【2026 Signal Transmission】Differences and connections between bus-type and 4–20mA instruments
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This post was last edited by The one on 2026-6-4 08:37. Bus-type instruments (such as HART, FF, Profibus-PA, etc.) and 4–20mA instruments are two major signal transmission methods in industrial automation; they are both functionally related to each other yet also have fundamental differences.
I. Key Differences 1. Signal Type 4-20mA instrumentation transmission: Analog signal (continuous current signal, transmitting only 1 variable) Bus-type transmission: Digital signal (in the form of data packets, capable of transmitting multiple variables + status)
2. Transmission content: The 4-20mA protocol can transmit only 1 main measurement value (such as flow rate or pressure). The bus-type protocol allows for the transmission of: 1. Multiple measurement values (such as pressure + temperature + density); 2. Instrument status (faults, calibration information); 3. Configuration parameters (range, damping)
3. Wiring method: 4-20mA instruments: two-wire/three-wire system (individual wiring for each instrument); Bus system: (multiple instruments connected in series on the same cable)
4. Wiring costs: For 4-20mA instruments, the greater the number of instruments, the higher the costs for cables and cable trays. In bus-type systems, multiple instruments share the same cables, which significantly reduces wiring costs (especially in large systems)
5. Interaction capability: 4-20mA instruments: one-way transmission (only from the instrument to the control system); local configuration via a manual operator is required. Bus-type instruments: two-way communication (control system ↔ instrument); remote configuration, calibration, and diagnosis are possible
6. Interference resistance: 4-20mA instruments: Analog signals are susceptible to electromagnetic interference (shielded cables are required); Bus-type: Digital signals have a verification mechanism, offering greater interference resistance
7. Expandability 4-20mA instruments: A single instrument can support only 1 variable; hardware addition is required for expansion. Bus-type: Functions can be expanded for a single instrument (such as enabling temperature compensation later on)
8. Cost variance: 4-20mA instruments: low cost of the instruments themselves, but high costs for wiring/maintenance; Bus-type instruments: high cost of the instruments themselves, but low costs for wiring/maintenance
II. Core Connections 1. Functional Overlap: Both have the core function of transmitting process variables (such as flow rate and pressure), with the ultimate goal of providing data to control systems for process monitoring and control.
2. Compatibility integration: Many bus-type instruments (such as HART instruments) support dual-mode transmission of both analog and digital signals: they can output an 4-20mA analog signal (for compatibility with traditional systems), while simultaneously transmitting digital signals over the same cable via the HART protocol (to enable bus functionality).