ProSoft Technology MVI56E-MNETC - Manual (Page 155)

MVI56E-MNETC ♦ ControlLogix Platform Reference
Modbus TCP/IP Client Enhanced Communication Module User Manual
ProSoft Technology, Inc. Page 155 of 183
February 3, 2013
5.5.11 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.
Adr Func Hi
Add Lo Add Quantity Byte
Cnt Hi
Data Lo
Data Hi
Data Lo
Data Error Check
Field
11 10 00 87 00 02 04 00 0A 01 02 CRC
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.
Adr Func Hi Addr Lo Addr Quantity Error Check Field
11 10 00 87 00 02 56
5.5.12 Modbus Exception Responses
When a Modbus Client sends a request to a server device, it expects a normal
response. One of four possible events can occur from the Client'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 Client program will eventually process a timeout
condition for the request.
 If the server receives the request, but detects a communication error (parity,
LRC, CRC, ...), no response is returned. The Client program will eventually
process a timeout condition for the request.
 If the server receives the request without a communication error, but cannot
handle it (for example, if the request is to read a non-existent output or
register), the server will return an exception response informing the Client of
the nature of the error.
The exception response message has two fields that differentiate it from a
normal response: