WHAT DO THE LEDs MEAN ON A SYSTEM 7 MPU BOARD?

 

For the most part, if you run System 6 Flipper and Game code, you can use the top two LEDs as

System 6 indicators, see:  System6 diags on System 7 MPU

 

If you run System 7 code, you need to know how to interpret the LEDs,

especially if the 7-segment on-board display is faulty (or the 74LS47 driver chip).

 

Let's look at the normal error indications from LEDs on a System 7 board:

 

System 7 MPU Board Self Test (Pressing the Diagnostic Button)

0 - Test Passed
1 - IC13 RAM Faulty
2 - IC16 RAM Faulty
3 - IC17 ROM 2 Faulty
4 - IC17 ROM 2 Faulty
5 - IC20 ROM I Faulty
6 - IC14 Game ROM 1 Faulty
7 - IC26 Game ROM 0 Faulty
8 - IC19 CMOS RAM or memory protect circuit faulty
9 - Coin-door closed, memory protect circuit faulty, or IC19 CMOS RAM Faulty.

I posted the following as a “TECH TIP” on RGP (rec.games.pinall  user: firepower)

Did some editing into HTML, but the content is basically the same.

 

This might be important to you when troubleshooting System 7 MPU boards.

I bought a faulty System 7 board and was trying to fix it.  At one  point the 7-segment

display stopped working, and began to show garbage characters.

So I added the 2 diagnostic LEDs for the Leon test ROM, as you might do...

 

But it was not that simple in System 7 land.   The information is displayed in a strange way.

 

On system 4-6 MPU boards, the LED cathodes are grounded and the IC2
(8T28 Tranceiver) raises a pin high to light the LED.  OK so far.

 

Then Williams changed the way the LEDs are implemented on System 7, probably due to

using the 7447 display driver and a 7 segment LED display.  

 

They wanted the display to be blank when no LEDs were lit. 

They also wanted a “0” to be displayed when the top two LEDs were lit. 

 

On System 7 the LEDs are kind of "reversed".   The LED anodes are always at +5v through the resistors,

and the LEDs will light when an input line to the 7447 is held low.  We are in backwards world.

 

In my tests, the logic turns OFF the LED for the number the display is indicating and all the other LEDs will stay lit.  

Not how I would design a binary LED indication.  It's a good thing the 7-Segment display usually works!

 

If you are running the diagnostics from the Williams ROMs,

or Leon's Test ROM and you want to make sense of the LEDs read on.  

It's probably easiest just to try and fix the 7-segment display!

Maybe replace a 7400 at IC33 or look closely at the signals at PIA IC18, as the 7447s don't go wrong that often.   

Mine was OK, it turned out to be a broken Address Line (A0) to the PIA IC18.

 

The 7-Segment display can be replaced with a KINGBRIGHT  SA03-12HDB  LED  0.3"  RED  DISPLAY.

 

For me this was a step on the journey:
I added 4 x LEDs and 4 x 150 ohm resistors below on the empty pads to
check this.  Top to bottom, the LEDs display A-B-C-D (the resulting display number shown in BCD).  

But an "Unlit LED" means a binary "1" for that position.  

If the board is locked up, you get all the LEDs lit and a "0" in the display, which is what I'm used to.

 

Numbers on the display are shown as "inverse binary" by the 4 LEDs.
In order from Top to Bottom A-B-C-D, with BINARY "0" for LED OFF and "1" for LED ON:

Display

Number

1st LED (Top)

IC34 pin7 (+1)

2nd LED

IC34 pin1 (+2)

3rd LED

IC34 pin2 (+4)

4th LED (Bottom)

IC34 pin6 (+8)

0

On

On

On

On

1

Off

On

On

On

2

On

Off

On

On

3

Off

Off

On

On

4

On

On

Off

On

5

Off

On

Off

On

6

On

Off

Off

On

7

Off

Off

Off

On

8

On

On

On

Off

9

Off

On

On

Off

 

“0” on constantly at power on can be an indication the board is locked up.

IC18 PA4 - PA7 will be high.  Pressing the Diagnostic switch will not change things.

 

Here are some tips with the later stages of Game ROM diagnostics on the system 7 MPU

Or with the Williams Test ROM in IC17, in order of probability:

8  - MPU board may be good- is it looking for a Driver board?
Then suspect that IC19 RAM is faulty or Finally a memory protect fault.


9 - First check for coin door closed (or pin 1J4-1 or 1J3-1 is being
grounded), then is IC7 faulty or another memory protect component faulty?

Finally is IC19 RAM faulty?

 

Another tip is with Leon's test ROM v3 
2 – So Top LED On, second LED Off.   This can mean "coin door closed",

or memory protect circuit bad.  Not just a bad 5101 RAM at IC19.

 

You can prove this by adding a jumper from 1J4-1 to Ground (1J4-2 is a good place).  

The test will stop at "2" and not continue until you remove the jumper.

Then it will proceed past "3" and back to flashing if IC19 and IC16 are good.

This may be a good check of your memory protect before putting it back in a game.

 

Adding those LEDs may be more confusing than you thought.   If you do add 2 LEDs, then add all four.