MVI56E-MNETC/MNETCXT ♦ ControlLogix Platform Reference
Modbus TCP/IP Client Enhanced Communication Module - Client/Server User Manual
ProSoft Technology, Inc. Page 145 of 172
September 20, 2018
Coils that are not programmed in the controller logic program are not
automatically cleared upon power up. Thus, if such a coil is set ON by function
code 15 and (even months later) an output is connected to that coil, the output is
hot.
5.4.11 Preset Multiple Registers (Function Code 16)
Query
Holding registers existing within the controller can have their contents changed
by this message (a maximum of 60 registers). 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
(16-bit for the 184/384 and 584); unused high order bits must be set to zero.
Note: Function codes 5, 6, 15, and 16 are the only messages that will be recognized as valid for
broadcast.
This is the structure of the message being sent out to the Modbus network. The byte values below
are in hexadecimal display.
Response
The normal response to a function 16 query is to echo the address, function
code, starting address and number of registers to be loaded.
5.4.12 Modbus Exception Responses
When a Modbus master sends a request to a server device, it expects a normal
response. One of four possible events can occur from the master's query:
If the server device receives the request without a communication error, and
can handle the query normally, it returns a normal response.
If the server does not receive the request due to a communication error, no
response is returned. The master program will eventually process a timeout
condition for the request.
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