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SYSTEM-20 BTU METER
7.2 DIAGNOSTIC FUNCTION CODE
The MODBUS implementation of the System-40 supports the use of function
code 08, Diagnostic.
When using function code 08, you must also specify the “sub-function” code
when sending the message to the System-40.
The following sub-functions are supported by the System-40:
Sub-function Code
Name
Hex
Decimal
00
00
Return Query Data
01
01
Restart Communications Option
04
04
Force Listen Only Mode
0B
11
Return Bus Message Count*
0C
12
Return Bus Communication Error Count*
0D
13
Return Bus Exception Error Count*
0E
14
Return Server Message Count1
0F
15
Return Server No Response Count1
11
17
Return Server Busy Count1
12
18
Return Bus Character Overrun Count1
* Count available in System-40 diagnostic menu page 1.
1 Count available in System-40 diagnostic menu page 2.
Description of Sub-functions: 00 Return Query Data
The data passed to the System-40 in the request data field is to be returned
(looped back) in the response. The entire response message should be identical
to the request.
01 Restart Communications Option
The System-40 serial line port will be initialized and restarted, and all of its
communications event counters are cleared. If the port is currently in Listen Only
Mode, no response is returned. This function is the only one that brings the port
out of Listen Only Mode. If the port is not currently in Listen Only Mode, a normal
response is returned. This occurs before the restart is executed.
04 Force Listen Only Mode
Forces the System-40 to its Listen Only Mode for MODBUS communications.
This isolates it from the other devices on the network, allowing them to continue
communicating without interruption from the addressed System-40. No response
is returned. When the remote device enters its Listen Only Mode, all active
communication controls are turned off. The Ready watchdog timer is allowed to
expire, locking the controls off. While the device is in this mode, any MODBUS
messages addressed to it or broadcast are monitored, but no actions will be
taken and no responses will be sent.
The only function that will be processed after the mode is entered will be the
Restart Communications Option function (function code 8, sub-function 1).