In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Coriolis wrote:

There are two different types of busses, the first is active driven, where a chip supplies the high voltage and low voltage. The second is open collector, in this system the voltage isnt supplied by any one chip but by a resitor tied between Vdd and the line. Any chip communicating on the line either leaves it alone (hi impedance) or sinks the line thereby pulling it down to gnd. These two states are what causes the line to be either high or low. The reason for open collector lines is when there is the possibility of more than one chip trying to use the same bus, if it were an actively driven bus line, and one chip is driving it high at the same time another chip is driving it active low, this creates a short circuit (low impedance path between Vdd and Gnd). Since this short is occuring internally inside two chips, one or both of the chips will burn out, something you definitely dont want to do.

This explains the reason for using open collector busses, do you need more detail on how its implemented in an SX program as well?

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