Operating instructions

Reference PTQ-MCM ♦ Quantum / Unity Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 119 of 139
April 29, 2008
8.6.6 Preset Single Register (Function Code 06)
Query
Function (06) allows the user to modify the contents of a holding register. Any
holding register that exists within the controller can have its contents changed by
this message. However, because the controller is actively scanning, it also can
alter the content of any holding register at any time. The values are provided in
binary up to the maximum capacity of the controller unused high order bits must
be set to zero. When used with slave address zero (Broadcast mode) all slave
controllers will load the specified register with the contents specified.
NOTE Functions 5, 6, 15 and 16 are the only messages that will be recognized as valid for
broadcast.
Adr Func Data Start Reg Hi Data Start Reg Lo Data #of Regs Hi Data #of Regs Lo Error Check Field
11 06 00 01 00 03 CRC
Response
The response to a preset single register request is to re-transmit the query
message after the register has been altered.
Adr Func Data Reg Hi Data Reg Lo Data Input Reg Hi Data Input Reg Lo Error Check Field
11 06 00 01 00 03 CRC
8.6.7 Diagnostics (Function Code 08)
MODBUS function code 08 provides a series of tests for checking the
communication system between a client (Master) device and a server (Slave), or
for checking various internal error conditions within a server.
The function uses a two-byte sub-function code field in the query to define the
type of test to be performed. The server echoes both the function code and sub-
function code in a normal response. Some of the diagnostics cause data to be
returned from the remote device in the data field of a normal response.
In general, issuing a diagnostic function to a remote device does not affect the
running of the user program in the remote device. User logic, like discrete and
registers, is not accessed by the diagnostics. Certain functions can optionally
reset error counters in the remote device.
A server device can, however, be forced into 'Listen Only Mode' in which it will
monitor the messages on the communications system but not respond to them.
This can affect the outcome of your application program if it depends upon any
further exchange of data with the remote device. Generally, the mode is forced to
remove a malfunctioning remote device from the communications system.
The following diagnostic functions are dedicated to serial line devices.
The normal response to the Return Query Data request is to loopback the same
data. The function code and sub-function codes are also echoed.