{"id":672,"date":"2022-02-16T23:07:11","date_gmt":"2022-02-16T19:07:11","guid":{"rendered":"https:\/\/www.jrtwine.com\/blog\/?p=672"},"modified":"2024-02-27T23:08:01","modified_gmt":"2024-02-27T19:08:01","slug":"theory-of-operation-of-the-shaq-27722-1-a23-led-display-board","status":"publish","type":"post","link":"https:\/\/jrtwine.com\/blog\/theory-of-operation-of-the-shaq-27722-1-a23-led-display-board\/","title":{"rendered":"Theory of Operation of the Shaq 27722-1 (A23) LED Display Board"},"content":{"rendered":"

Theory of Operation of the Shaq 27722-1 (A23) LED Display Board (4-player score board).Theory of\u00a0 Operation of the Shaq 27722-1 (A23)<\/b> LED Display Board (4-player score board).<\/b><\/h1>\n

Theory of Operation<\/b><\/h2>\n

This is a brief technical introduction to the operation of the 27722-1 (A23) board.\u00a0 It will cover inputs and behavior of the board.<\/span><\/p>\n

Inputs<\/span><\/h2>\n

This board accepts the following three control inputs:<\/span><\/p>\n

    \n
  1. Master Reset (<\/span>\/MR<\/b>)<\/span><\/li>\n
  2. 8-Bit Data (<\/span>DX0<\/b>–<\/span>DX7<\/b> – pulled up to <\/span>+5v<\/b>\/<\/span>VCC<\/b>)<\/span><\/li>\n
  3. Enable\/Strobe for the latches (<\/span>AX4<\/b>)<\/span><\/li>\n<\/ol>\n

    It also accepts <\/span>+5<\/b>, common <\/span>GND<\/b>, <\/span>+12<\/b> and the <\/span>+12V GND <\/b>(both <\/span>GND<\/b>s are connected together).\u00a0<\/span><\/p>\n

    The incoming 8-bit data is split into two 4-bit words with the <\/span>low<\/span><\/i> word (<\/span>DX0<\/b>–<\/span>DX3<\/b>) used for specifying the value to display on a particular LED digit via the \u20184511 (<\/span>BCD to 7-segment latch\/decoder\/driver<\/span><\/i>), with <\/span>DX0<\/b>, <\/span>DX1<\/b>, <\/span>DX2<\/b> and <\/span>DX3<\/b> connected to inputs <\/span>A<\/b>, <\/span>B<\/b>, <\/span>C<\/b> and <\/span>D<\/b> (or <\/span>D1<\/b>, <\/span>D2<\/b>, <\/span>D3<\/b>, and <\/span>D4<\/b>), respectively.\u00a0 The <\/span>high<\/span><\/i> word (<\/span>DX4<\/b>–<\/span>DX7<\/b>) is used for digit selection via the \u20184514 (<\/span>4 to 16-Line Latch\/Decoder\/Demultiplexer<\/span><\/i>), with <\/span>DX4<\/b>, <\/span>DX5<\/b>, <\/span>DX6<\/b> and <\/span>DX7<\/b> connected to inputs <\/span>A<\/b>, <\/span>B<\/b>, <\/span>C<\/b> and <\/span>D<\/b>, respectively.\u00a0\u00a0<\/span><\/p>\n

    Latches for the corresponding chips are controlled via the <\/span>AX4<\/b> signal, which is directly connected to the 4511\u2019s <\/span>\/LE<\/b> input and also goes through an inverter whose output is connected to the 4514\u2019s <\/span>LE<\/b>.\u00a0\u00a0<\/span><\/p>\n

    The <\/span>\/MR<\/b> signal is connected to the 4511\u2019s <\/span>\/BI<\/b> signal, which is used to blank the displays whenever the game is in reset.<\/span><\/p>\n

    <\/p>\n

    Outputs<\/span><\/h2>\n

    Outputs from the \u20184511 go into a ULN2803 (8 channel Darlington array) whose corresponding outputs are connected to specific segments of all of the LED digits via current limiting resistors.\u00a0\u00a0<\/span><\/p>\n

