Modbus addressing problem

The leading numeral 3 is not used in the Modbus message packet as an address, rather it is used to tell the Modbus master,(the internal driver in Kepware), to use the function code 04 in its message which tells the device that the data of interest is an 'input register', not an output register.

To identify whether the data is in an input register or holding register, sometimes the Modbus master uses the leading numeral 3xxxxx or 4xxxxx to determine that, or sometimes uses a method like 'select function', where one selects "input register" (which uses function code 03) or "holding register" (which uses function code 04). [Note that the function code 03 addresses the 4xxxxx data, the function code 04 addresses the 3xxxxx data.]

In both cases the xxxxx value is an indexed value from either zero (address) or one (register number), although the terminology of address or register is not officially defined and sometimes is interchanged.

Kepware is using 6 digit addressing, 3xxxxx. If you enter 30001, you're going to ask the slave for data at 330001. I suspect you're supposed to enter 300001, because you want the first input register (00001), even though the slave shows you 5 digit addresses (30001) in its documentation.

If you must actually enter the leading numeral 3 in Kepware to define the 'address', then try inserting an extra zero and use the six digit 300001 or 300019.

Or look for where in Kepware you select for the type of data needed, input or holding register and just use 00001 or 00019.

Hello,

For MODBUS addressing 30001 and 300001 access the same register.
3 = Input register
0001 or 00001 = register 1.

If you enter 300001 in Kepware you will be accessing register 30001 in the device. The invalid reading is caused by some other issue, not 5/6 digit addressing.

danw & Mark-
Thank you very very much for your Information .... I'll try again and find what is the wrong ... I'll post the results later .

This topic is rather old and I suppose it has been resolved. Nonetheless I am having similar problem with Kepserver v6.5 connecting with Modbus TCP. I is continously reporting an "Exception =2 Block range = 400103 to 400103" "Bad address in Block". The only problem that I can see is that it uses the 6 digit instead 5 digit. I have tested the Modbus Server with QModMaster and it is up and running; so the problem comes from KepserverEX (it is a nightmare).

Many thanks for your comments, they are the only useful information I found on the web.

Andres1989 said:

This topic is rather old and I suppose it has been resolved. Nonetheless I am having similar problem with Kepserver v6.5 connecting with Modbus TCP. I is continously reporting an "Exception =2 Block range = 400103 to 400103" "Bad address in Block". The only problem that I can see is that it uses the 6 digit instead 5 digit. I have tested the Modbus Server with QModMaster and it is up and running; so the problem comes from KepserverEX (it is a nightmare).

Many thanks for your comments, they are the only useful information I found on the web.

Click to expand...

While you may have similar components (Kepware, modbus etc), you should post this a new question rather than continuing on a years old thread.