Mitsubishi Electric MELSEC iQ-R08ENCPU - Diagnostics (FC: 08 H)

3 MODBUS STANDARD FUNCTION DETAILS
3.8 Diagnostics (FC: 08H)
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3.8 Diagnostics (FC: 08H)
Executes the various diagnostics and checks the C24 status and communication status.
In the diagnostics, a sub-function code is specified after a function code with a request message.
Checking communication conditions using a diagnostic counter
When using the MODBUS slave function, the number of error occurrences during communication is stored in a diagnostic
counter.
The communication status of MODBUS can be checked by a diagnostic counter of the MODBUS standard function.
■ Diagnostic counter
*1 The bus message count is in an exclusive relationship with the bus communication error count.
■ Count range
0000H to FFFFH are counted.
The count is stopped if it has reached FFFFH. To continue, clear the counter.
■ Methods for clearing a counter
A counter can be cleared by any of the following methods:
• Turning the power OFF and ON
• Resetting a CPU module
• Clearing the buffer memory to '0' by using a sequence program
• Receiving the following MODBUS standard functions
Counter Buffer memory MODBUS standard function
Type Description CH1 CH2
Bus message count
*1
Counts the number of messages detected on the line. 32512
(7F00H)
32576
(7F40H)
Page 28 Return bus message
count (sub-function code: 0BH)
Bus communication error
count
*1
Counts the number of the following error messages detected on
the line.
• CRC/LRC error message
• Overrun/parity error
• Short frame (less than 3 bytes)
• Character overrun (256 bytes or more)
Messages other than the above are counted by the bus
message count.
32513
(7F01H)
32577
(7F41H)
Page 29 Return bus
communication error count (sub-
function code: 0CH)
Exception error count Counts the number of exception error occurrences.
(Including broadcast messages)
32522
(7F0AH)
32586
(7F4AH)
Page 30 Return bus exception
error count (sub-function code: 0DH)
Slave message count Counts the number of times that messages addressed to the
host were processed.
(Including when broadcast request messages were received)
32518
(7F06H)
32582
(7F46H)
Page 31 Return slave message
count (sub-function code: 0EH)
Slave no-response count Counts the number of times that broadcast request messages
were received.
32519
(7F07H)
32583
(7F47H)
Page 32 Return slave no response
count (sub-function code: 0FH)
Slave NAK count Refers to the number of times that NAK responses were
received in a master device.
It is always '0' in C24s.
32520
(7F08H)
32584
(7F48H)
Page 33 Return slave NAK count
(sub-function code: 10H)
Slave busy count Refers to the number of times that busy responses were
received in a master device.
It is always '0' in C24s.
32521
(7F09H)
32585
(7F49H)
Page 34 Return slave busy count
(sub-function code: 11H)
Character overrun count Counts the number of times that the request message size
exceeded the upper limit.
32514
(7F02H)
32578
(7F42H)
Page 35 Return bus character
overrun count (sub-function code: 12H)
Page 36 Return IOP overrun error
count (sub-function code: 13H)
Counter to be cleared MODBUS standard function
All diagnostic counters shown above Page 23 Restart communications option (sub-function code: 01H)
Page 27 Clear counters and diagnostic register (sub-function code: 0AH)
Character overrun count Page 37 Clear overrun counter and flag (sub-function code: 14H)