Modbus Recommendations

Looking for any recommendations, as I have very little experience with modbus.

I have 4 units, each with 2 stand alone single loop controllers that have modbus capability. I need to get data from these controllers to our BAS system.

My initial thought is to network the 8 controllers and connect them to a Micrologix 1100, then our OPC server can get the tag values over ethernet.

The thing is, the units are portable in theory,(though I don't expect them to be moved much if at all) so if I could keep them separate, that would be better. But I don't want to put in 4 ML1100. My question is, is there a cheaper way to get the modbus data to a remote OPC server. I know the PLC will do it, but is it really necessary? There is no control required, only monitoring.

The bridges I've seen are in the same price range as ML1100, and I'm not clear what would act as the master if no PLC is there?

Thanks for any suggestions,

Rob

Not enough information has been given.

What sort of physical communications do your remote devices support? RS-232, RS-485, Ethernet? That will make a huge difference in the answer to your question.

I am guessing that the devices are serial and you want to convert to ethernet?

check with lantronix or digi. both are solid options and very easy to setup.

also, if your devices support rs485 you can network them all together on a 485 bus then connect them to a single lantronix or digi.

another option if the devices only support rs232 and are within 50 or so feet of each other is to use a black box splitter, which will allow you to tie all of the devices in then use a single digi or lantronix to collect the data.

post what you have for a better description.

yep, sorry. the devices are RS485. thanks for the replies,

rob

An ML1100 with a NET-AIC is how we usually do it

Yeah, I don't think I can get any lower cost than that, just thought I'd ask. I don't think I will need the AIC for the 1100 though, if we use the 1763-NC01 cable.

Thanks again,

Rob

Have a look at this:
http://www.bb-elec.com/bb-elec/literature/MES1x_2106ds.pdf

I think it will do what you want for much less than a MLX1100. Not 100% sure but these guys are supposed to have great technical support so I'd call them and run the application by them to see what they recommend.

naegely said:

Yeah, I don't think I can get any lower cost than that, just thought I'd ask. I don't think I will need the AIC for the 1100 though, if we use the 1763-NC01 cable.

Thanks again,

Rob

Click to expand...

You're right. All you need is the NC01.

I wouldnt use a 1100 just for this. thats overkill. wire the 485 devices together, give them each a different rtu address, put a 120 ohm resistor on each end, and use a lantronix or digi or any other ethernet to serial converter. the only way i would use the micrologix would be if you wanted a hardware firewall, or if your opc server only handled ab protocol.

I looked at the Digi One IA, and it's about $150 less than an ML1100, so could save a little. But that's where my ignorance of Modbus is a problem. So my 8 devices are slaves and are daisy chained to a serial/enet converter. What is the master, i.e. what initiates data transfer from the devices?

naegely said:

I looked at the Digi One IA, and it's about $150 less than an ML1100, so could save a little. But that's where my ignorance of Modbus is a problem. So my 8 devices are slaves and are daisy chained to a serial/enet converter. What is the master, i.e. what initiates data transfer from the devices?

Click to expand...

Your OPC server.

Thanks Norm, I will give them a call

OK, thanks Mickey, that makes sense. So a tag configured in the OPC server with Modbus TCP driver should grab the data. Sorry, like I said, haven't worked with modbus much at all.