Where the IS220PDOAH1A Fits in a Mark VIe Cabinet

Think of the IS220PDOAH1A as the relay-driving half of a Mark VIe I/O chain: it takes commands from the <R>, <S>, or <T> controller over the I/O Ethernet network (IONet) and turns them into relay outputs on a discrete output terminal board. It plugs directly onto that terminal board through a DC-37 pin connector — there's no card rack involved — and it uses dual RJ-45 Ethernet ports to reach the controller, with a 3-pin connector supplying 28 V dc power through a soft-start, current-limited input.

Inside the pack, a processor board shared across every Mark VIe distributed I/O pack works with an acquisition board built specifically to drive and monitor relays: up to 12 relay outputs, with feedback read back from the terminal board. The terminal board determines the relay technology in play — TRLYH1B, C, D, and F boards carry electromagnetic relays, while TRLYH1E boards use solid-state relays instead. On power-up, GE's design intentionally holds every output disabled until the pack's internal self-tests complete, with the enable circuit kept independent from the command circuit so relays cannot be accidentally driven during initialization.

Troubleshooting Before You Order a Replacement

• Field wiring, the relay hardware on the terminal board, blown fuses, and loose terminals are the most common source of a relay that won't operate correctly — check these before assuming the pack has failed.
• Swap the suspect pack onto a known-good terminal board, or install a known-good pack on the suspect board, and see which one the problem follows. Whichever it is, that's the part to replace.
• Remember that outputs are held off during the pack's power-up self-test sequence — a brief period with no relay response after a power cycle is expected behavior, not a defect.
• Verify the terminal board's relay type (electromagnetic vs. solid-state) matches what your application expects; a mismatch in relay type or wiring can produce symptoms that look like a bad pack.
• Check IONet cabling, connectors, and switch health before replacing hardware — a network problem and a failed pack look the same from the operator screen.
• In TMR systems, identify which core (R, S, or T) the alarm is actually tied to before pulling any single pack out of a working triad.

Decoding This Part Number

IS220 identifies a Mark VIe distributed I/O pack, PDOA the discrete output function, H1 the design generation, and the trailing letter the hardware/processor revision. All of these need to match your terminal board and control software configuration before you order — a similar-looking part number is not automatically a drop-in replacement. See our GE Speedtronic Part Number Decoder for the full breakdown.

Related Hardware

PDOA mounts on TRLY-family relay terminal boards. Systems built around a combined input/output terminal board instead use the related PDIO pack, which handles both discrete inputs and relay outputs from a single assembly. Our GE Speedtronic Turbine Control Boards collection covers the broader Mark V/VI/VIe range if you need to cross-check other boards on the same panel.

What to Send Us for a Quote

• The complete part number and suffix (IS220PDOAH1A)
• A photo of the label
• Turbine model and Mark control generation
• Quantity needed
• How urgent the request is

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