The APX 7500 can support up to four control heads.
Any configuration of two or more control heads requires all control heads to be remote mount, as having one of the control heads mated directly to the transceiver (dash mount) means that the CAN connector(s) for additional heads would be otherwise inaccessible.
You can connect up to two control heads directly to the transceiver, or you can daisy chain heads to each other; e.g. you have one head connected to the transceiver, with an additional head connected to the first head, for up to four total control heads. It all depends on the layout and how far from the transceiver each head would be.
The goal is to achieve the shortest possible aggregate length of cabling. In other words, if one head would be mounted close to the transceiver, with the second head mounted at some distance, then plug them both into the transceiver. However if both heads would be mounted at some distance from the transceiver, but close to one another, than it would be more ideal to connect one head to the transceiver, with the second head connected to the first head.
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