RS485 Connnection to DV-6 VE4006P2

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Thread Starter

Bhaskar Mittal

Need to connect the Toshiba G9 (4 nos.), G7 (2 nos.) and ABB ASD-800 (2 nos.) on a 485 network (Modbus RTU).

This is to communicate (send one way signal to Delta V DCS card VE4006P2.

One option is to use XLTR-1000 as interface card.

If the hardware connection details and software configuration can be suggested, that will help.

Distance of total cable length will be within 2000 ft.
 
Modbus is not plug-and-play even if Emerson's laughable marketing spiel, "Plug your Serial Interface into any available slot in the I/O interface carrier, connect the third - party device, power up and play", tries to claim it is.

Modbus is a project. Congratulations, you're the project engineer!

You gotta get the Modbus manual for the drive (a Modbus slave), find the Modbus map and figure out which data you need to read from the drive, which data you need to write to the drive.

You gotta get the manual for serial card and the function block (or whatever the DCS programs in) for the Modbus master and figure out where the write data comes from and where the read data goes to. Then do the programming configuration and logic do the successive read/write transactions.

Except for comm settings, the slaves are passive, they just report data when queried or accept a valid write when addressed to them (once the wiring is correct).

Each drive is configured with its own unique integer ID number (can't use zero). The serial comm parameters have to match on all devices: baud rate, 8 bit word, parity, 1 stop bit.

2 wire A/B or (+)/(-) connections are not defined by specification and differ from device to device. Although A should wire to A, when you mix vendors, A might go B. If it's backwards it won't work, but won't damage the drivers.

I like CAT 5 cable for 485, but you need shielded cable when working with drives.

There should be a terminating resistor at the extreme far ends of the network (usually the DCS and last drive on the daisy chained multidrop).

I stick a 485 repeater/isolator at the DCS end to protect the master's serial card and another repeater/isolator half way down the multidrop, just in case there's a really bad electrical fault on the comm line that takes out the 485 drivers, the loss is limited to only half the field devices. (I live in 3 season thunderstorm lightning land).

The programming configuration is on the master DCS end.

Start with one drive, get it working. Read a known non-zero value to make sure you get that exact value, because there's the dreaded one-offset (addresses start at zero, registers start at one, no one agrees on what is an address, what is a register) where you get a value from an adjacent address/register. Not good. Try your writes, make sure they do what they should.

Then add the other drives of the same brand, one by one. When all those are working, add one drive from a different vendor to see how well the network tolerates the addition of an alien. In the sandbox, not all kids play well with others. In 485, sometimes aliens do not play well with the locals.

There's anecdotal experiences cited here (one recently with Modbus energy meters) where adding a different vendor device crashed the entire network. Sometimes it happens. It shouldn't, but it does and given the relative time cost of figuring out primary cause versus getting it working on deadline, most people opt to bite the bullet and buy another network card and have multiple 485 networks. (that's really great thing about Ethernet, those little black box magnetics that isolate every node, but 485 uses a 'common' which isn't very common over geographical distances). It appears that your DCS card has two serial networks, which probably can be redundant or two separate networks, so bingo, there's the 2nd network just sitting there waiting to play.

You might luck out and find someone who's actually done that combination of devices who will share his programming and connection experiences but with a mix of 3 drive vendors and a DCS, I think you've got your work cut out for you.
 
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