Modbus RTU with SCADA master connected
Answered
Hello, i am connecting a netbiter to a network that already have a SCADA system, i installed the netbiter between slaves and start pulling data.
Small amount of timeout errors and alot of CRC errors were received after start reading data, data points are 40 point from 5 devices, 5 or 6 timeouts, 40 CRC and 100 valid was observed in netbiter status page after an hour in each device.
On Argos dashboard some data point appears and then disappear then reappear again.
Also on the SCADA system it start doing the same as on Argos dashboards, data missing and coming back .
No device provide all requested data from 1 query, their is usually 1,2 or 3 points missing in the reply, i decreased the quantity of data to 10 in a dashboard and the same happen
i switched off the netbiter to keep the customer SCADA working with no alarms..
Please advise, Is this a noise problem or a multi master problem and what should i do?
George
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Official comment
Hi George
As we are also discussing this in a support ticket I shall share here what we have advised there.
This looks like a multi-master problem. Remember that Modbus RTU is a single master protocol as only one device can be communicating on the half-duplex bus at any time. The SCADA is the current controller of the bus, it sends a request to a device and then waits for the response.
If a Netbiter is introduced onto the same RS485 bus then it is also sending requests to the devices on the bus and these different communications are colliding with each other and are being misinterpreted by the two masters.
Our suggestion is the use an Anybus X-gateway AB7899 (Modbus RTU device - Modbus RTU device) to act as a decoupler, the SCADA will need to be reconfigured to send the data that the Netbiter wants to read to the gateway and then the Netbiter will only communicate with the other "side" of the gateway in a second isolated RS485 bus with just the two nodes. Thus preventing any collisions on the bus from having two masters active.
There is no easy solution that does not require the SCADA to be reconfigured.
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Since you are getting occasional CRC errors, it sounds like it may be a connection problem. Keep in mind that RS485 buses should be terminated on both ends and the max bus length is dependent on the baud rate. Also, consider any sources of interference.
See also:
https://pdfserv.maximintegrated.com/en/an/AN3884.pdf
https://www.analog.com/media/en/technical-documentation/product-selector-card/rs485fe.pdf
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