I have a controller that has a RS-422 output.

Can I use it to only send data to another device that has RS-485?

  • \$\begingroup\$ Is there a particular communications protocol involved (Modbus for example), or are you just asking about the hardware interface? \$\endgroup\$ – Tut Mar 4 '15 at 12:09

RS-422 & RS-485 are similar in that they both use balanced, differential signalling. All of the RS-422 & RS-485 systems that I have seen or worked with use 5V levels.

A RS-422 driver is an output-only device. It will happily feed a RS-485 device so long as that RS-485 device is only ever receiving data. You will have a data collision if the RS-485 device ever tries to transmit on that pair.

The best way to think of this is that RS-422 is intended for systems that use separate Tx & Rx signal paths. That is: there is a single sender on each of the two data pairs (one sender at each end of the link).

Note that RS-422 can have multiple listeners.

RS-485 is designed for multi-drop systems where multiple devices can talk and listen to the same single conductor pair.

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An RS422 is very similar to an RS485 except that the 422 is not intended as a true multi-point system and does not always use addressing (as does a 485 system). Though a 422 driver can transmit to multiple 422 devices in a one way format.

So while the 422 signals and voltages can be made to match a 485 system you would need to manually add an address prefix to the 422 signal to properly address the 485 device.

So the basic answer is yes, (if you operate it correctly).

One caveat is that some 422 drivers cannot be switched off, so you should insure that the 485 device is not able to transmit or answer back on the same line.

You also obviously need to use the same duplex mode for both the 422 and the 485 device.

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    \$\begingroup\$ Um, EIA422 and 485 are physical layer specifications, there is no addressing. That is done at a higher level, or not, as you wish. And of course 485 is always half duplex or simplex, it can't do full duplex. \$\endgroup\$ – tomnexus Mar 4 '15 at 10:27
  • \$\begingroup\$ RS485 is capable of half or full duplex (2 or 4 wire systems). digital.ni.com/public.nsf/allkb/… \$\endgroup\$ – Nedd Mar 4 '15 at 12:00
  • \$\begingroup\$ An RS422 system would be considered as always half duplex as there is no return data path. \$\endgroup\$ – Nedd Mar 4 '15 at 12:23
  • \$\begingroup\$ While addressing is not part of the RS485 protocol most RS485 devices will operate with a unique address, unless they have a special master/slave mode or listener mode. \$\endgroup\$ – Nedd Mar 4 '15 at 12:39
  • \$\begingroup\$ Good references thanks; I had an over-simplified view of 422/485. But there is no addressing specified or required, that's up to the user. I guess there are two questions here, Physical layer, which will work if OP has a Talk-Only device that doesn't need to be interrogated, and The Rest of the Layers, which are not defined in the question. Talking to a PC, no problem, to a piece of control gear, not likely. \$\endgroup\$ – tomnexus Mar 4 '15 at 12:59

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