MODBUS communication and total time scan


Thread Starter


Both analog and discrete values are fetched from a PLC via RS485 2W MODBUS. How much time does it take to read value for analog (numeric) value and digital (holding register) value till the values are displayed in DCS system?

I read somewhere that it takes 5 sec for numeric and 1 sec for discrete value.

What effect it will have if several devices are put using multidrop.
Can anyone confirm this?
You need to provide a bit more information! Biggest factor in determining the update time is the communications speed (baud rate). Modbus RTU is a reasonably compact protocol, you can request up to 127 consecutive registers of data using an eight (8) byte command; the response could be up to 259 bytes long. At 19,200, 8, N, 1 that string would take less than 2 seconds to transmit.

Patrick Lansdorf


I would actually say that most influential factor is/can be the response time of the slave. In some cases up to 300ms.

I have a excel sheet, that calculates the line times for Modbus RTU.

let me know if you would like to have it.

Patrick L
[email protected]
Thanks for your Quick response. attached here more information .
Baud rate:9600
Request : 32
Stop Bit : 1
Length of cable around 1km
Master:Honeywell DCS through SI card
Slave:IVT OilMist System

Fred Loveless

At 9600 baud it takes about 1 msec to transmit 1 piece of data on the wire. A request for holding registers is going to to have 8 bytes. The number of requests depends upon the amount of data and how the master has to request it. a request uses a staring register and a total number of registers. If the requesting 32 registers from contiguous addresses then the response would be 64 bytes (2 bytes per registers) plus 5 bytes for header and footer. So at a minimum it will take about 77 msec on the wire for that request. The other factor is the amount of time it takes for the device to process the request and send the response back. I have seen that take 300-400 msec or more but usually it is less than that.

I assume this is a HPM you are talking about....

The Honeywell serial FTA is pretty fast actually. But there are things that can cause the comms to give the impression that its slow:

a. if you have array points built that is writing data to a slave through the same FTA and you are not careful that the push to the array point is only done on change of state (rather then constant push every scan) then your ready array gets buffered until all writes are complete.

b. if you are displaying the data on a GUS display or the like - then depending upon the NIM loading/LCN loading/GUS display size and how often that parameter is updated it might give you a false impression that comms is slow. Check in native window first.

c. if you have error on the link causing retries. You can tell this from the LED status on the FTA (blink rate)