Appendix B ______________________________________________________ MODBUS Reference
VAISALA ______________________________________________________________________ 191
Diagnostic Sub-Functions
DMT340 supports some of the MODBUS diagnostic functions described
in the MODBUS Application Protocol Specification V1.1b. These
diagnostic functions are accessed with function code 08. See Table 41
below for details.
Table 41
DMT340 MODBUS Diagnostics
Code Sub-Function
Name Notes
00 (0x00)
Return Query Data
01 (0x01)
Restart Communications
Option
Cancels the “Listen Only” mode.
Data field must be set to “00 00”
for Restart Communications
Option. Sub-Function 01 with
data field “FF 00” is rejected.
02 (0x02)
Return Diagnostic Register
Non-zero if there are active
transmitter errors.
For same error information, see
status registers 0516 and 0517.
The value of the Diagnostic
Register is a logical OR of these
two status registers.
04 (0x04)
Force Listen Only Mode
Puts device “off-line”
10 (0x0A)
Clear Counters and
Diagnostic Register
Diagnostic Register cannot be
cleared
11 (0x0B)
Return Bus Message Count
Total number of messages seen
on the bus/interface
12 (0x0C)
Return Bus Communication
Error Count
Number of messages with bad
MODBUS RTU CRC or
MODBUS TCP frame
13 (0x0D)
Return Bus Exception Error
Count
Number of MODBUS exception
responses sent
14 (0x0E)
Return Slave Message Count Number of MODBUS messages
processed
15 (0x0F)
Return Slave No Response
Count
Number of MODBUS messages
received without sending a
response. This happens for
received broadcast messages
(MODBUS RTU only) or when
DMT340 is in the “Listen Only”
mode.
Although MODBUS Diagnostics are standardized only for serial line
devices, DMT340 supports the same diagnostic functions also on
MODBUS TCP.
NOTE
Resetting, powering up the transmitter, or reselecting the MODBUS
mode (by serial command or with the user interface) resets all MODBUS
diagnostic counters and cancels any “Listen Only” mode.
