1794-ID2 — FLEX I/O 2-Channel Intrinsically Safe Digital Input Module

The 1794-ID2 is a 2-channel intrinsically safe (IS) digital input module for the 1794 FLEX I/O platform. It interfaces directly with field devices located in hazardous areas classified as Zone 1 / Zone 21 (gas and dust) under IEC/ATEX standards or Class I Division 1 under NEC. The module itself is installed in the safe area — typically the control panel — and limits the electrical energy it can deliver to the hazardous area field circuit to levels that cannot ignite a flammable atmosphere even under fault conditions.

Two input channels per module reflects the nature of intrinsically safe installations: each IS circuit requires individual current-limiting protection, and the wiring discipline around IS circuits (segregated cable runs, documented system drawings, installation certification) makes high-density channel counts less practical than in standard I/O. The 1794-ID2 brings IS compatibility to the FLEX I/O distributed architecture, allowing IS field devices to connect into the same FLEX I/O node as standard digital and analog I/O without needing a separate IS barrier panel.

Specifications

Parameter	Value
Part Number	1794-ID2
Platform	FLEX I/O (1794)
Input Channels	2 (individually isolated, IS-rated)
Hazardous Area Rating	ATEX Zone 1 / Zone 21; IECEx; Class I Div 1 (NEC)
Input Voltage (safe area)	24V DC (module powered from backplane)
Max Output Voltage to Field (Uo)	≤ 28V DC
Max Output Current to Field (Io)	≤ 93 mA
Max Power Output to Field (Po)	≤ 660 mW
Input Impedance	~5 kΩ per channel
Channel Isolation	Individually isolated (channel-to-channel and channel-to-backplane)
Backplane Current (5V)	120 mA
Operating Temperature	0°C to 55°C (safe area, module location)
Standards	ATEX II 2G/2D, IECEx, UL Class I Div 1, CE

Wiring a Zone 1 Installation

IS wiring must follow strict discipline to maintain the certification. Requirements that apply to installations using the 1794-ID2:

Cable segregation: IS field wiring must be segregated from non-IS wiring throughout the entire cable run. Run IS cables in separate conduits or cable trays with at least 50 mm separation from other circuits, or use IS cable with a suitable jacket and color coding (blue is the international convention for IS wiring).
No earth loops: the IS circuit must have only one earth connection, typically at the safe area end. Multiple earth connections create potential paths for ground loop currents that can exceed the circuit's energy limits.
Barrier entity parameters: the module's Uo, Io, Po, and Co (capacitance) parameters must be compatible with the connected field device's Um, Im, Pm, and Ci parameters. This compatibility check must be verified and documented — it's a mandatory part of the IS system certification process.
Cable capacitance and inductance limits: IS circuits have maximum permissible cable capacitance and inductance values. Long cable runs to Zone 1 must be checked against the module's entity parameters to ensure the cable's parasitic capacitance and inductance don't exceed the certified limits.

Installing an IS system without proper documentation and a qualified IS system assessment does not constitute a certified installation, regardless of whether IS-rated components are used. An ATEX/IECEx-certified system requires a formal system description, entity parameter compatibility check, installation inspection, and documentation that satisfies the applicable hazardous area classification authority.

FAQ

Q: Does the 1794-ID2 need to be located in the hazardous area?

No. The module installs in the safe area — the control panel or FLEX I/O enclosure outside the hazardous zone. Only the field devices and the interconnecting cable enter the hazardous area. The module's IS circuitry limits the energy that reaches the hazardous area through that cable.

Q: Can the 1794-ID2 be used for Zone 2 / Division 2 applications?

Yes. Equipment certified for Zone 1 / Div 1 is also suitable for the less stringent Zone 2 / Div 2 classification. Zone 1 certification is the more demanding standard; applying it to a Zone 2 installation provides a higher safety margin than required.

Q: What types of field devices can connect to this module?

Any passive IS-rated field device whose entity parameters are compatible with the module's output parameters — proximity switches, pushbuttons, limit switches, float switches, and similar two-wire devices. Active devices with their own power supplies require separate entity parameter analysis and may not be compatible with a simple IS input module.

Q: Can the two input channels connect to field devices in different hazardous zones?

The two channels are individually isolated, so they can theoretically serve devices in different zones. However, the IS circuit documentation and entity parameter compatibility must be verified independently for each circuit. In practice, mixing zones on one module is unusual and should be reviewed by the hazardous area safety engineer responsible for the installation.

Q: Is a separate zener barrier or galvanic isolator required alongside this module?

No. The IS protection circuitry is integrated into the 1794-ID2 itself. Separate zener barriers or isolators are not required and should not be added in series — doing so would change the entity parameters of the circuit and potentially invalidate the IS certification for that circuit.

1794-ID2 Brand New FLEX I/O Input Module 1794ID2