Reference MVI56-MCM ♦ ControlLogix Platform
Modbus Communication Module
ProSoft Technology, Inc. Page 131 of 159
July 24, 2008
The forcing of a coil via MODBUS function 5 will be accomplished regardless of
whether the addressed coil is disabled or not. (In ProSoft products, the coil is
only affected if the necessary ladder logic is implemented).
Note: The Modbus protocol does not include standard functions for testing or changing the
DISABLE state of discrete inputs or outputs. Where applicable, this may be accomplished via
device specific Program commands. (In ProSoft products, this is only accomplished through ladder
logic programming).
Coils that are reprogrammed in the controller logic program are not automatically
cleared upon power up. Thus, if such a coil is set ON by function Code 5 and
(even months later), an output is connected to that coil, the output will be "hot".
8.9.6 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.
Ad
r
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
8.9.7 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.
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