What is the keybus on a DSC PC1616, DSC PC1832, and DSC PC1864?

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In a lot of literature that talks about the DSC PowerSeries panels, you will see the word 'keybus' but you may not really get a good explanation as to what that means or what significance it has. The keybus is a 4-wire connection that is used for keypads and other modules. On the main circuit board it will be labeled using the four standard alarm wire colors: Red, Black, Yellow, and Green.

Red is the positive power terminal to the circuit. It is connected internally on the panel to the Auxiliary + terminal and the combined current cannot exceed 700mA. Black is the common terminal or the - 12 volt direct current. You will want to leave a 10 percent buffer as well to ensure that you do not exceed the current draw limit. If you do exceed it the system will not function. You can reference each device's installation manual to see what the maximum current draw is. If the combined current draw is going to be too great you will need to add an additional power supply board such as the PC5204.

The Yellow and Green terminals are the data out and data in terminals. This is how the main alarm circuit board and other connected devices communicate back and forth with each other. 

The keybus is supervised as well. This means that anything connected to the keybus is detected by the main board and then saved. If at any point the main board loses communication with a module or keypad on the keybus you will get a supervision trouble condition. You can then go and find what the issue is and resolve it. Or if you purposefully took something off of the system you will need to reset the keybus supervision in programming using section 902. After about a minute, to give the system time to detect everything on the keybus, you can enter section 903 to confirm that the system sees everything that you expect it to see.

DSC recommends that the minimum wire gauge to use is 22 while the maximum is 18 gauge. They also state that shielded wire should not be used for the keybus. Modules on the keybus can all be on home runs to the panel or connected in a parallel series or T-tapped. Or any combination of those wiring options. But the wire length to any one module cannot be more than 1000 feet and the total keybus wiring should not be more than 3000 feet.