Practical Modbus limit


Thread Starter

Bill Townsley

Hi all,

I am connecting a series of MTS tank level gauges together using their modbus version of the instrument. Does anyone know what the practical limit for the number od instruments on the highway would be. I have found out that if we use IS, then maximum number of instruments is only five. However, if we go with explosion proof housing instead, the addressible limit is 255 devices - which would be ridiculous at 4800 baud.

Thanks for any information.
Yes you are right when you say that it is ridiculous to address 255 devices at 4800 baud.
The number of devices that can be connected practically depends on your application. If you have to do only monitoring of information without any control strategy to be implemented any place and if the rate at which data is to be acquired is not important, then practically, I have been able to connect upto 70 units in a daisy chain loop. Again in this case, I had to read only two
registers and write maximum two registers from and to each device. Each scan typically required 45 to 50 seconds to execute. As a control strategy, we had implemented an interrupt based writing from our SCADA system, so that if there
is a user command to write into any device, the loop would be broken in between and the device ID for which the writing command is issued is polled for data transfer. Again as I mentioned, here the update rates and command execution is not very critical and hence acceptable. In other cases, where response times is an issue, we have limited ourselves to max four devices per loop.
Hope that above is of some help for you to decide
The Modbus protocol are told us the limit is 247 device and default parameter
is 9600,8,1 even.

Bill Townsley

Thanks for the information. My application is on a tank farm using modbus-enabled level gauges. Update time is not an issue. I might set
the typical upper limit to 16 devices - mainly so I don't lose the entire site if the modbus link fails...
You can theoretically connect 255 Modbus units together using RS485: 31 per segment with a repeater between segments. The 255 limit is determined by the size of the node address in the Modbus packet. The difference in the number of devices connected with explosion proof is probably because the explosion proof will contain its own signal isolator and power supply, i.e.. repeater, and will not rely on the source to supply the signal to all of the devices within the explosion proof enclosure.

To determine the number of devices that you can realistically connect at 4800 baud. You will need to calculate the required response rate per MTS
device, cut in half (messaging goes both ways) and then subtract a fudge factor that will approximate the response lag of each device attached. If the required response interval is too short for what the network can provide, you will have to split the network, using two or three (what ever is required) communications or custom applications modules. In a food processing system that I had done, we used two networks. One that connected fifteen tank level gauges and one for two process lines, each containing ten
devices. We were able to prioritize the messaging in order to meet the required response times.

Hope that this helps. Look to the ISA web site for information on RS485/422 and the modicon web site for information on the modbus protocol.