Modbus Communications to a Scadapack 474

[AlexNolan]

I have a SCADAPack 474 sitting on my desk.

It's set up with a fairly simple program including a number of Modbus AO points (DNPAO_200, Modbus DINT 40400/40401) these points exist as modbus read/write targets, Exclusive control is OFF, Logic variable Type is T_SPx70_DINT, they are not being written to in the controller logic and they can be written to in the SPx70 Online Diagnostics. 

Modbus is set as a communications protocol Via IP and Serial on ports 2&3 for testing purposes. the Scadapack is the modbus slave in this case and will be read from another SCADApack over radio link. 

On my test rig i have a USB485 converter connected to the serial ports using Mdbus (a quite reliable diagnostic tool), Remote connect attached via TCP and all of the scadapack indicators clearly visible.

Modbus reads function correctly, there's a bunch of points set up as modbus read points and they read correctly over Modbus IP and Modbus 485 via Port 2 and 485 via port 3. So the serial isn't the problem, the IP comms isn't the problem and the modbus addressing isn't the problem. 

But when I write the modbus value for any of the modbus Points the datagram goes out over the serial (or TCP) and returns with a confirmation response.  But the point inside the SCADApack does not change value. 

Is this a simple setting which isn't correctly set or something more complicated?

[Joel_Weder · Accepted solution]

Hi Alex,

I have Mdbus too (Calta used to do work for us when I was at BP many years ago), though more often these days I use Modbus Poll and Modbus Slave. I doubt that a failure of Mdbus itself is the issue, if you're not getting communication failures. The most common issue I've dealt with is a mis-configuration of the database point Start value or the Number of registers to write. Two comments, though my memories of working with Mdbus are many years old:

1) You need to write two consecutive registers to send a float to a Holding register.

2) If "Modicon Format Float" is checked, I seem to recall that register numbers start at 0. (e.g. 40001 would be register 0) If not checked, accessing register 40001 would be a value of 1.

If my comments don't help, I suggest you reach out to our global support team:

1-888-226-6876 or

SupportRO@se.com

[AlexNolan]

That isn't quite the solution. 

I have repoked the SCADApack this morning and the solution is that the SCADAPack does not function as a pure Modbus slave. if it did all of the normal registers would accept whatever messages you threw at them and do whatever you commanded. 

IE if you have a block of three Dint points [40300,40301][40302,40303][40304,40305] and you made a single register write to [40303] then on a proper modbus slave the register [40303] should update the with the value you wrote. the fact that it's actually the Most Significant register in a DINT shouldn't matter. 

but with (New) SCADApack it does. 

Thus single register writes don't work when you're writing to Dints, and register block writes don't work if you don't start on the right address. 

the comms message will say that the message succeeded and will give a good response but the write won't actually take effect on the point itself.