    Outputs <\/span>Q0<\/b>–<\/span>Q11<\/b> from the \u20184514 go into two 7417s (Hex Buffers with Open-Collector Outputs) and the outputs from the 7417\u2019s are used to drive one of twelve MPS-U45 transistors that power (or <\/span>enable<\/span><\/i>) each individual digit. Digits are shown one at a time in a multiplexed fashion.<\/span><\/p>\n

    Operation<\/span><\/h2>\n

    The display values are programmed by asserting the <\/span>AX4<\/b> signal to enable the latches, setting the data for the digit and its value, and deasserting <\/span>AX4<\/b> which \u201clocks in\u201d the data.\u00a0 The programmed digit and value will remain allowing the game to perform other tasks until it is ready to program the next digit and value.<\/span><\/p>\n

    Plain English(?) Version<\/b><\/h2>\n

    OK – so the <\/span>DX0<\/b>–<\/span>DX7<\/b> signals coming into the board are an 8-bit <\/span>message<\/span><\/i>, which would normally be considered a single <\/span>byte<\/span><\/i> of data.\u00a0 But think of the message it as <\/span>two<\/i><\/b> 4-bit words<\/span><\/i> \u2013 the lower 4 bits (<\/span>DX0<\/b>–<\/span>DX3<\/b>) make up one word and the upper 4 bits (<\/span>DX4<\/b>–<\/span>DX7<\/b>) make up the other 4-bit word.<\/span><\/p>\n

    Each 4-bit word one can hold a binary value ranging from zero (all 4 bits off or <\/span>0000<\/b>) to fifteen (all 4 bits on or <\/span>1111<\/b>).\u00a0 We will call the lower word <\/span>VALUE<\/b> and the upper word <\/span>DIGIT<\/b>.\u00a0<\/span><\/p>\n

    The VALUE Word<\/b><\/h3>\n

    This data word is connected to the four data inputs of a 74HC4511 (\u20184511), which is a <\/span>BCD to 7-Segment Decoder<\/b>.\u00a0 It takes a 4-bit binary value on its inputs (<\/span>D1<\/b>, <\/span>D2<\/b>, <\/span>D3<\/b>, <\/span>D4<\/b> on the schematic) and converts it to a set of signals on its output pins (<\/span>A<\/b>–<\/span>F<\/b>) that correspond to the LEDs of a 7-segment display that should be turned on in order to display that value.\u00a0 For example, if you place a value of <\/span>3<\/b> on the inputs (<\/span>0011<\/b>), then the <\/span>A<\/b>–<\/span>F<\/b> outputs will look like (<\/span>1111001<\/b>), which would be used to light up segments <\/span>A<\/b>, <\/span>B<\/b>, <\/span>C<\/b>, <\/span>D<\/b> and <\/span>G<\/b>:<\/span><\/p>\n

    \u00a0<\/span><\/p>\n

    The <\/span>A<\/b>–<\/span>F<\/b> outputs from the \u20184511 are connected to a set of Darlington transistors (ULN2803) to enhance the drive strength and the outputs from the Darlington array are connected in <\/span>parallel<\/span><\/i> to the corresponding segments on <\/span>all<\/span><\/i> of the digits in the display.\u00a0 So how come all of the digits do not show the same value?\u00a0 We are getting to that next!\u00a0<\/span><\/p>\n

    The DIGIT Word<\/b><\/h3>\n

    This word is connected to a 74HC4514 (\u20184514), which is a \u201cstraight\u201d <\/span>4-to-16 Decoder<\/b>.\u00a0 It takes a 4-bit binary value as input and sets one of the corresponding 16 output pins high.\u00a0 For example, if you place a value of <\/span>5<\/b> on the inputs (<\/span>0101<\/b>), output pin <\/span>5<\/b> (<\/span>Y5<\/b> on the schematic) would be set high.\u00a0 These outputs (only twelve are used) are each connected to a corresponding digit and are used to select which digit of the display will be turned <\/span>on<\/span><\/i> and will light up the segments as specified by the output of the \u20184511.<\/span><\/p>\n

