1746-NO8I New Factory Sealed SLC 500 Analog Current Output Module 1746NO8I

1746-NO8I — SLC 500 8-Channel Isolated Analog Output Module

The 1746-NO8I is an 8-channel isolated analog output module for the SLC 500 platform. Each channel outputs an independent voltage or current signal — 0–10V, ±10V, or 4–20 mA, configurable per channel — to drive process actuators, drive speed references, or instrument setpoints. The "I" designation indicates individual channel isolation: each output floats independently from the others and from the SLC backplane, allowing connections to field devices on different electrical systems without ground loop interference.

Specifications

Isolated Outputs in SLC 500 Process Systems

SLC 500 systems frequently interface with VFDs, valve positioners, and instrumentation from multiple vendors and electrical zones across an installation. A non-isolated analog output module shares a common reference across all channels — if connected devices have different ground potentials, the resulting common-mode voltage corrupts the output signal accuracy or, in severe cases, damages the output circuitry. The 1746-NO8I avoids this entirely: each of the eight channels operates as its own floating circuit, immune to ground potential differences on any other channel or the backplane.

This matters in retrofit and expansion projects where new drives or instruments are added to an existing SLC 500 system, and the new equipment's grounding can't be guaranteed to match the original installation's reference. Isolated outputs remove that uncertainty from the design — each output simply does its job regardless of what's happening electrically on the others.

Wiring and Load Considerations

• For current mode (4–20 mA) channels, an external 24V DC loop supply must be connected — the module doesn't source loop current internally. Verify the loop supply voltage provides adequate headroom: at 20 mA through 500 Ω maximum load, the voltage drop is 10V, leaving 14V of margin from a 24V supply for cable resistance and the receiving device's own input impedance.
• For voltage mode channels, minimum load resistance should be verified against the connected device — driving too low an impedance load can cause output clipping or inaccurate voltage delivery.
• Use individually shielded twisted-pair cable per channel for runs exceeding a few meters. Ground the shield at the panel end only.
• Configure each channel's fault state (hold last value, go to zero, go to a defined value) in RSLogix 500 to match the connected device's fail-safe requirement before commissioning.

FAQ

Q: Can different channels be configured for different output types simultaneously?

Yes. Each of the eight channels is configured independently — some can be 4–20 mA while others run 0–10V or ±10V, in any combination needed for the application.

Q: Does isolation affect the module's accuracy compared to a non-isolated output module?

No. The ±0.2% accuracy specification is independent of the isolation architecture. Isolation affects noise immunity and ground loop rejection, not the fundamental conversion accuracy.

Q: Can one external 24V supply power all eight current-mode loops?

Yes, provided the supply has adequate current capacity for all channels operating at full 20 mA simultaneously (160 mA total minimum). Note that sharing one supply across all channels ties their loop commons together, which reduces the channel-to-channel isolation benefit for those specific channels.

Q: What happens to the outputs if the SLC processor goes into fault?

Output behavior on processor fault is configurable per channel in RSLogix 500 — hold last value, ramp to zero, or go to a user-defined value. Set this according to the safe state required by each connected device before the system goes into production.