Question: common pins on the Bourns absolute encoder - Page 1
Forum section: electronics / projects

author: teedee
author_rank: Newbie
timestamp: November 29, 2014, 11:14:11 am
content: Hi,I'm reading the datasheet of this encoder:http://www.mouser.com/ds/2/54/ACE-73551.pdfand I'm having a hard time to understand the two C pins. Are they the same pin? The datasheet has a little note next to one of the C pin (the one between pins 2&3) saying: "common" is either on a decoded "select" pin (active low) or at GND. So I suspect "common" here refers to the C pin between pins 2&3. What about the C pin between pins 6&7? I wasn't able to find any info in the datasheet to tell me what this pin is for.Thanks in advance for your help.TD.

author: SL4P
author_rank: Super Contributor
timestamp: November 29, 2014, 10:23:41 pm
content: Connection pin C in most notationis C.If they served different functions, they would be C1 and C2 (or something else).EnjoyEDIT: I guess I have to ask - why not use a simple bi-phase encoder?Cheaper, less pins and no absolute hard position to be worried abut?

author: teedee
author_rank: Newbie
timestamp: November 29, 2014, 11:21:00 pm
content: Thanks SL4P.I wonder why they would need to make another pin C that does exactly the same thing though. I assume these 2 C pins are wired together, aren't they?About not using the bi-phase encoder, it is mainly because of the power saving purpose since with mechanical absolute encoders, I only need to maintain a minimum TTL voltage to detect the changes on the output pins and I don't have to do further computation to scan the bits.

author: SL4P
author_rank: Super Contributor
timestamp: November 30, 2014, 01:31:16 am
content: Yes and Yes.Second identical pin - aids in PCB layout (or some devices to sink more current -not this one I hope!)The bi-phase will use maybe three more lines of code than reading a simple port value - and gives you(almost)infinitely more flexibility in the output. no constraints in range, easy to detect speed, some encoders have on-axis push-button, (usually) smaller footprint, etc.etc... and probably cheaper.Interrupt driven it will respond much more predictably than polling the port bits...I'm not suggesting this is wrong for your application, but I'd invariably be looking at a bi-phase encoder first --- or a linear taper pot on an a/d port (for absolute position if it's that important)...!Tell us more!

author: rs20
author_rank: Super Contributor
timestamp: November 30, 2014, 04:56:13 am
content: Very clever device -- I was wondering why they didn't use standard Grey codes... but if you look at the bit sequences, p8 is the same as p7 but with a simple angular offset, and the same for the rest. This is not true of grey code. Nevertheless, the important property that the subsequent codes different by only a single bit is maintained. As they explain in the datasheet, this allows them to use multiple contactors on the same pattern, rather than a separate pattern per bit.

author: SL4P
author_rank: Super Contributor
timestamp: November 30, 2014, 06:42:19 am
content: I wasn't really paying attention to the code sequence earlier - but the non-continuous output pattern also means you have to dedicate a chunk of memory to a look up table - unless your code specifically swallows each switch position whole.Bourns also indicate this in the datasheet.An index pin once-per-revolution would have been useful too...

author: rs20
author_rank: Super Contributor
timestamp: November 30, 2014, 10:49:45 am
content: Quote from: SL4P on November 30, 2014, 06:42:19 amMemory is a lot cheaper than contactors. For an application that requires absolute positioning, this is an excellent solution. I'm not sure which such applications would particularly benefit from a once-per-revolution pin? You can just derive that off your lookup table.

author: SL4P
author_rank: Super Contributor
timestamp: November 30, 2014, 11:03:57 am
content: Agree.Contactors? Sorry missed the reference.An index pin can help program identify if user has spun the dial - without that little bit extra code to check wrap around.No big deal. As I said above, I like this encoder switch (reminds me of the old 1970's CB radio 40-channel switches!), but if the objective is replacement of a rotary switch / input encoder, I can't see where this particular hard-coded gizmo justifies the extra pins, footprint or code. That's all.Using 8 bits to represent 7 bit values just seems 'wrong' to me! But that's just me.

author: rs20
author_rank: Super Contributor
timestamp: November 30, 2014, 11:10:54 am
content: Oh yes, I misspoke when I said "contactors". Actually not sure what the correct terminology is, but with a gray coded scheme, each pin has its own dedicated coded ring it slides on. With the design in question, the pins are all contacting the same one coded ring, just at different positions around the ring. Those coded rings cost money (and physical space), I presume.Perhaps it's impossible to get away with this with only 7 bits? Fun task, trying to prove that!

author: teedee
author_rank: Newbie
timestamp: November 30, 2014, 04:32:48 pm
content: Thanks all for your feedback.Like rs20 said, the application I'm working on need absolute positioning. Also, it doesn't work on a high revolution speed anyway, maybe one revolution every hour. And I don't need to consistently read the value either, only when it changes value. The thing I need most is reasonable resolution (at least 3 degrees for every step) and power efficiency. So this encoder is the best I could find out there for the purpose. The only thing I don't really like is that the encoder is a bit bulky. I wish they could make something more compact. I guess if someone designs an encoder with similar output mechanism but using stacked discs, instead of a single disc, that will make the encoder smaller.

author: rs20
author_rank: Super Contributor
timestamp: November 30, 2014, 10:18:16 pm
content: Just to be sure, you realise the real meaning of "absolute positioning" is that when this widget is first powered up, it knows exactly where it is straight away? Any old rotary encoder can achieve absolute positioning when coupled with firmware that keeps track of what's happened in the past, and calibrates itself at turn on. (As SL4P has been hinting at).

author: teedee
author_rank: Newbie
timestamp: December 01, 2014, 05:16:06 am
content: Yeah, I know that and it's actually what I needed