AD MODBUS network -> Part Selection -> DCM vs adapter

I've got a couple of PLCs from AD. It started with a D0-06, and I realized I could save a fair bit of wiring by using a D0-05 to run the remote control panel. (3 wires on RS-232 being much easier to run than 14 signal wires plus power) Initially, I thought about a PIC, but the PLC solution was significantly cheaper than the PIC once I factored in the writer, the board prototyping, and the programming software. ($100 for one D0-05 vs $500 for CCS-C, $100 for a writer, $50 in parts, and about 4 weeks of development time.)

The RS-232 link works nicely for the control panel. I can read any of the inputs on the remote panel, and I can send lighting signals so that only the valid options are illuminated. It's saved a lot of pins on the PLC, and I'm extremely happy with the way it works. (I'm running MODBUS as the protocol with the built-in MRX / MWX blocks)

Anyway, the design has changed a little, and I now have to add a second remote PLC to control an interlock. I'm going to use another D0-05. This means I can't use RS-232 and have to move to RS-422 or RS-485 because it's now a multiple-slave system. However, the D0-05 doesn't support 422/485. I have to buy an adapter of some kind. My options are:

1. F2-UNICON -> $240
2. FA-ISOCON -> $300
3. DO-DCM -> $360 (free shipping)

The UNICON and ISOCON change the RS-422/485 signals to the RS-232 signals that the D0-05 supports. The ISOCON provides isolation from em interference, so unless there's a compelling reason to save the $60, I'd choose the ISOCON over the UNICON.

The D0-DCM inserts into the D0-05 modules and will not require any other adapters. I've got the available space in the D0-05 modules. I don't know if it provides any isolation against stray em noise. There will be a 3HP motor attached to the system, but it's going to be on a seperate power supply, isolated by contactors.

Everything will be outdoors, near the ocean. I'm planning on using RS-485 as the physical layer simply because it uses less wiring.

Which should I go with? The D0-DCM communication module, or the ISOCON RS-232 / RS-485 adapter? What problems or issues will I encounter with these modules?

Thank you.

MASEngr said:

1. F2-UNICON -> $240
2. FA-ISOCON -> $300
3. DO-DCM -> $360 (free shipping)

Click to expand...

Option #4. A DL06 instead of the DL05 $199 it supports RS485 natively.

Personally, I would go for option #2 because of the ease of wiring and optical isolation.

I would also recommend 4 wire RS-485/RS-422 if it is an option. Much easier to deal with 2 differential pairs than dealing with the echo suppression delay/issues of 2 wire RS-485 (single differential pair).

#2 is the option that AD recommended, too. He was, shall we say, quite emphatic about using the ISOCON instead of the UNICON. I'm not sure if I can justify the 4-wire plan, since my boss found a spool of surplus 4-wire (2 twisted pairs) in a local telco's dumpster. (It's in perfect shape.)

#4 won't work due to space constraints. It's a pretty good plan, though.

Tech Support is right. You don't want to use the F2-UNICON for this type of application.

The FA-ISOCON is the right convertor for this application.