Honeywell VISTA 20P vs VISTA 21iP

This video illustrates the similarities and the differences between the Vista-20P and the Vista-21iP:

These two panels are very similar. Both the VISTA 21iP and the VISTA 20P support up to 48 total zones of protection. Both have 8 hardwired zones built into their control panels. Both can use what is known as zone doubling in order to use more than the standard 8 zones without needing a zone expander. Zone doubling turns the 8 hardwired zone into 15. Also, both panels can be retro-fitted with wireless expansion modules to expand their total number of zones.

These panels are really only different with regard to the communicators they use to achieve either IP communication or GSM communication. The first way in which they're different is the VISTA 21iP's built-in IP communicator. Out of the box, the 21iP is ready to be monitored over the internet. To accomplish this with the 20P, it must be first outfitted with the 7847i AlarmNet communicator, or the IGSMV4G for dual path communication. If you want dual path monitoring with a 21iP, you can simply add the VISTA-GSM4G

If you are interested in cellular communication with the VISTA 20P, you will need to install either the GSMV4G (which requires a transformer for power) or the GSMX4G (which uses panel power). The ViISTA 21iP requires a VISTA-GSM4G for cellular alarm monitoring.