AI Summary
- Status relays in 2026 are diagnostic I/O flags — not just coil outputs — across Allen-Bradley, Siemens, and Mitsubishi platforms.
- Status numbers now follow bit-mapped conventions tied to module health, comms, and fault conditions, rather than vendor legacy coil indexes.
- Modern PLCs expose status through tag-based monitoring, HMI indicators, and CIP / Profinet diagnostic profiles — no multimeter required.
- Indicator LEDs follow standardized state tables readable from engineering software in seconds, even on legacy Allen-Bradley 1756 modules.
- KOEED sources both current and EOL status relays; send your BOM to Moritta@KOEED.COM for a 24-hour quote.
PLC status relays in 2026 are diagnostic-grade I/O mirrors with bit-mapped status codes, LED indicators, and CIP / Profinet diagnostics — vastly more observable than 2024-era designs.
Engineering Notes · PLC Diagnostics · Updated 2026-06-30
By KOEED Engineering Team · 2026-06-30 · 8 min read · Engineering Notes
PLC status relays used to be passive coil-driven outputs. In 2026 they are diagnostic mirrors of the CPU, with bit-mapped status numbers, LED state tables, and CIP / Profinet diagnostic profiles that surface faults to Allen-Bradley and Siemens engineers in seconds — without touching a multimeter.
TL;DR
- A PLC status relay is an output flag the CPU sets to mirror an internal condition (run, fault, battery-low, comms-loss) for downstream hardware.
- Status numbers in 2026 are tag bit positions in IEC 61131-3 structures, not legacy NO / NC coil numbers.
- Allen-Bradley (1756, 1769) and Siemens (S7-1200 / 1500) now expose status via Studio 5000 / TIA Portal tags, plus on-module LED state tables.
What changed since 2024
Two years ago, a “PLC status relay” usually meant a physical output contact driven by ladder logic to indicate a machine state. Maintenance teams had to read fault codes from a handheld HMI and trace ladder rungs to find the source.
In 2026, three shifts have changed the practice:
-
Tag-based status. Studio 5000 v36+ and TIA Portal V19+ map every status flag to a named tag, not a coil address.
-
CIP / Profinet diagnostic profiles. Vendor diagnostic standards now ride on every status relay, exposing health to the controller and the asset-management layer.
-
LED state tables. Each module ships with a printed LED-state matrix so field techs can read a single blink pattern and route the fault without a laptop.
> Tip
When reading an LED pattern on an Allen-Bradley 1756-EN2T or Siemens 6ES7 315-2EH14, always cross-check the tag ModuleStatus in the controller scope. LEDs can lie when the module is hot-swapped mid-fault.
Status numbers in 2026: from coil number to bit position
The original 2024 article treated “status number” as a vendor-defined coil index (e.g. status 1 = NO, status 2 = NC). That model still exists for backwards compatibility with Allen-Bradley SLC 500 and Mitsubishi MELSEC-A systems, but new platforms take a different path.
The 2026 reference model uses a status-word structure shared across IEC 61131-3 platforms:
| Bit position |
Status meaning |
Typical LED color |
| Bit 0 |
Run / OK |
Solid green |
| Bit 1 |
Fault |
Solid red |
| Bit 2 |
Battery low |
Amber blink |
| Bit 3 |
Forced I/O |
Yellow |
| Bit 4 |
Comms loss |
Red blink |
A “status number of 1” in 2026 usually means bit 0 set, not a vendor coil. This shift matters when a 2024-era Allen-Bradley SLC 500 is replaced by a 1756-L73: the field team must re-learn where to read the same condition.
Vendor snapshot: Allen-Bradley, Siemens, Mitsubishi
Allen-Bradley (1756 / 1769 / 5069)
Status relays in ControlLogix and CompactLogix are exposed as tag attributes on the module object. The ModuleInfo tag carries run / fault / battery / comms bits, and the on-module OK LED mirrors bit 0. The AI Diagnostic Tool can cross-reference a 1756-EN2T indicator pattern to its tag value in seconds.
Siemens (S7-1200 / S7-1500)
S7 status is delivered through the diagnostic buffer and the LED system data block in TIA Portal. The 6ES7 315-2EH14 and 6ES7 317 CPUs expose 16-bit status words that map directly to the bit table above; older Siemens S7-300 CPUs retain a legacy status-byte view that TIA Portal still surfaces.
Mitsubishi (MELSEC-iQ-R / Q)
Mitsubishi keeps a coil-style status relay for backwards compatibility but adds GX Works3 diagnostic tags. Engineers reading a Mitsubishi Q-series CPU in 2026 will see both the legacy M-coil status and the new diagnostic-word status; the LED table matches the AB / Siemens convention.
Field procedure: reading a status number in three steps
-
Identify the module. Read the catalog number on the front faceplate (e.g.
1756-EN2T).
-
Read the LED state table. The QR code on the side of the module links to a 2026-era diagnostic PDF with the blink-pattern matrix for that catalog number.
-
Cross-check with the controller scope. In Studio 5000 / TIA Portal, watch the
ModuleStatus tag live. If the LED says OK but the tag shows fault, you have a hot-swap event — re-seat the module.
! Warning
Never trust an LED alone in a safety circuit. Safety relays (Allen-Bradley 1756-EN2T safety partner, Siemens F-CPU) require a separate diagnostic read from the safety tag — see IEC 61508 SIL diagnostics in your platform manual before clearing a fault.
Sourcing notes from KOEED
KOEED keeps both current and EOL status-relay modules in stock across Allen-Bradley, Siemens, Mitsubishi, Omron, and Schneider. If your legacy PLC uses the 2024-era coil-style status number, send the BOM to Moritta@KOEED.COM and we will cross-reference the 2026 tag-based equivalent alongside the legacy part. Worldwide shipping, datasheet on request, 24-hour quote turnaround on business days.
Working on a PLC diagnostics project?
Send your BOM (legacy or 2026 tag-based) to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — all in one quote within 24 hours.
Send My BOM →
Frequently asked questions
What is a PLC status relay in 2026?
A PLC status relay in 2026 is a diagnostic-grade output (or tag-mapped flag) the controller sets to mirror an internal condition — run, fault, battery low, or comms loss. It feeds both hardware indicators and software tags.
How do status numbers work today?
Modern platforms use bit-mapped status words — bit 0 = Run, bit 1 = Fault, bit 2 = Battery low, and so on. Legacy coil numbers (status 1 = NO, status 2 = NC) still exist on SLC 500 and MELSEC-A but are no longer the default on new builds.
Which Allen-Bradley modules expose tag-based status?
ControlLogix 1756 modules (L7x CPUs, EN2T, IB16IF, OB16E) and CompactLogix 1769 modules expose status via the ModuleInfo tag. Studio 5000 v36 or newer is required to view the diagnostic bit positions.
Do Siemens S7-300 status relays still use the old numbering?
S7-300 retains the legacy status-byte convention (e.g. status 1 = NO), but TIA Portal V19+ maps the same flags into a 16-bit diagnostic word. Engineers can read either view, but the bit-mapped view is the recommended 2026 practice.
Can KOEED cross-reference a 2024-era status relay to its 2026 equivalent?
Yes. Send the original part number and the platform family to Moritta@KOEED.COM and we will return a side-by-side legacy / tag-based quote within 24 hours, with worldwide shipping options.
KOEED Engineering Team
Industrial automation editors at KOEED. Writes about PLC sourcing, cross-reference, and legacy system support. Reach the team at Moritta@KOEED.COM.
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