Image Map Image Map
Page 3 of 3 FirstFirst 123
Results 21 to 29 of 29

Thread: Anyone good with logic gates and BJT Transistors??

  1. #21
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,010

    Default

    I have also had a look at the two matrix organisations for the two keypad and I don't think you can use a simple lookup table in the form of an EPROM, PLD etc.

    If using a micro controller you will also have to ascertain the scan frequency of the control board and ensure that your interface solution can work well within this. You will also have to "roll your own" keypad de-bounce logic etc.

    Not sure in the US what the prevailing Standards and Regulations would be for elecromagnetic susceptibility etc. What you don't want to happen is for everything to work OK in your office and then to fail when you install it in a factory or when someone decides to use their mobile phone when purchasing a drink (or whatever your vending machines dispense).

    You will also have to consider using the watchdog timer just in case the micro controller crashes. If it doesn't recover, presumably your vending machine is useless!

    Dave

  2. #22
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,185

    Default

    Quote Originally Posted by daver2 View Post
    I have also had a look at the two matrix organisations for the two keypad and I don't think you can use a simple lookup table in the form of an EPROM, PLD etc.
    Why not? Unless I'm missing something the old keyboard can generate part of the 12-bit address and the 'new' scan lines the rest, and one of four output bits goes to the 'input' (this assumes the left numbers are the scan lines).

    No?
    Last edited by MikeS; October 10th, 2019 at 09:36 AM.

  3. #23

    Default

    MikeS: A simple PLD solution *MIGHT* be possible but first one needs to know which of the output keypad lines generate the scans and which are detectors. After that you can check whether they can be used for scan lines on a "naked" input keypad with PLD translation.

    A potential problem with a microcontroller solution is the two or three clock domains. Either performing it's own scan or tapping into existing input scan logic is fairly straightforward so long as it retains the current scan status. However, unless it uses twelve relay-style outputs there is a potential problem. Detecting the output-side scan and generating a response is fairly straightforward *BUT* there will be a delay. Unless the downstream circuit is carefully checked, there remains a question of where during the scan interval a "closure" detected. Early detection during the delay period creates a major problem.

    Assuming TTL-level inputs and outputs, I still believe a single PLD (possibly CPLD) device can perform a reliable low-cost translation. A "naked" input keypad could us the output scan lines as it's input multiplex triggers.

    The PLD would have twelve SR flip-flops holding the last scanned state of the input keys which can then be used to generate immediate responses to the output scan requests. Each flip-flop is SET on multiplex+closure and RESET on multiplex+open. This way they retain the last scanned input value. Appropriate outputs (likely open-drain) are generated based on the output scan signals. Debounce logic should already be incorporated into the existing output circuit.

  4. #24
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,010

    Default

    MikeS,

    Have you got a concept sketch for your proposal?

    I think I may have chosen the other set of numbers for the scan lines...

    Dave

  5. #25
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,185

    Default

    [QUOTE=WSM;590241]MikeS: A simple PLD solution *MIGHT* be possible but first one needs to know which of the output keypad lines generate the scans and which are detectors. [quote]

    Absolutely; we need to at least know which is which and the voltage levels involved.
    The PLD would have twelve SR flip-flops holding the last scanned state of the input keys which can then be used to generate immediate responses to the output scan requests. Each flip-flop is SET on multiplex+closure and RESET on multiplex+open. This way they retain the last scanned input value. Appropriate outputs (likely open-drain) are generated based on the output scan signals. Debounce logic should already be incorporated into the existing output circuit.
    Sounds like we're on the same track, although the EPROM wouldn't (need to?) latch the keypress values.

    Either way, I think it would be trivial to implement on a cheap single chip once you've worked out the truth table, without having to worry about clock synchronization, spurious resets, etc.

    But it would be interesting to implement on a BASIC Stamp with relays etc.; definitely more "fun"...

  6. #26
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    32,259
    Blog Entries
    18

    Default

    And now you understand what I posted what I did about getting details before offering suggestions.

    Been caught in that trap before--it gets embarrassing and expensive.

  7. #27

    Default

    Quote Originally Posted by Chuck(G) View Post
    And now you understand what I posted what I did about getting details before offering suggestions.

    Been caught in that trap before--it gets embarrassing and expensive.
    But you can walk out! ��

  8. #28
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    32,259
    Blog Entries
    18

    Default

    No, I specifically requested more information. Scan patterns, voltage polarities, etc. would be useful.

    I try not to go into things blindly. You're welcome to, however.

  9. #29

    Default

    Quote Originally Posted by Chuck(G) View Post
    No, I specifically requested more information. Scan patterns, voltage polarities, etc. would be useful.

    I try not to go into things blindly. You're welcome to, however.
    No suspicious minds round here today...

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •