H1D: The Later VAIC Generation, and Why That Matters

GE revised the VAIC card partway through its production life, and the IS200VAICH1D marks the newer generation. The practical difference is output drive capability: where the earlier H1B/H1C generation is rated for up to 500 Ω of load on a 20 mA output over 1,000 ft of #18 wire, the H1D generation raises that to roughly 800 Ω over the same run, with up to 18 V available at the terminal board screw terminals. That matters if you have long cable runs to a valve positioner or actuator — it's also why H1D requires a TBAIH1C-or-later terminal board, or any revision of STAI, and is not backward-compatible with an older TBAIH1B or DTAI terminal board. Check what terminal board is already installed before ordering.

Function: Analog I/O Between the Field and the Mark VI Controller

Physically, the IS200VAICH1D is a VME Analog Input/Output card that plugs into the Mark VI VME rack. It handles 20 analog inputs and 4 analog outputs using an onboard analog multiplexer, A/D converter, D/A converter, and signal conditioning circuitry. Field transducers wire into the terminal board, not the VAIC itself; the terminal board links to the card over a cable, and the VAIC converts what it reads into digital values sent across the VME backplane to the VCMI communications card, which passes them to the controller.

Simplex and TMR Operation

VAIC works in both simplex and Triple Modular Redundant configurations. In TMR, the terminal board fans a single input out to three VAICs in the R, S, and T racks, and each analog output is produced by all three cards and voted to a median value — if one card develops a fault, the remaining two continue driving the correct output. In simplex systems, a single VAIC handles the full load from its terminal board with no voting.

What to Check Before You Condemn the Card

• A channel pegged high, pegged low, or frozen is more often a wiring or transducer problem at the terminal board than a card failure — check there first.
• Hardware limit faults (fixed thresholds) stop a single channel from being scanned; system limit faults (configurable) usually flag a process condition. Neither one, by itself, means the card is bad.
• On TMR panels, a channel that deviates from the voted median is flagged individually — trace that specific signal path before assuming the card itself is at fault.
• If you just swapped in a replacement and got a hardware incompatibility alarm, recheck the terminal board generation against the compatibility notes above — an H1D card paired with an old TBAIH1B is a mismatch, not a defect.

Compressor Stall Protection Built Into the Firmware

VAIC firmware also runs gas turbine compressor stall detection at 200 Hz on its first four analog channels, using either a two- or three-pressure-transducer algorithm depending on turbine size, tracking compressor discharge pressure and its rate of change to trip a shutdown signal to the controller if needed. If you're seeing stall trips you don't trust, look at the pressure transducers before you look at the card.

Decoding the Rest of the Part Number

IS200VAICH1D IS200VAICH1DAA: the letters following H1D (here, AA) track further revisions and should be matched exactly against the physical label when ordering. For the complete breakdown of every segment of a Mark VI part number, see our part number decoder guide.

Related Boards

STAI and TBAIH1C-or-later terminal boards pair with this generation; VCMI carries the data onward to the controller. See our Mark VI IS200 boards collection for the rest of the panel.

Ordering a Replacement? Send Us This

• Full part number and suffix, IS200VAICH1DAA, with a label photo if possible
• Terminal board currently installed (TBAI/STAI revision)
• Turbine model, panel configuration (simplex/TMR), and site
• Urgency — unit down or planned spares

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