Arduino UNO 5v.
The voltage value on pins 13 & 14 with GND from pin 8 of SN74LS247 is 9.1v.
Now measure the voltage of any segment in relation to the GND.
I told you wrong, I was using the wrong pin on SN74LS247. It's between 4v and 5v when GND is on SN74LS247 and and POS is on pin 13 & 14.
It's still about 2v when POS is on pin 13 & 14 and GND is on any lighted segment.
Pretty close. I’m not sure what flat ribbon and j2 are though?
I’m using an Arduino Uno R3.
Yes and the flat ribbon is delivering the negative GND signals to DISP2. The other end of the ribbon is connected to another pcb in the bingo console. The other two SN74LS247s driving DISP2 are also on that other pcb
When a pin is configured as an INPUT with pinMode(), and read with digitalRead(), the Arduino (ATmega) will report HIGH if:
- a voltage greater than 3.0V is present at the pin (5V boards)
- a voltage greater than 2.0V volts is present at the pin (3.3V boards)
REF: constants - Arduino Reference
Try this circuit.
Use an arduino port as an input without PULL_UP.
I’m going to try this this morning.
Supposing it works, will I need to have two resistors between every pin and every pin on the arduino? That’s 28 resistors. I don’t have that many!
Yes, you are right.
Wouldn’t I really only need 14 5k resistors and one 10k since there’s one ground and 14 pins on DISP2?
And that of course, is the only sensible answer!
Especially as there are resistors between the 74LS247 segment outputs and the LED, so looking at the voltages on the LED itself is the wrong place.
No, this is a resistive divider, so you need 14 resistors of 5K and 14 resistors of 10K.
Did you test with a port?
And did it work?
PS: Is it not possible to retrieve the data as suggested by Mr. Wawa?
It's the best way to do it.
Look, I'm calling "XY Problem" on this one.
It was all very well until #9 when the real nature of the circuit was published.
We do encounter from time to time, someone who is trying to read from a display multiplexed with a special-purpose chip such as a Voltmeter where the single chip drives both digits and segments and even includes the current-limiting elements. You have no choice but to make do with detecting what voltages you can on the display.
We now know this is completely the opposite situation. We have BCD to 7 segment encoder chips driving the display. You want to know what the binary value corresponding to the display digit is? It's right there in front of you, no need to decode anything! The only slight problem is that it is TTL levels rather than 5 V logic, but this is relatively trivial and in general even a 5 V Arduino will cope. Just connect it to the inputs of the 75LS247. Look up the datasheet to find them 
And by the way. 
It's not multiplexed! 
If the display driver is a 74LS247 the maximum voltage of the system is 5V so there are no worries there.
That is because it is not a schematic, it is simply a list of connectors to the board.
Nooooooooooo!
Yes back up here.
Number one is to connect the ground of your display to the ground of the Arduino. You know that pin 8 of the 74LS247 is ground so use your meter in continuity mode to find out where that is on a conveniently connector you can solder to. Then make your measurements again. There are no negative voltages involved like I said and connecting a common anode to ground only worked because your two boards are floating with respect to each other.
However, Wawa has it right in reply #16, just measure the four input pins on the two 74LS247 chips. That is just 8 connections as opposed to 14 trying to measure the segments.
You sure? Because there is one 74LS247 driving two 7 segment displays. That suggests multiplexing to me.
@wawa pedal @Grumpy_Mike I like this suggestion!
There are 4 SN74LS247s involved here. Two (pictured above) drive DISP1, and the other two are on a different PCB (not pictured) and their signals come into THIS PCB via the ribbon cable.
I have located the two SN74LS247 chips driving DISP2. As you said there are 8 total connections off those two chips. Help me with the Arduino connections: I'd connect these 8 connections to 8 Arduino pins. So far so good?
Then I only need to connect one ground from one of the SN74LS247s to Arduino GND or both SN74LS247s grounds to Arduino GND? Or, if I've located another GND on the PCB, can I connect that to Arduino GND? It's the grounds that are messing me up... #n00b
Given the correct color and given the above images and test results, this has to be a LTD-6640E common cathode display as shown in the datasheet in the link you've provided ... see circuit C on page 3.
EDIT: For convenience:
No you are misunderstanding. You have 4 INPUT pins and 8 OUTPUT pins from this chip. Please look at the data sheet.
No that is not what the picture of the board shows. I may have missed this but why have you not mentioned two boards before? Just start with one board.
Once you find any ground it will be the same on both boards and connect that ground to ANY ground on the Arduino board. Disconnect all other wiring then make your measurements again, there will be no negative voltages.
Forget about voltage dividers if you are using a 5V Arduino. What one are you using?