Our company is a bit new to this industry, so looking for some advice. I added some 2-wire smoke detectors to an existing system (Ademco Vista 128FBP) and they do not work. They power up with the red LED on solid. The existing detectors work fine on the same loop. I looked at the existing detectors when I ordered the new ones, and thought I got the same thing. They look the same. I installed Honeywell 5193SD detectors. Doing some research, it looks like they may not be compatible, but I’m not sure. The install manual calls for various models of System Sensor detectors, which look identical, but perhaps are not the same. Any insights would be greatly appreciated.
First, the 5193’s are an addressable detector. Are you connecting them directly to a 2-wire zone or the polling loop?
I am connecting them to zone 1 on the Vista panel - which is a smoke detector zone by default I believe. They are not connected to the polling loop.
There’s your problem. The smokes won’t work on a conventional zone.
The 5193 addressable smoke detector cannot function on the hardwired zone.
According to the 128FBP manual I found on line these are the compatible smoke detectors that can work on Zone 1.
[attachment=1]128FBP_hardwired_SD.jpg[/attachment]
According to the same manual these smoke addressable detectors are compatible with the 128FBP polling loop.
I have not found any literature saying the 5193SD is compatible with the 128FBP.
[attachment=0]128FBP_addressable_SD.jpg[/attachment]
Thank you all very much. I went into this thinking it was a simple add on to an existing system. Lesson learned. I can get the correct detectors and install them easy enough. I think the mounting base for the System Sensor detectors they have in place is the same one as in the 5193 - so maybe I’ll get lucky and just plug the replacements in.
I personally am primarily a phone tech, and I am learning my way into this field. I really appreciate the assist. I’m sure I’ll be back often! 
And the 2W-B’s are a lot less expensive! Do yourself a favor too and after you get everything installed - take one detector off at a time and verify you get a trouble on each, then test, test, test.