    The LED displays pull a decent amount of current and cannot be driven directly by the TTL output from the \u20184514, so a transistor is used.\u00a0 The outputs from the \u20184514 go into a pair of \u20187417 buffers to help increase the drive strength (and possibly act as a layer of protection).\u00a0 The outputs from the buffers are used to drive the transistors which act as switches to turn each digit on and off based on the output of the \u20184514, hence a specific digit is turned on based on the value of <\/span>DIGIT<\/b> word (<\/span>DX4<\/b>–<\/span>DX7<\/b>).\u00a0 If you are ever bench testing one of these boards, be aware that keeping a single digit left on for too long will cause the associated transistor to heat up!\u00a0<\/span><\/p>\n

    An example of the data values and the expected results:<\/span><\/p>\n\n\n\n\n\n\n\n
    VALUE: DX0 – DX3<\/b><\/td>\nDIGIT: DX4 \u2013 DX7<\/b><\/td>\nEffect<\/b><\/td>\n<\/tr>\n
    0001<\/b><\/td>\n0011<\/b><\/td>\nDigit <\/span>3<\/b> should display <\/span>1<\/b><\/td>\n<\/tr>\n
    1000<\/b><\/td>\n0001<\/b><\/td>\nDigit <\/span>1<\/b> should display <\/span>8<\/b><\/td>\n<\/tr>\n
    0111<\/b><\/td>\n1100<\/b><\/td>\nWhat do you think should happen here?<\/span><\/i><\/td>\n<\/tr>\n
    0110<\/b><\/td>\n0111<\/b><\/td>\nWhat do you think should happen here?<\/span><\/i><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

    The AX4 Signal<\/b><\/h3>\n

    These two decoders are not just basic decoders, they are <\/span>latches<\/span><\/i>.\u00a0 Most functional chips respond directly to their inputs and have no storage or memory \u2013 when their inputs change or go away, their outputs respond shortly thereafter.\u00a0 Latches allow for a chip to continue to behave as if its inputs were still present and stable even if they have been removed or are changing.<\/span><\/p>\n

    This is where the <\/span>AX4<\/b> signal comes in.\u00a0 It is connected to both the <\/span>\/LE<\/b> (<\/span>active low <\/span><\/i>L<\/b>atch <\/span>E<\/b>nable \u2013 an overbar above or slash in front of a signal indicates that it is <\/span>asserted<\/span><\/i> or <\/span>active<\/span><\/i> when the signal is low<\/strong> as opposed to high<\/strong>) of the \u20184511 and is also connected to an inverter whose output connects to the <\/span>LE<\/b> (active <\/span>high<\/span><\/i> Latch Enable) of the \u20184514.\u00a0 When <\/span>AX4<\/b> is pulled low, the latches on <\/span>both<\/span><\/i> chips are enabled and the inputs provided will be \u201cremembered\u201d when <\/span>AX4<\/b> is brought high, so the effects of the last set <\/span>DIGIT<\/b> and <\/span>VALUE<\/b> values will remain.<\/span><\/p>\n

    The \/MR Signal<\/b><\/h3>\n

    The <\/span>\/MR<\/b> signal is a blanking signal that simply <\/span>turns off<\/span><\/i> all outputs from the \u20184514.\u00a0 It allows the score display to ignore any data on the data lines which may be inconsistent while the game is in reset.\u00a0<\/span><\/p>\n

    Multiplexing<\/b><\/h3>\n

    The display works by having the game set the value for the first digit, then the second, then the third, etc. until the last digit is set and then it starts over again with digit one.\u00a0 This means that at any one time, only <\/span>ONE<\/i><\/b> digit is lit up at any time (this technique is called \u201cmultiplexing\u201d).\u00a0 But thanks to human <\/span>persistence of vision<\/span><\/i>, the display will look like all of its digits are on.\u00a0 The benefits of this are that less components are required \u2013 each digit shares the same 7-segment decoder, and the total power required is reduced because only one digit is ever powered at any time.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"

    Theory of Operation of the Shaq 27722-1 (A23) LED Display Board (4-player score board).Theory of\u00a0 Operation of the Shaq 27722-1 (A23) LED Display Board (4-player score board). Theory of Operation This is a brief technical introduction to the operation of the 27722-1 (A23) board.\u00a0 It will cover inputs and behavior of the board. Inputs This […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3,101],"tags":[20,21],"class_list":["post-672","post","type-post","status-publish","format-standard","hentry","category-repair-logs","category-gottlieb","tag-arcade","tag-repair"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t