I-O Display Systems Basic I/O Product - Manual (Page 23)

Setup & Installation 2
2-6(Vol.1)
RS-485 Programming:
The BASIC I/O will work equally well when connected to either 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 is designed to
handle this control automatically. However, 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 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 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 on the network receives the instruction and begin to decode it. That
particular BASIC I/O addressed begins to construct a response.
5) Once the carriage return is of the instruction is received, the BASIC I/O begins to
transmit a response.
6) The Host receives the response and takes the appropriate action.
This interaction is heavily dependant on asynchronous timing. Usually, the Host software has
no real means of determining that the instruction has been completely sent. This means that
the program must calculate the