Micrologix 1400 with modbus 4-wire system

[jbutcher]

Has anyone ever connected a micrologix 1400 to a 4-wire RS485 modbus network? I know about the 1761-NET-AIC but it apparently only works with 2-wire RS485 (ideally DH485). I'm connecting to a multidrop network of 3 FMC Microloads and they do not work over Modbus 2-wire.

Google turns up 485LDRC9 < http://www.bb-elec.com/Products/Datasheets/485LDRC9_3713ds.pdf > as a possible 485-to-232 converter. Has anyone tried this or something similar? Am I missing something?

Thanks.

 

[diat150]

you can try connecting your txa/rxa together and txb/rxb together. a's are usually negative and b's positive, but I would try switching them if it doesnt work the first go around. this wont work on some devices, it it is worth a shot. outside of that you will need some sort of converter, b&b electronics usually has good prices on that kind of stuff.

 

[rguimond]

1763-NC01 breakout connectors are all I ever use, although I have "wasted" 1761-NET-AIC modules before I realized I didn't need them - the ML1100 and ML1400 models have 485 pins on the mini-DIN connector.

Likely, you can connect the RX+ / TX+ and RX- / TX- conductors together.

 

[tallgirl]

I'll third connecting RX+/TX+ and RX-/TX-. Modbus is multi-drop, open collector and there should only ever be one transmitter on the line.

Four-wire RS-485 is for full-duplex communications, which Modbus doesn't generally support. One gotcha for tying the RX and TX lines together is you won't be able to do full-duplex. Which you likely weren't anyway. Another gotcha is if the device has its receiver enabled while transmitting. You'll find out real fast if it doesn't work when the device floods the bus the first time it transmits a Modbus response that is also a valid Modbus command.

 

[jbutcher]

BnB's 485LDRC9 RS-232 to 485 converter did the trick. Was a little worried about timing between the two sides but the converter has baud dip switches and it woks like a charm. Biggest hurdle was byte-swapping between the MicroLoad and the MicroLogix 1400 - they don't agree on endian-ness.

I've never encountered something in practice that couldn't go from 4 to 2 wire (full duplex to half) before this. Called and verified with FMC that this is the case with MicroLoads as of April 2014. Tech support mentioned that there is a firmware patch coming 'soon' that will address this but I'm using the converter in the meantime. Also verified with Allen Bradley that they don't support 4 wire, full-duplex (see technote 591526).