Notifier SDX-551 Invalid Reply

Hi everyone,

I have a Notifier SDX-551 (older CLIP based photoelectric smoke detector) on my 640 demo system that keeps giving me an INVREP (invalid reply) trouble, despite the fact that the detector is polling and appears to be communicating with the control panel. The only other device on this loop is an MMX-1 CLIP monitor module, which works just fine. The panel’s protocol is set for CLIP, so that should not be the issue. This is the only device I have that does this. Does anyone know what’s going on? Thanks again.

If all connections are tight and voltage present time of trouble, replacement needed. Fsp-851 or 951-IV

sometimes autoprogram will fix this type of issue

A brief glance at the documentation indicates the SDX-551 has a thermal detector variant (SDX-551TH). A combination smoke/heat detector must be programmed into the panel as such or you will get WRONG TYPE/INVALID REPLY trouble. Could also be a programming error. Also possible: your panel is too new to poll this legacy device, the device is malfunctioning, or it is making a poor electrical connection.
What panel are you using?

My panel is an NFS-640. It does confuse me as this panel was connected to older CLIP devices (SDX-551’s and BGX-101L’s) from an old AFP-400 system when it was installed in the building it came from, so my SDX-551 SHOULD be working without any issue…could it just be a bad detector head? All of the electrical connections on the loop are tight as well.

I had a peek at the installation manual for the NFS-640, looks like the SDX-551 should be compatible… Check the panel programming. Does it think there’s supposed to be a different device there, like a pull station or module? If programming all checks out, try a different SDX-551 detector and see if it gives the same fault.
If the programming is correct, my guess is the detector may be faulty.

Well, I did have the point type set up for a photoelectric smoke detector, so I’m assuming the head could be bad. I’d have to order another one to see if its a bad head since I only have one of the 551’s at this point. Once I do that I’ll test it out, see what happens. If the issue comes back with a new head, could it be a possible issue with the firmware or panel itself?

I would definitely try a new head first. The SDX-551s are pretty abundant on sites like eBay, probably can be had for under $25. The manual lists the SDX-551 as compatible using the CLIP protocol so that SDX should work unless there is a version of the software that dropped CLIP support entirely (and you’ve already said you’re using an MMX in CLIP protocol so we know this isn’t the case). Definitely let’s try a new sensor first.