Cryptic DMM Logging? - Page 1
Forum section: electronics / beginners

author: cubemike99
author_rank: Contributor
timestamp: January 29, 2014, 03:17:24 pm
content: This is the output of the IR led on my BM257. I got this waveform using a reverse biased photodiode and a Saleae analyzer. Is there an obvious way that ~15 bytes are encoded in this, possibly in something like manchester code? Initially I thought that the data might be on one of the standard IR carriers, but shouldn't the Logic be able to pick up a 32kHz square wave? Thanks for the help.

author: richard.cs
author_rank: Super Contributor
timestamp: January 29, 2014, 03:20:11 pm
content: Don't dismiss the possibility that it's outputing 7-segment patterns, I have a DMM that does that from its RS232 port, clearly added as an afterthought.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 29, 2014, 04:58:39 pm
content: The kits that are available for the BM25x series comprise of Opto=>RS232, RS232=>USB. The fact that there is an RS232 section in there suggests that the data will be straight-forward 7/8 bit protocol. You need to look at the data on something that will accurately resolve the timing, or find a way of synchronising your logic analyser clock with the data stream.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: January 29, 2014, 05:14:09 pm
content: Quote from: richard.cs on January 29, 2014, 03:20:11 pmI recently bought a new meter and while researching noticed that many of them do that. There's source on github for drivers. An afterthought and maybe the quickest, simplest, cheapest implementation. Weird at first but sufficient at least. Some of them have the IR port external.I want to see a DMM with a built-in LED projector, not sure what color. I think eInk would be nice too.

author: cubemike99
author_rank: Contributor
timestamp: January 29, 2014, 05:21:54 pm
content: Quote from: Andy Watson on January 29, 2014, 04:58:39 pmI actually do have another capture that looks a lot better. I got this using an IR receiver (probably 32 kHz). The thing is, this capture is not easily repeatable. When data is being transmitted from the meter, it's sent in groups of 8 bursts. Using just a photodiode, these 8 bursts are identical but, as you can see from the previous picture, they look very weird. Using the receiver, they the pulsewidths are much more even, but the bursts are quite different from each other. Here is one of the more complex bursts I saw.Interestingly, the three equal spikes at the beginning are in both bursts.

author: sync
author_rank: Frequent Contributor
timestamp: January 29, 2014, 05:27:22 pm
content: It very likely that the IR is not 32kHz modulated and that it's just RS232 encoded. Use your photo diode and do RS232 decoding with your logic analyzer.btw:http://www.brymen.com.tw/product-html/software-download/Protocols/

author: cubemike99
author_rank: Contributor
timestamp: January 29, 2014, 09:49:18 pm
content: I did some more playing around with the bias resistor values, and I can get a repeatable, reasonable waveform for a given display state with a plain photodiode. The issue, though, is getting getting signal exactly as it is sent by the IR led to my cheap logic analyzer. Slightly different circuits produce slightly different waveforms, and I can't probe the IR led directly since my analyzer's hi threshold is below the led voltage. I have no way of knowing if the data that I read is what's really there. I'll keep playing around with my current setup and see if that gets me anywhere.

author: cubemike99
author_rank: Contributor
timestamp: January 30, 2014, 04:07:39 pm
content: I'm now getting very repeatable waveforms using the photodiode and my analyzer is even able to consistently read the rs232 data! The problem now is that either the meter or the analyzer has bad timing: I said before that every "packet" in the 8 "packet" burst is identical, though now that seems not the case. There are very slight variations between the packets in a single burst: one or two rs232 characters may be different (FF vs F8) or a character is missing. I have two theories: either the meter takes readings much faster than it displays them and always send the current actual value, which due to PS noise, etc, causes the reading to fluctuate slightly, or the photodiode is not set up correctly to catch such brief pulses (the meter transmits at ~18000 baud). Until I can somehow get the data to be more consistent, there's too much variation to turn the data into a reading.I had the idea today to use an op amp and voltage divider as a crude comparator to try and measure the IR led voltage directly to know for certain what a good transmission should look like. I think after probing the led directly, I'll keep the op amp to buffer the photodiode signal, as before it was going straight into the analyzer.