Scantronic 9651: Cracked?

The Scantronic alarm panel + keypad I mentioned in my previous post have now arrived! I have hooked up power and a backup battery and the kit seems to work great - right away I got down to investigating the mysteries of the keypad protocol that I introduced in my last post on this topic.

Circuit board

So there it is, the main control unit of the Scantronic 9651. Right now I’m not exactly powering it up correctly; in an effort to avoid killing myself I’ve opted not to use the inbuilt magnetic transformer (240 VAC to 20 VAC - yes VAC again - The base unit does run on AC). I have decided to use my bench power supply combined with a 2.4Ah sealed lead-acid battery to power the panel for now, and it seems to work fine despite the “MAINS FAIL” messages displayed occasionally.

Test setup

As you can see above, my initial testing of the panel involves my handy little DSO Nano. This is an excellent little device (actually one of the earlier versions of the now numerous handheld digital oscilloscopes produced by Seeed Studio) and it allows me to see the digital waveforms produced by the panel and the keypad in real time, as well as recording the waveforms for viewing later on a PC or Mac.

Testing with a DSO Nano Scope

I started by recording some of the frequently occurring signals produced on the “DATA” line and trying to decipher them. It looks to me like a PWM (Pulse Width Modulation) waveform, of which the clock will of course be provided on the “CLK” pin.

Right there I start to get a bit stumped - the DSO I have only lets me display one channel (I.e. one waveform) at a time, and I need both the clock and the PWM signal to start to really understand what is going on. Basically - I’m looking for a logic analyser. These are cool little devices that let you analyse which lines of a logical system are high or low (1 or 0) and record them, show them alongside one another, track their changes and much more. The proper lab-based ones are, as always with proper lab-based things, expensive. The knock-off USB based ones are cheap, but the software looks terrible. I need to chat to some people that know about these things before I can make a purchase and not run the risk of buying something completely and utterly pants or wasting my money on some 50 kilogram box of bolts from a 1990s electronics lab.

I’ll keep looking.

Damien Walsh

I'm a 27 year old Computer Science graduate working in software & electronics development. I like electronics, programming, a variety of weird music and I sort of have a thing for motorbikes too.

Manchester, UK