I-O Display Systems Basic I/O Product manual RS-422, RS-485

Setup & Installation 2

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.

2-5(Vol.1)