一、为什么工业现场用 4-20mA
- 抗干扰能力强：电流信号受线路电阻与感应噪声影响小，适合远距离传输。
- 可识别断线：正常信号最小为 4 mA，读到 0 mA 即可判断为断线或掉电，而 0-20 mA 无法区分「零值」与「故障」。
- 二线制可同时供电：两根线既供电又传输信号，布线成本低。
二、二线制、三线制、四线制
- 二线制：两根线同时承担供电与信号，现场仪表由回路供电（常见 24 VDC）。接线最少，但仪表功耗受限。
- 三线制：电源正、信号正（共用一根）与公共端，功耗较大的传感器常用。
- 四线制：电源与信号完全独立，适合功耗较大的分析仪器。
接线前必须核对仪表说明书的端子定义，不同厂家命名（如 OUT、+、−、COM）不完全一致。
三、布线与接地规范
- 分开敷设：信号电缆与变频器输出、电机动力电缆分开走线；不同桥架或保持间距，无法避免时交叉应为 90°。
- 屏蔽层单端接地：模拟信号电缆的屏蔽层一般在控制室侧单点接地，两端接地会形成地环路引入干扰。
- 避免地环路：当仪表与控制系统接地点存在电位差时，会引起共模干扰，可用信号隔离器解决。
- 电缆选型：模拟量信号用对绞屏蔽电缆；长距离 RS485 用特性阻抗匹配的屏蔽双绞线。
- 端子紧固与防水：户外接线盒做好防水密封，防止受潮导致绝缘下降。
四、RS485 组网要点
- 手拉手布线：避免星型分支，主干串联各节点。
- 终端电阻：总线两端加终端电阻（常见 120 Ω），减少反射。
- 极性一致：A/B（或 D+/D−）命名各厂家不统一，接反会导致通信失败，需按说明书确认。
- 参数一致：所有节点的波特率、数据位、停止位、校验位必须一致，站号不能重复。
- 隔离与接地：长距离或跨配电区域时考虑隔离，避免共模电压超出允许范围。
五、故障速查
- 读数跳变：多为干扰。检查是否与动力电缆同槽、屏蔽层是否单端接地、是否存在地环路。
- 无信号（0 mA）：检查供电电压、极性、回路是否断开、端子是否松动。
- 恒为满量程：传感器故障、输入短路、或实际超出量程。
- 读数偏高/偏低固定偏差：量程设置错误、未做标定、或未做温度补偿。
- RS485 通信不上：先查 A/B 极性与波特率，再查站号、终端电阻与地址冲突。
- 多台设备时好时坏：检查总线拓扑、分支长度与接地。
提供仪表型号、信号类型与现场布线情况，我们可以协助核对接线方式与配套电缆、隔离器、终端电阻等附件。
1. Why industry uses 4-20 mA
- Noise immunity: a current signal is little affected by line resistance and induced noise, so it travels far.
- Broken-loop detection: the minimum live signal is 4 mA, so 0 mA means a broken loop or lost power - something 0-20 mA cannot distinguish from a genuine zero.
- Two-wire power: two conductors carry both power and signal, keeping wiring cheap.
2. Two-wire, three-wire, four-wire
- Two-wire: the same two conductors supply power and carry the signal; the loop powers the instrument (commonly 24 VDC). Minimum wiring, but instrument power is limited.
- Three-wire: supply positive, signal positive (shared) and common; used by sensors with higher consumption.
- Four-wire: supply and signal fully separate; typical of larger analysers.
Always check the terminal definitions in the manual - naming (OUT, +, -, COM) is not consistent between manufacturers.
3. Wiring and earthing practice
- Separate routes: keep signal cable away from variable-frequency drive outputs and motor power cable; use separate trays or spacing, and cross at 90 degrees where separation is impossible.
- Earth the shield at one end: for analogue signals the shield is normally earthed at the control-room end only; earthing both ends creates a ground loop that injects noise.
- Avoid ground loops: a potential difference between instrument and control system earths causes common-mode interference; a signal isolator solves it.
- Cable choice: twisted-pair screened cable for analogue; impedance-matched screened twisted pair for long RS485 runs.
- Terminals and sealing: seal outdoor junction boxes against water, since damp degrades insulation.
4. RS485 network points
- Daisy-chain: avoid star branches; run one trunk through all nodes.
- Termination: fit terminating resistors at both ends of the bus (commonly 120 ohms) to reduce reflections.
- Consistent polarity: A/B (or D+/D-) naming is not standardised; reversed polarity kills communication, so confirm from the manual.
- Consistent parameters: baud rate, data bits, stop bits and parity must match on every node, and station addresses must be unique.
- Isolation and earthing: consider isolation over long distances or across distribution areas to keep common-mode voltage within limits.
5. Troubleshooting
- Jumping readings: interference. Check sharing a tray with power cable, single-point shield earthing and ground loops.
- No signal (0 mA): check supply voltage, polarity, loop continuity and terminal tightness.
- Permanently full scale: sensor fault, shorted input, or a genuinely out-of-range value.
- Constant offset high or low: wrong range setting, no calibration, or missing temperature compensation.
- No RS485 communication: check A/B polarity and baud rate first, then station address, termination and address conflicts.
- Intermittent faults with several devices: check bus topology, branch lengths and earthing.
Give us the instrument model, signal type and site wiring and we will help confirm the wiring method and accessories such as cable, isolators and terminating resistors.