Chapter 6. Serial I/O, SNP & RTU Protocols222 PACSystems* RX7i, RX3i and RSTi-EP CPU Reference Manual GFK-2222AD6.2.14 Read Bytes Function (4402)This function causes one or more characters to be read from the specified port. The characters are readfrom the internal input buffer and placed in the specified input data area. The function returns both thenumber of characters retrieved and the number of unprocessed characters still in the input buffer. If zerocharacters of input are requested, only the number of unprocessed characters in the input buffer isreturned.If insufficient characters are available to satisfy the request and a non-zero value is specified for thenumber of characters to read, the status of the operation is not complete until either sufficientcharacters have been received or the time-out interval expires. In either of those conditions, the portstatus indicates the reason for completion of the read operation. The status word is not updated untilthe 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 requestedamount 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 notbeen read from the internal input buffer remains and it can be retrieved with a subsequent read request.Example Command Block for the Read Bytes FunctionValue(decimal)Value(hexadecimal)MeaningAddress 0005 0005 Data block lengthAddress +1 0000 0000 NOWAIT modeAddress +2 0008 0008 Status word memory type (%R)Address +3 0000 0000 Status word address minus 1 (%R0001)Address +4 0000 0000 Not usedAddress +5 0000 0000 Not usedAddress +6 4402 1132 Read bytes commandAddress +7 0030 001E Read time-out (30 seconds)Address +8 0005 0005 Number of bytes to readAddress +9 0008 0008 Input data memory type (%R).Address +10 0100 0064 Input data memory address (%R0100)
