Bandwidth Calculation for Modbus system


Thread Starter


A Modbus RTU/Modbus TCP gateway is connected to 10 devices. SCADA PC is connected to the gateway through an existing LAN.
I need to calculate the bandwidth required in the LAN to carry data between the gateway and SCADA PC.

Bus Baud rate is 9600 bps.
I believe that the bandwidth will be limited to 9600 bps since the gateway is not buffering data.
But I should include the headers of TCP/IP packets also in the calculation?

Would you please help me on that.

I don't have any idea how Modbus RTU data is encapsulated in the Modbus TCP packets.

Thanks in advance

Lynn August Linse

Your bottleneck will be the multi-drop RS-485. At 9600 baud I would not expect to 'sustain' more than 3 or 4 polls per second. Not saying you couldn't cram 10 per second through (if small messages), but you need to plan for the worst, not the lucky-best. Plus any 1 node going offline could cause this to drop to less than 1 per second.

Compared to serial, the Ethernet limitations "don't exist" ... unless you are running over a low-speed DSL line or back haul.

Patrick Lansdorf


There are 2 types of gateways, transparent and non-transparent.
With a transparent gateway, the Modbus TCP master will send a ModbusTCP command to the gateway and the gateway will strip the ethernet frame and relay a Modbus RTU command to the Modbus RTU slave. The Modbus RTU slave will respond to the gateway and the gateway will add ethernet frame and respond to the Modbus TCP master.
The non-transparent gateway usually has a dual port memory and a separate Modbus RTU master. This type of gateway has a scan list of preconfigured Modbus RTU commands and uses the dual port memory to share data with a Modbus TCP interface, so that the Modbus TCP master will read/write to the Modbus TCP slave interface of the gateway.

I have an excel sheet that will calculate the Modbus RTU line time. email me at: pal(at) and I will send it to you.