HD 557 S1:1990

Overview

HD 557 S1:1990 - adopted as SIST HD 557.S1:2002 - defines the use of binary direct voltage signals in industrial process measurement and control systems. The standard applies to non-multiplexed, two‑wire binary direct voltage signals of a nominal 24 V level, used to transmit binary information between elements of process control installations. It provides a common reference for suppliers, system designers and operators to ensure compatible signalling and reliable information exchange in industrial automation.

Key topics

The standard focuses on the fundamental aspects needed for consistent implementation of two‑wire 24 V binary signalling in process control:

• Signal type and topology: non‑multiplexed, two‑wire binary direct voltage signals.
• Nominal voltage level: designation of 24 V as the reference system level for these binary signals.
• Functional scope: transmission of binary (on/off, logic) information between control, measurement and field elements.
• Interoperability intent: establishing a shared baseline so devices from different manufacturers can exchange binary status or command signals reliably.
• Application context: industrial process measurement and control systems (sensors, actuators, controllers, I/O modules).

Note: The document extract gives the scope and identification; it does not include specific thresholds, wiring practices, or electrical limits. For detailed electrical parameters or installation rules, consult the full standard text or complementary electrical standards.

Applications

This standard is practical for situations where robust, simple binary signalling is required:

• Field instrumentation: status output signals from sensors or switches to controllers or PLCs.
• Discrete control: on/off command lines from controllers to actuators, relays, or valves.
• I/O module interfacing: defining compatible signal expectations for digital inputs and outputs in process automation racks.
• Retrofit projects: replacing or integrating legacy two‑wire 24 V binary loops into modern control systems.
• Control system design and procurement: specifying signal types to ensure equipment compatibility and reduce integration risk.

Who should use this standard

• Control and instrumentation engineers specifying signal interfaces
• Process automation system integrators and OEMs
• Instrument manufacturers supplying digital outputs/inputs
• Maintenance and commissioning personnel ensuring correct wiring and signal behavior
• Procurement teams writing technical requirements for control hardware

Related standards

• Other CLC/HD and IEC standards covering industrial process measurement, control systems and electrical installation practices are typically used together with HD 557 S1 to define detailed electrical characteristics, wiring methods and safety requirements.

Keywords: HD 557 S1:1990, SIST HD 557.S1:2002, binary direct voltage signals, 24 V binary signals, two‑wire binary signals, process measurement and control.