I-O Display Systems Basic I/O Product - Manual (Page 59)

Installation 2
2-5(Vol.2)
RS-485 Programming:
The BASIC I/O AD will work equally well when connected as RS-422 or RS-485. However
special host programming considerations may be necessary when implementing an RS-485
network. Unlike RS-422 where both the transmit and Receive signals have their own
differential pair of conductors, RS-485 utilizes only one differential pair. The single pair of
conductors is used bidirectionally and handles both transmit and receive signals. In order for
this to be possible, the transmitter for each device on this type of network must be enabled
and disabled whenever a message is to be sent. The transmitter for the BASIC I/O AD is
designed to handle this control automatically, the transmitter control for most popular RS-485
cards that are installed in the Host computer must be controlled by the user program. This
control is not straight forward and may impact the overall system throughput with inherent
delay periods. The following is a typical instruction/ response transaction between a host
computer and a BASIC I/O AD using RS-485.
1) The Host computer enables its RS-485 transmitter (usually via the RTS line)
2) The Host then sends an instruction to the BASIC I/O AD in the form of an ASCII
printable string.
3) Once the Host determines that the string has been completely sent, the RS-485
transmitter is disabled.
4) Every BASIC I/O AD on the network receives the instruction and begin to decode
it. That particular BASIC I/O AD addressed begins to construct a response.
5) Once the carriage return is of the instruction is received, then begins to transmit a
response.
6) The Host receives the response and takes the appropriate action.