Theory of Operation of the Shaq 27722-1 (A23) LED Display Board (4-player score board).Theory of 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. It will cover inputs and behavior of the board.
Inputs
This board accepts the following three control inputs:
- Master Reset (/MR)
- 8-Bit Data (DX0–DX7 – pulled up to +5v/VCC)
- Enable/Strobe for the latches (AX4)
It also accepts +5, common GND, +12 and the +12V GND (both GNDs are connected together).
The incoming 8-bit data is split into two 4-bit words with the low word (DX0–DX3) used for specifying the value to display on a particular LED digit via the ‘4511 (BCD to 7-segment latch/decoder/driver), with DX0, DX1, DX2 and DX3 connected to inputs A, B, C and D (or D1, D2, D3, and D4), respectively. The high word (DX4–DX7) is used for digit selection via the ‘4514 (4 to 16-Line Latch/Decoder/Demultiplexer), with DX4, DX5, DX6 and DX7 connected to inputs A, B, C and D, respectively.
Latches for the corresponding chips are controlled via the AX4 signal, which is directly connected to the 4511’s /LE input and also goes through an inverter whose output is connected to the 4514’s LE.
The /MR signal is connected to the 4511’s /BI signal, which is used to blank the displays whenever the game is in reset.
Outputs
Outputs from the ‘4511 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.
Outputs Q0–Q11 from the ‘4514 go into two 7417s (Hex Buffers with Open-Collector Outputs) and the outputs from the 7417’s are used to drive one of twelve MPS-U45 transistors that power (or enable) each individual digit. Digits are shown one at a time in a multiplexed fashion.
Operation
The display values are programmed by asserting the AX4 signal to enable the latches, setting the data for the digit and its value, and deasserting AX4 which “locks in” the data. 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.
Plain English(?) Version
OK – so the DX0–DX7 signals coming into the board are an 8-bit message, which would normally be considered a single byte of data. But think of the message it as two 4-bit words – the lower 4 bits (DX0–DX3) make up one word and the upper 4 bits (DX4–DX7) make up the other 4-bit word.
Each 4-bit word one can hold a binary value ranging from zero (all 4 bits off or 0000) to fifteen (all 4 bits on or 1111). We will call the lower word VALUE and the upper word DIGIT.
The VALUE Word
This data word is connected to the four data inputs of a 74HC4511 (‘4511), which is a BCD to 7-Segment Decoder. It takes a 4-bit binary value on its inputs (D1, D2, D3, D4 on the schematic) and converts it to a set of signals on its output pins (A–F) that correspond to the LEDs of a 7-segment display that should be turned on in order to display that value. For example, if you place a value of 3 on the inputs (0011), then the A–F outputs will look like (1111001), which would be used to light up segments A, B, C, D and G:
The A–F outputs from the ‘4511 are connected to a set of Darlington transistors (ULN2803) to enhance the drive strength and the outputs from the Darlington array are connected in parallel to the corresponding segments on all of the digits in the display. So how come all of the digits do not show the same value? We are getting to that next!
The DIGIT Word
This word is connected to a 74HC4514 (‘4514), which is a “straight” 4-to-16 Decoder. It takes a 4-bit binary value as input and sets one of the corresponding 16 output pins high. For example, if you place a value of 5 on the inputs (0101), output pin 5 (Y5 on the schematic) would be set high. 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 on and will light up the segments as specified by the output of the ‘4511.
The LED displays pull a decent amount of current and cannot be driven directly by the TTL output from the ‘4514, so a transistor is used. The outputs from the ‘4514 go into a pair of ‘7417 buffers to help increase the drive strength (and possibly act as a layer of protection). 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 ‘4514, hence a specific digit is turned on based on the value of DIGIT word (DX4–DX7). 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!
An example of the data values and the expected results:
| VALUE: DX0 – DX3 | DIGIT: DX4 – DX7 | Effect |
| 0001 | 0011 | Digit 3 should display 1 |
| 1000 | 0001 | Digit 1 should display 8 |
| 0111 | 1100 | What do you think should happen here? |
| 0110 | 0111 | What do you think should happen here? |
The AX4 Signal
These two decoders are not just basic decoders, they are latches. Most functional chips respond directly to their inputs and have no storage or memory – when their inputs change or go away, their outputs respond shortly thereafter. 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.
This is where the AX4 signal comes in. It is connected to both the /LE (active low Latch Enable – an overbar above or slash in front of a signal indicates that it is asserted or active when the signal is low as opposed to high) of the ‘4511 and is also connected to an inverter whose output connects to the LE (active high Latch Enable) of the ‘4514. When AX4 is pulled low, the latches on both chips are enabled and the inputs provided will be “remembered” when AX4 is brought high, so the effects of the last set DIGIT and VALUE values will remain.
The /MR Signal
The /MR signal is a blanking signal that simply turns off all outputs from the ‘4514. It allows the score display to ignore any data on the data lines which may be inconsistent while the game is in reset.
Multiplexing
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. This means that at any one time, only ONE digit is lit up at any time (this technique is called “multiplexing”). But thanks to human persistence of vision, the display will look like all of its digits are on. The benefits of this are that less components are required – 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.