-backplane
Isolation Voltage	250V AC continuous
External Loop Supply	20.4–26.4V DC (user-supplied, required)
Backplane Current (5V)	125 mA
Operating Temperature	0°C to 60°C
Standards	IEC 61131-2, UL 508, CE

Why Channel Isolation Matters

Isolation breaks the electrical path between output channels and between the output loop and the backplane. In practice this means:

Two output channels can drive devices on completely different electrical systems simultaneously, with no signal interaction between loops.
A fault or ground shift on one loop — say, a short-circuit in a valve positioner — doesn't affect the other three channels.
Ground loop currents, which would offset a non-isolated 4–20 mA signal by several mA, have no path to flow between channels or to the backplane.

In a single-vendor, single-panel installation with a clean common reference, isolation may add little value. In a multi-vendor distributed process system, it's the reason the loops work cleanly from day one.

Loop Wiring Checklist

Before powering up, run through these points for each channel:

External supply connected — each channel requires a 20.4–26.4V DC loop supply. The module does not draw output current from the backplane rail.
Total loop resistance calculated — add cable resistance (both conductors) plus field device input impedance. Must be under 870 Ω at 24V supply. For long runs, verify with actual cable specs.
Shielded cable used — individually shielded twisted pair per channel. Ground the shield at the panel end only to prevent forming a shield loop.
Fault state configured — set each channel's behavior on controller fault (hold last value / go to defined mA / ramp to 0) to match the fail-safe requirement of the connected device.
Loop check performed — command each output to 4 mA and 20 mA from the controller and verify at the field device before closing the loop.

FAQ

Q: Why does this module need a separate 24V supply when the controller already has a 24V backplane?

The backplane 24V is for module logic, not loop current. Isolated output channels must have their loop power separate from the backplane rail — mixing them would defeat the isolation. Each channel's external supply terminal is independent, so one supply can power all four loops in parallel.

Q: What happens to outputs if the controller faults?

Each channel's fault behavior is configurable independently in Studio 5000 — options are hold last value, go to a programmed mA level, or ramp to zero. Set it to match the safe state of the connected field device.

Q: Can one external 24V supply power all four output loops?

Yes, a single supply can feed all four loop supply terminals in parallel provided it has enough current capacity. At 20 mA maximum per channel, all four at full output draw 80 mA total — well within a standard 24V/1A supply. Note that doing so ties the four loop supply commons together, which partially reduces the channel-to-channel isolation benefit.

Q: How long a cable run is practical?

Several hundred meters is common in process plants. At 0.1 Ω/m for 0.5 mm² cable, a 200 m run (both conductors) adds about 40 Ω to the loop — well within the 870 Ω limit. For very long runs or heavier cable, calculate actual loop impedance including the field device input resistance.

Q: Is the module compatible with HART devices?

The 1769-OF4CI is a standard 4–20 mA output module and does not support HART communication natively. HART devices can still receive their 4–20 mA setpoint signal, but the HART superimposed signal cannot be read or transmitted by this module. A HART-capable analog output module is needed for full HART integration.

1769-OF4CI New Sealed Output Module 1769OF4CI