Are RS485 Full-Duplex interface same with RS422 interface?


closed as unclear what you're asking by Dmitry Grigoryev, uint128_t, Voltage Spike, Brian Carlton, clabacchio Jul 7 '17 at 12:43

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    \$\begingroup\$ Could you share some context please? \$\endgroup\$ – user17592 May 20 '13 at 12:30
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    \$\begingroup\$ Provide some background a good well written question will generate good answers, this question could use some work \$\endgroup\$ – Voltage Spike Jul 6 '17 at 15:14

RS-422 and RS-485 use the same electrical interface specification.

If you use that interface to build a bidirectional link on a single pair of wires, it's called RS-485, and it supports half-duplex operation.

If you use that interface to build two unidirectional links on two pairs of wires, it's called RS-422, and it supports full-duplex operation. This is sometimes mistakenly referred to as "full-duplex RS-485", but that's really a nonsense phrase.

  • \$\begingroup\$ You are the first person on the internet to clarify this explicitly. Thank you very much indeed. \$\endgroup\$ – hkBattousai Aug 6 at 14:20

RS-422 is a point to point communication connection. You can build it with one pair of wires to allow unidirectional communication between two parties (simplex, rarely used). With two pairs of wire it allows for bidirectional, duplex communication.

RS-485 is a bus structured communication setup that can potentially support multi master communication. It allows for multiple parties to communicate in full duplex if two pairs of wires are used. If only one pair of wire is present the communication flow is half duplex.

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    \$\begingroup\$ Why would one use RS-485 with full duplex? You'll have one master that sends a message (with ID) and wait for the answer of that slave? If you'll send a message before that slave responds, there will potentially be two slaves talking on the line, which is bad. So sending and receiving shouldn't be at the same time, in a bus system? \$\endgroup\$ – Paul May 10 '16 at 11:20
  • \$\begingroup\$ You would use it when the slave wants to talk continuously until asked to stop. With half duplex, the slave will have to release the line to see if master wants to say anything. With full duplex it does not have to do this. \$\endgroup\$ – Dojo Feb 22 '18 at 17:12
  • \$\begingroup\$ TI has a guide for it ti.com/lit/ug/sllu197/sllu197.pdf \$\endgroup\$ – Dojo Feb 22 '18 at 17:13

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