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Communications Instructions
Publication 1763-RM001B-EN-P - April 2007
The maximum amount of data that can be transferred via a MSG
instruction is 103 words (120 words for Modbus commands) and is
determined by the destination data type. The destination data type is
defined by the type of message: read or write.
• For Read Messages: When a read message is used, the destination
file is the data file in the local or originating processor.
Input, output, string, and RTC file types are not valid
TIP
for read messages.
• For Write Messages: When a write message is used, the destination
file is the data file in the target processor.
The maximum number of elements that can be transmitted or received are
shown in the following table. You cannot cross file types when sending
messages. For example, you cannot read a timer into an integer file and
you cannot write counters to a timer file. The only exceptions to this rule
are that:
• long integer data can be read from or written to bit or integer files,
and
• RTC files can be written to integer files.
The table below is not intended to illustrate file
TIP
compatibility, only the maximum number of elements
that can be exchanged in each case.
Message Type
File Type
485CIF
O, I, B, N
L
T, C, R
(1) (2)
ST
500CPU
O, I, B, N
(3)
F
, L
T, C, R
RTC
(1)
ST
Element Size Maximum Number
of Elements per Message
1-word
103
2-word
51
3-word
34
42-word
(3)
2 (write only
)
1-word
103
2-word
51
3-word
34
8-word
1 (write only)
42-word
2
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