I-O Display Systems Basic I/O Product - Manual (Page 60)

Installation 2
2-6(Vol.2)
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 approximate time necessary to transmit the entire instruction
before the RS-485 driver is disabled. Since the BASIC I/O AD can respond very quickly to
the instruction, the Host must disable the driver as soon as possible in order to receive the
BASIC I/O ADs response. RS-485 communications can be tricky at best and should be
seriously considered before being adopted. Third party software users should make sure that
the package they have chosen supports the particular RS-485 communications card to be used.
Communication Wiring:
The Host to first BASIC I/O AD can be RS-422 or RS-485. Most Host computers come
equipped with an RS-232 serial port. A choice must be made to either equip the host with an
RS-422 or RS-485 card or to use an external RS-232 to RS-422/ 485 converter such as the
duTec BaudMASTER.
For ranges less than 5000 feet, both RS-422 and RS-485 networks can operate in multidrop
mode. For ranges greater than 5000 feet, RS-422 (NOT RS-485) networks can operate in
repeater mode. In this mode, the distance between individual units can be up to 5000 feet. The
trade-off for using the repeat mode is that the powering down of any single unit disables
communications with all units further “downstream” from the host.
A network of BASIC I/O AD must be made up of units which are configured as either all
multidrop or all repeat.