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GFK-1541B Chapter 4 Programming Modbus/TCP Channel Commands 4-23
4
Communications Status Words
The COMMREQ Status word (CRS word) reports status in the format shown below. The CRS
word location is specified in Words 3 and 4 of the Command Block.
Hex Format
High
Low
00 00
Minor Error Codes (high byte)
Success and Major Error Codes (low byte)
CRS Word in
There are several points to remember when interpreting the contents of the COMMREQ Status
word:
1.
Display the Status Words in hexadecimal form
to more easily differentiate the high and low
bytes. A good way to do this is to use a MOVE WORD function block to display the
hexadecimal value within the ladder program.
2. The Ethernet Interface will never send a zero for the COMMREQ Status word to the PLC
CPU. The user program should zero the COMMREQ Status word
before
issuing the
COMMREQ function and then check for a non-zero value indicating that the Ethernet
Interface is responding to the COMMREQ. A good way to do this is to use a MOVE WORD
function block to zero the CRS word.
3. A status code of 1 in the low byte and 0 in the high byte indicates that the request was
successful. All other non-zero values indicate errors. Refer to the tables below for a
complete listing of major and minor error codes.
Major Error Codes
The following tables list the error codes that are reported in the COMMREQ Status word after the
execution of a COMMREQ function.
Error Status Major Error Description
Hexadecimal
01H Successful Completion.
87H The client PLC CPU’s Service Request Queue is full.
8BH Illegal Service Request. The requested service is either not defined or
not supported. (This value is returned in lieu of the actual service
request error (01H), to avoid confusion with the normal successful
COMMREQ completion.
90H Client API Error. See the following table of Minor Error codes.
91H Modbus/TCP Error Codes at server. An error was detected at the
Modbus/TCP server. See the following table for the Minor Error codes.