RS-485 and Modbus Serial Wiring Reference
Distinguish the RS-485 electrical layer from Modbus RTU, review bus and stub concepts, and reconcile vendor terminal labels before checking a serial link.
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Separate electrical and protocol layers
RS-485 describes an electrical interface. Modbus RTU describes serial message framing and behavior used on a suitable link. Matching a terminal label does not set the Modbus address, serial format, or device register map.
| Layer / field | What to obtain from the documents |
|---|---|
| Electrical interface | RS-485 variant, transceiver limits, reference/common arrangement, isolation, and documented cable requirements. |
| Wiring topology | Two-wire or four-wire arrangement, trunk/stub limits, termination and biasing design. |
| Serial settings | Baud rate, data bits, parity, stop bits, and the device’s supported modes. |
| Modbus settings | Device address, role, RTU/other mode, function support, and the exact register map. |
| Register interpretation | Address convention, data type, scaling, signedness, and multi-register ordering from the product manual. |
Bus topology concept
A transmission-line design must account for cable impedance, length, data rate, transceiver capability, stubs, termination, and biasing. Do not add a termination at every device merely because the device has a termination option. Star wiring or arbitrary long branches can behave differently from a documented trunk arrangement.
Reconcile terminal labels
Manufacturers may use A/B, +/−, D0/D1, or other labels. Confirm the function and polarity definition from both device manuals instead of assuming every vendor’s A/B names match. The Modbus serial guide defines its own naming convention, which must be mapped to the actual products.
Signal common/reference, protective earth, and cable shield are different roles. Their permitted connections and grounding depend on the equipment and link design. A two-wire differential pair does not mean every system can omit all reference, isolation, or surge-protection considerations.
Record link information
- List each device model, address, supported serial mode, and settings.
- Record each manufacturer’s terminal labels and the verified function mapping.
- Retain cable type, topology, physical ends, termination/bias settings, reference/shield arrangement, and isolation information.
- Use the documented commissioning method to separate communication, addressing, and register-interpretation problems.
This page does not define a universal A-to-+ wiring rule, prescribe shield grounding, or generate register values. Store observed results and unresolved differences in the test record and job card. Ethernet pin assignments are a different task covered by the T568 reference.