In the control cabinets of gas turbines and steam turbines, if you see modules with model numbers starting with "IS220Y" and marked with "SIL" on the panel, that's GE's Mark VIeS safety control system. It's the SIL-certified version of Mark VIe, having passed exida's IEC 61508 evaluation. It achieves SIL 3 in random faults with HFT=1 (triple redundancy), and also meets SIL 3 requirements in system faults. Below, we've grouped these 8 models by function, explaining their roles and how to match them.

What is Mark VIeS?
Mark VIeS is a safety-oriented turbine control system developed by GE based on the Mark VIe architecture. Compared to standard Mark VIe, its core difference is that all I/O packets and controllers are SIL certified, and the naming convention uses "Y" instead of "P" for identification—for example, the standard analog I/O packet is IS220PAICH1A, and the corresponding SIL version is IS220YAICS1A. The system architecture adopts TMR (two out of three) redundancy, with the controller and I/O packets connected to three IONet networks: R, S, and T, respectively. Each safety I/O packet has its own processor, and critical protection functions (such as overspeed protection) can be completed locally within the I/O packet without waiting for controller intervention.

Eight Module Classifications and Functions

1.Safety Controller
IS420UCSBS1A — Mark VIeS UCSB Controller, 600 MHz, SIL
This is the core of the Mark VIeS safety controller. A 600MHz processor performs logic operations for Safety Instrumented Functions (SIFs). Power supply is 28Vdc, with a maximum current of 1.1A. It shares similar hardware with the standard Mark VIe controller IS420UCSBH1A, but differs in firmware and certification—the UCSBS1A is specifically designed for SIL applications.
In a TMR configuration, three UCSBS1As are connected to the R, S, and T IONets respectively, achieving SIL 3 through a 2-out-of-3 voting method.
2.Discrete Input (Contact Input)
IS200STCIS2A — Mark VIeS, Contact Input TB, 24V DC
IS220YDIAS1A — Mark VIeS, Input Pack Contact In, SIL
This is a combination of a "terminal board + I/O pack," representing the standard discrete input configuration for Mark VIeS.
The IS200STCIS2A is a terminal board, mounted on a DIN rail, responsible for wiring, isolation, and initial conditioning of field signals. It supports 24V DC contact input using Euro-style box-type terminals.
The IS220YDIAS1A is an I/O pack, plugged into the terminal board, responsible for signal digitization and IONet communication. It is a 24-channel contact-closed input module with group isolation, supporting three rated voltage ranges: 24V/48V/125V DC. It operates at 28Vdc and draws 0.23A.
Compatibility: The IS220YDIAS1A must be used with the IS200STCIS2A (or IS200STCIS1A, IS400STCIS2A, etc.). The terminal board handles wiring, while the I/O pack handles processing and communication; both are indispensable.
3.Relay Output:
IS200SRLYS2A — Mark VIeS, Relay Output TB
IS220YDOAS1A — I/O Pack, Relay Out, SIL
This is a Mark VIeS relay output combination.
The IS200SRLYS2A is a relay output terminal board, providing the physical interface for relay contacts and feedback detection circuitry.
The IS220YDOAS1A is a 12-channel relay output I/O package, SIL certified. Output status is confirmed through the feedback circuitry of the terminal board, enabling output loop diagnostics. Power supply: 28Vdc, current: 0.71A.
Compatibility: The IS220YDOAS1A is used with the IS200SRLYS2A (or IS200TRLYS1F, IS200TRLYS2F, etc.). The feedback circuitry on the terminal board detects whether the relays operate correctly, which is crucial for SIL system output diagnostics.
4.Analog Input/Output (Analog I/O)
IS200STAIS2A — Mark VIeS, Analog Input TB
IS220YAICS1A — Mark VIeS, I/O Pak for Analog, SIL
This is a Mark VIeS analog I/O combination.
The IS200STAIS2A is an analog input terminal board, Simplex configuration, supporting 10 analog inputs and 2 analog outputs. The 10 inputs support two-wire, three-wire, four-wire, and externally powered transmitters, with input ranges including voltage and 4-20mA. The 2 outputs are 0-20mA, one of which can be jumpered to 0-200mA. An onboard ID chip identifies the package model. Maximum lead resistance is 15 ohms, and maximum cable length is 300 meters.
The IS220YAICS1A is an analog I/O package, SIL certified. It features 10 analog inputs (voltage, 4-20mA) and 2 4-20mA analog outputs, supporting single-package and triple-package configurations. Power supply is 28Vdc, with a current of 0.50A.
Compatibility: The IS220YAICS1A is used with the IS200STAIS2A (or IS200STAIS1A, IS200TBAIS1C, etc.). The terminal board handles signal wiring, and the I/O package handles A/D conversion and D/A output.
5.Turbine Protection:
IS200TPROS1C — TPROS1C Term BD Protective – SIL
This is Mark VIeS' backup turbine protection terminal board, SIL certified. It is used with the IS220YPROS1A (backup turbine protection I/O package) to form a PPRO (Backup Turbine Protection) system.
Features: Receives 3 speed signals, E-stop emergency stop signals, and 7 contact inputs. Drives 3 trip relay outputs. Overspeed protection is performed locally in the I/O package, independent of the controller. Provides feedback detection for the trip relay output circuit. Supports TMR and Simplex configurations. Compatibility: The IS220YPROS1A is used with the IS200TPROS1C (or IS200TREAS1A, IS200SPROS1A). The three YPROs are connected to the R, S, and T networks respectively, forming a two-out-of-three protection configuration.

How to Select a Model
1.Determine the Safety Level
If only SIL 2 is needed, Simplex or Dual configuration is sufficient.
If SIL 3 is required, TMR is mandatory, and three sets of controllers and I/O packages are necessary.
2.Confirm Voltage Rating
YDIAS1A supports 24V/48V/125V DC input voltages. Confirm the voltage required at the site when ordering.
The voltage of the terminal board and I/O package must match.
3.Confirm Terminal Board Model
Each I/O package can only be paired with a specific terminal board; they cannot be mixed. For example, YDIAS1A cannot be plugged into the standard Mark VIe STCIH1A; it must be paired with STCIS1A/2A. In the terminal board model number, "S" indicates the SIL version, and "H" indicates the standard version.
4. Consider Environmental Requirements
If there are explosion-proof requirements on site, confirm that the module's ATEX marking conforms to the site area classification. The standard module's operating temperature is -30°C to +65°C.

Main Application Areas
1. Gas Turbine Control
Sequential control, combustion regulation, load control, and safety interlocks such as overspeed protection, vibration protection, and temperature protection for gas turbines.
2. Steam Turbine Control
Speed regulation, valve control, and safety functions such as overspeed protection and shaft protection for steam turbines.
3. Combined Cycle Units
Coordinated control of gas turbines and steam turbines, and a plant-wide safety interlock system.
4. Mechanical Drive Applications
Drive control and safety protection for critical rotating equipment such as compressors and pumps.