A two-wire RS-485 half-duplex device can connect directly to converter terminals labeled A and B. If the converter instead exposes separate receive and transmit pairs, bridge the matching positive terminals and the matching negative terminals before connecting the two bus conductors. Confirm the terminal naming at both ends because the evidence does not establish whether A corresponds to + or − for every device.
Identify the Converter Interface
The described MAX487-based interface has power, ground, A, and B terminals. In that arrangement, A and B are the two half-duplex data conductors; power and ground are separate connections and must not be treated as additional differential data terminals.
| Converter terminals | Half-duplex connection |
|---|---|
| A and B | Connect the two bus conductors directly, preserving the corresponding labels at the remote device. |
| RxD+, RxD−, TxD+, TxD− | Bridge RxD+ to TxD+ and bridge RxD− to TxD−, producing one positive and one negative bus connection. |
Wire the Two-Wire Bus
- Remove power before changing the wiring.
- If the converter provides A and B, connect one cable core to A and the other to B.
- If it provides separate receive and transmit pairs, jumper RxD+ to TxD+ and connect that junction to RS-485+; jumper RxD− to TxD− and connect that junction to RS-485−.
- At each additional node, continue the same two conductors without exchanging their assigned labels.
- Restore power and verify bidirectional communication.
Resolve Label and Ground Ambiguity
The evidence supports adding a ground conductor to a two-wire multidrop network but does not define its connection limits or whether the specific devices require it. A two-core cable carries only A and B; if the installation needs a ground reference, provide a separate ground conductor and follow the device documentation.
Do not infer polarity solely from the letters A and B. The available evidence gives explicit +/− bridging only for converters labeled RxD and TxD; it does not prove a universal mapping between A/B and +/−. When labels differ between devices, verify each manufacturer’s terminal definition before energizing the link.
Verify the Half-Duplex Link
Test communication in both directions after wiring. If communication fails, first confirm that A and B were not exchanged, then confirm that converters with separate receive and transmit terminals have both positive terminals bridged together and both negative terminals bridged together. For a multidrop installation, verify that every node remains connected to the same two conductors.
FAQ
Can I connect a two-core RS-485 cable directly to A and B?
Yes, when the converter exposes A and B as its half-duplex data terminals. Connect one core to A and the other to B, maintaining the same conductor assignments at the remote device.
How do I convert separate RS-485 RxD and TxD terminals to two-wire half-duplex?
Jumper RxD+ to TxD+ for RS-485+, then jumper RxD− to TxD− for RS-485−. Connect the two cable conductors to those two junctions.
Does a two-wire RS-485 network need a ground conductor?
The evidence indicates that a ground line is usually added, but it does not establish a universal requirement or connection method. If required by the connected devices, use a separate ground conductor because the two data cores are already assigned to A and B.