Chapter 6. Serial I/O, SNP & RTU Protocols
222 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD
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.
Example Command Block for the Read Bytes Function
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