I-O Display Systems Basic I/O Product - Manual (Page 22)

Setup & Installation 2
2-5(Vol.1)
The serial communications link between a Host computer and a network of BASIC I/Os is
made up of either a single (RS-485 half-duplex) or Dual (RS-422 full duplex) shielded twisted
pair (s) of wires whose shields are connected to a signal common conductor. This
communications link should in turn have an overall shield which is isolated from the signals
(including signal ground) and connected to earth or chassis ground at one location. The most
common cause of difficulty experienced by customers in the field is improperly installed
communications wiring.
RS-422:
Advantages:
Easier to implement in software since host driver need not be controlled.
Can be either Multidrop or Repeat
No turn-around delay required.
Disadvantages:
Requires five conductor wire instead of three
RS-485:
Advantages:
Needs only 3 wire conductor
Disadvantages:
Host 485 driver control must be implemented requiring tricky serial port manipulations
Can only be Multidrop
Usually requires turn around delay implementation.