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4 wire modbus RTU 1
- Thread starter Alms88
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In all these years I've messed with Modbus I never had an RS-422 unit or I would have tried to make 422 work with 485.
No, RS-422 and RS-485 are not different names for the same thing. Their electrical characteristics are similar, but not identical. Technically RS-485 is half-duplex with tri-stating for change-over from transmit-to-receive; 4-wire RS-422 is capable of full duplex, although the Modbus RTU protocol by definition can not operate full duplex.
If it were me, I'd probably try making both work together by jumpering (T+) to (R+), (T-) to (R-) on the 422 side and connecting the 485 (+) to the 422 (+), 485 (-) to 422 (-) to see if it works.
But if you want a security blanket, there are commercial 422-to-485 converter modules with some nice isolation specs:
Link
I'd pay the $20 premium for the CON-485/422-REP over the REP-485/422-9E because it has screw terminals for the DC power supply instead of a coax plug. Just what I want: a call-back because the coax power plug comes out and the converter turns off.
The diagrams show the use of dual converters, one on each end, but duplicity is for the surge protection isolation, not necessary for communications. A single converter would do what you need to 422 to 485.
No, RS-422 and RS-485 are not different names for the same thing. Their electrical characteristics are similar, but not identical. Technically RS-485 is half-duplex with tri-stating for change-over from transmit-to-receive; 4-wire RS-422 is capable of full duplex, although the Modbus RTU protocol by definition can not operate full duplex.
If it were me, I'd probably try making both work together by jumpering (T+) to (R+), (T-) to (R-) on the 422 side and connecting the 485 (+) to the 422 (+), 485 (-) to 422 (-) to see if it works.
But if you want a security blanket, there are commercial 422-to-485 converter modules with some nice isolation specs:
Link
I'd pay the $20 premium for the CON-485/422-REP over the REP-485/422-9E because it has screw terminals for the DC power supply instead of a coax plug. Just what I want: a call-back because the coax power plug comes out and the converter turns off.
The diagrams show the use of dual converters, one on each end, but duplicity is for the surge protection isolation, not necessary for communications. A single converter would do what you need to 422 to 485.
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