PACSystems™ RX3i and RSTi-EP CPU Reference Manual Section 6
GFK-2222AK October 2019
Serial I/O, SNP & RTU Protocols 156
6.2.14 Read Bytes Function (4402)
This function causes one or more characters to be read from the specified port. The
characters are read from the internal input buffer and placed in the specified input data
area. The function returns both the number of characters retrieved and the number of
unprocessed characters still in the input buffer. If zero characters of input are requested,
only the number of unprocessed characters in the input buffer is returned.
If insufficient characters are available to satisfy the request and a non-zero value is
specified for the number of characters to read, the status of the operation is not complete
until either sufficient characters have been received or the time-out interval expires. In
either of those conditions, the port status indicates the reason for completion of the read
operation. The status word is not updated until the read operation is complete (either
due to timeout or when all the data has been received).
If the time-out interval is set to zero, the COMMREQ remains pending until it has received
the requested amount of data, or until it is cancelled.
If this COMMREQ fails for any reason, no data is returned to the input data area. Any data
that has not been read from the internal input buffer remains and it can be retrieved with
a subsequent read request.
6.2.14.1 Example Command Block for the Read Bytes Function
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