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Saturday, March 17, 2018
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Modbus 485 Distribution
New to Modbus networks....looking for advise to add a slave to existing network.

Hello forum,

I have never dealt with Modbus. I have an application where there is an existing Modbus 485 socket being used on a ScadaPack32. I will be adding some equipment into the building that has the option of Modbus 485. I was going to use 4-20mA signals back to the PLC but am now wondering if I should jump onto the existing Modbus.

My question is how to you distribute a Modbus network? Is it like Ethernet where you have a managed (or unmanaged) switch to connect devices. (I know you can also daisy chain them)

Does a Modbus 485 network switch (or splitter) exist and is this how you would normally add to an existing Modbus 485 network?

Thank you for your input.

If there is an existing serial Modbus network, you should confirm that adding one or more devices to it will not slow down the bus's throughput to the point where overall performance is affected. The Modbus master polls each device in turn, gets a response and moves on to the next device. Each poll and response takes time, which adds up as devices are added.

Serial modbus uses 'daisy chain' wiring, all devices in parallel, one after another. That can convenient or inconvenient, depending on the installation and geographic layout.

You should be aware of the failure modes that can affect not just one device, but all devices on the bus. A device's 485 driver can fail and tie up the entire bus. Many field devices (Modbus slaves) ground their 485 through the case ground rather than a 485 signal ground, which makes 485 subject to ground loops. Some 485 ports are isolated, many (most?) are not. I only saw this once, but a 485 bus with six slaves was accidently exposed to 115Vac, which took out all the devices on the bus.

Serial-to-ethernet converter/gateways are not inexpensive, but Ethernet is electrically isolated by design. If I were 'networking' a large number of serial Modbus devices, I'd look at putting some small number, like 3-5, on one gateway and then using Ethernet for remaining network connections to leverage the ease of hub-and-spoke Ethernet topology/cabling.

There are RS-485 hubs that are active repeaters that repeat the input on all the outputs, which can mimic hub-and-spoke topology, but I have no experience with one. A hub would be a slave node on a network, just like any other slave.