Should I unplug my 15P before replacing a keypad?

Yes. Always power down the alarm system before adding powered devices to it. This is global logic that you will find with computers and/or any electronics. Power down when installing devices that will draw from the same power supply. In order to power down the Honeywell VISTA-15P you need to remove the AC transformer and DC backup battery. First unplug a single lead from the backup battery. This generally sits in the bottom of metal cabinet that houses the alarm panel. If its an older battery sometimes there may be corrosion that makes it difficult to pull the lead off. Use a set of needle-nose pliers to squeeze the female spade and open it up a bit. Then remove the lead. Once the battery is disconnected we want to remove the transformer from wall power.

The transformer is a beige plastic power block (also called a "wall wort") that is plugged into an AC wall receptacle. Generally it is screwed into the plate to prevent the heavy block from falling out. Even though at first glance tracing this wire may seem to be leading into your wall cavity the transformer is plugged into an outlet on the exterior of the wall. Track it down and unplug it at the wall outlet. If you cannot find the transformer you can disconnect the wires at the panel on terminals 1 and 2. Keep in mind, this is not recommended and you are dealing with live power. It is low voltage but it can damage the board if it shorts on any of the circuitry.

After the panel is powered down completely you can land your new keypad connections. Keypads require 4 wires: positive power (red), negative power (black), data in (green), data out (yellow or white). On the keypad there may be icons that just show + and - for power connections. These are straightforward. + is positive power and should connect to the ECP 12VDC power output on terminal 5. This is usually a red wire. The - symbol is your ground and should be a black wire connected to terminal 4. This is the panel's ECP ground. Next you have an upward facing triangle with a "G" beside it. This is the data input on terminal 6 on the panel. The downward facing triangle with a "Y" is the data output on terminal 7. The G and Y stand for green and yellow respectively. These are common color standards used in 4-wire cables when connecting keypads.

When all 4 connections have been tightened down on the keypad and the panel please check for any shorts across the terminals on either end. Assuming there are no shorts just power up the panel by connecting your transformer first. Then connect the battery lead. Make sure to screw the transformer back into the wall plate to prevent it from falling out later.