I have a MB+ PLC system in-place. Now, mgmt. want to monitor some equipment that has on-board RS-485 connected, ModBus protocal connections. So, I know that I need a Bridge unit to interconnect these. My question is this: do I need a MB <--> MP+ bridge (like the bm85c002), or do I need a general (programmable) rs485 <--> MB+ bridge (bm85s485) ??

Basically, will my 485 line (comming from the equipment) just plug-up to the "serial" port of the 'c002, or is that port rs232 only??

I've found documentation on pretty much every OTHER aspect of the bridges -- EEK!

 

As long as your equipment supports Modbus protocol, use the BM85C002 BridgeMux. Since the equipment connection is RS485, use a port-powered RS232/RS485 converter, such as a Black Box IC620A. You can't plug it directly in to the Modbus port, since the port needs RTS and CTS jumpered, just like a Modbus programming cable.

There's another option that's much less expensive on the hardware side, but adds programming time. Use a Momentum processor with RS232 and RS485 ports (171 CCC 780 10 or 171 CCS 780 00) with a Modbus Plus option adapter (172 PNN 210 22) on the least expensive I/O base. Get the RS485 T-connector and terminator, too. Write a PLC program using XMIT (984) or XXMIT (IEC) to talk to your equipment, and program your other PLCs to read and write the corresponding addresses in the new PLC. BTW, the XMIT and XXMIT command word value for 2-wire RS845 is 8448.

 

The NW-BM85S485 will do the trick. Put your ModBus RS485 device in the BM85's look-up table & then just use the MSTR instruction from the ModBus Plus/PLC side. However, depending on your PLC CPU you might have a lower-cost option. Modicon's Momentum PLC comes with an instruction called XMIT (some of them even come with a ModBus RS485 port), which allows the PLC to behave as a ModBus Master through its serial port. Other Modicon PLCs require you to buy this instruction and install it yourself. Once done it behaves as part of the PLC OS. All you then need is a ModBus RS232 to RS485 converter (B&B ELE, NR&D...) & you're all set. The flexibility of ModBus & the multiple flavors of Modicon PLCs give you several additional options but these two might be the most "straight-forward."

 

You could use the standard BM85 with an RS232-485 converter (from Black Box or other similar vendors). I would not advise using the programmable BM85 with built in RS485 unless you are an experienced embedded systems programmer. It is really designed to allow custom serial devices to access a Modbus+ netowrk.

You could also use a third party product like the one from Panel-Tec. It has two combo 232/485 ports and is available with a wide variety of protocols pre-installed, including Modbus. Check out

http://www.panel-tec.com/MD3000.htm

 

MB+ is a token passing peer-to-peer, and although the spec seems to mimic RS-485 and urban legands abound about standard RS-485 devices working with MB+, MB+ is not RS-485.

Modbus on RS-485 is master/Slave.

You should be able to just connect your RS-485 Modbus devices to any of the 4 BM85 serial ports - they have (or used to have) both an RS-232 and an RS-485 version. The only complexity is who will poll whom? MB+ is peer-to-peer, so every node as the ability to poll the others. Modbus RS-485 will be either slaves or Masters. Your devices likely are slaves, which means they'll just sit there waiting for someone else to ask them Modbus questions. If so, your MB+ PLC can use the MAST block to create MB+ qeuries, that the BM85 will convert ("bridge") to Modbus/RTU for your RS-485 slaves to answer.

You'd only want a "programmable" BM85 if you needed to write your own "C" code to map or "gateway" some non-Modbus serial protocol into Modbus.

Best Regards

Lynn August Linse, [email protected] IA Firmware Specialist, Digi Int'l (www.digi.com)

 

Check out http://www.hms-networks.com/products/abcmbp.shtml

Never used this, but it looks like it might be a simple solution.