What is the meaning of Modbus Exception Code 7 ?


Thread Starter

Hassan Tariq

We have 15 devices connected to 1 serial to ethernet converter and then this converter is connected to the computer.The protocol these devices are using is Modbus.The problem is when we try to establish the connection with these devices using Modbus TCP/IP driver from the computer,all the device works fine except 1.This one device shows the error "device block request [ 401200 to 401213] responded with exception 7". I have checked the modbus specifications which shows the description of this code is "Read Exception status".But this description is not useful to find out the exact reason of the problem.Can any one help me out to find out the possible reason for this problem and how can we solve it.


Hassan Tariq

The slave cannot perform the program function received in the query. This code is returned for an unsuccessful programming request using function code 13 or 14 decimal. The master should request diagnostic or error information from the slave.

Lynn August Linse

> But this description is not useful to find out the exact reason of the problem. <

You need to contact the manufacture - Modbus exceptions (other than perhaps code 1 & 2) are not standard enough to be generically useful.

I've seen vendors create dozens of their own exception codes using numbers they felt were free or underutilized.

You may discover exception code #7 on this product means the firmware is out-of-date or a temperature sensor is loose.

But if I had to hazard a QUESS & your device is NOT a Modicon PLC, then I'd say this unit might have some write or read-protect enabled and the #7 is implied to mean "You can't do what you just asked".
you need to check the parity bet, the master and the slave. I just faced this exception 2 hours ago with modbus RTU over TCP/IP.