The master/slave feature allows two units to be connected to one modem in the field. The modem cable used is split and connected to both units. One unit is set to the Master (ID1) and the other the slave (ID2) however up to 64 slave units may be connected (ID2-65).  THe units that are connected MUST be of the same version.

Here is the steps used in centurion to link to the site units remotely.

1) Click on the Serial ID Number on the main status screen.
2) The window asking Master / Slave will appear.
3) Select which counter you want to talk to.
4) System will link to it.

It should be noted that all of this ONLY works with counters that are generally similar to each other as follows:

Unicorn, Phoenix, Pegasus - V2.80 to V4.5x
Unicorn, Phoenix, Pegasus - V2.39 to V2.4x

Do not use a Master/Slave setup that mixes these two types with direct dial. It will work with the auto-poller, but not with direct dial programming so it should be avoided altogether.