Are all grounds the same

[Spaz Monkey]

So I'm working on the rewiring of my cab (long story).  Looking at the wiring, the previous owner has wired the video ground to pin 28 on the jamma harness.  Is that ok?  Could I technically take all my grounds, including the video ground. and wire them to pin 1/a on the harness?

[fjl]

Usually all grounds are the same but its not always so. I wouldn't mix them from PCB to monitor unless I was really certain it was okay.

[SavannahLion]

After giving it a little bit of thought.

It is usually true that ground is ground and you can technically tie all the grounds to pin 1 but....

I think it's probably not something you really want to do. Ground needs to be as wide as possible to allow the current to flow as easily as possible. This is why there are 8 pins + audio and video grounds. You don't want to create a bottle neck to force the current to flow through. In this case, the bottle neck would be the small contact point of the single pin on the JAMMA connector.

The reverse is somewhat true with the +5v, +12v and -5v lines. There are numerous pins to ensure the least resistence.

Is there a particular reason why you can't connect video ground to pin 14?

[shardian]

In theory yes, but in practical use, not always. I'll detail you the example in my gottlieb pin.

On this pin, the grounding was not designed very well. Over time, the various grounds connections degrade and such other long term effects. Some grounds are hard connected to a metal ground on the cabinet, some are pcb grounds that connect thru the wiring harness. All of these grounds are located at various places throughout the pin. The problem is that depending on how well the grounds are connected, potential differences can arise that can play havoc with electrical components such as the boards and coils due to increased baseline voltage.

Hope that helps you out a little bit.

[Pac-Fan]

Anything raw metal, exposed to the user or sometimes not, should be connected to the incomming frame ground, via the center pin of the power cord going into the cabinet, aka earth ground.

This includes the exposed metal on things like the: Coin door, control panel, speaker/vent grilles, power switch metal plate, fluorescent lamp frame, isolation transformer housing, monitor frame, back door kill switch support plate, etc..



Anything not exposed to the user and part of a signal loop back to the gameboard should have their grounds shared and connected across all ground points on the board's harness.

This includes everything like: Joystick and button leafs/microswitches, Trackball/spinner optical boards, service switches, coin switches, etc..


As a generality, the video cable (output) signal grounds normally are separate from the 'input' grounds, and you can safely just send the 1 thin trace to the monitor, because the amount of current is so small driving the video, the return lead doesn't need to be great.  In some cases it might be that that ground is separate from the other signal grounds, in other cases, the grounds are crossed anyway so you can connect ground (for video) to any ground on the connector.

However -- Given a separate video ground, I would never puposely cross a video ground to the main input grounds. You could end up with interference if they are designed to be isolated.  And more importantly -- NEVER run the main input grounds into a single video ground trace as you will definitely end up with too much current into a single small ground connection and fry the connector or board.

[grantspain]

the video ground is not the same as voltage ground or switch ground,if you connect the video ground to pin 28 you will get a some strange issue's
connect it to the correct pin

[xar256]

the video ground is not the same as voltage ground or switch ground,if you connect the video ground to pin 28 you will get a some strange issue's
connect it to the correct pin

I can vouch for that as I just dealt with this issue last week.  The guy who did the conversion on my cocktail cab, hooked up the video ground to the metal chasis instead of the pin on the neckboard.  This caused a wavy type effect on the right side of my screen. 
Moved the wire over the the correct pin on the neck board, and now the picture is straight and stable.    

[Spaz Monkey]

Long story short.  Had a guy wire a multi-board setup.  Saw that on one of the harnesses, the video ground was missing.  It showed up as pin 28 on the harness.