Jamma to SCART questions

[venkman]

hi there,

i'm refurbishing a cab i bought. all the wiring is a mess - i think it was a silverline cab so the wiring isn't even standard JAMMA. i'm going to rewire it all.. (i'm sure i'll have questions on that when the time comes) .. but i'm also *removing to give away/sell the broken monitor and putting in a 19" wharfdale tv with a scart port.

i'm following the instructions over at http://www.gameland.gr/geoanas/realarcade.htm

and this diagram:


to make this:


the question:
in the diagram it has 5, 9, 13, 18 and 21 connected to the ground on the board. So when i make the cable should i join the scart connector's 5, 9, 13, 18 and 21 with extra little wires so that it's a loop, or will it be ok to just have them terminate there in the scart plug? the other end of these just connects with the board's ground of course..

[Nipedley]

You mean have all the appropriate ground pins in the scart connected together then 1 wire coming from that to the board? That's fine, you can do it that way since ultimately all the pins will be connected to ground whether they are wired individually or not. I connected them all together into one wire for my jamma->scart adaptor and it worked just fine.

[venkman]

yeah i mean they all connect into the one ground cable at the the board end, but do i need to string the ground together ar the scart plug end?

i'll try to illustrate it..

[venkman]

i'm still unsure about this..


i found this:
"Logic ground is ground in reference to your DC power source... your 5VDC, 12VDC & -5VDC... whether it comes from a linear power supply, switching supply or plain old battery power! No, it's not like your automobile where any metal surface is actually grounded, so you can't run a wire from your coin door frame up to a switch on your CP looking for logic ground & expect it to close the circuit for you.

The PCBs ground traces are all linked together on both sides of the board, so any place that is designated as ground on the PCB, is just that,  logic ground, and unites all logic ground into one common termination back to the supply grounding point at your source of DC power."

so.. still not sure if i need to 'loop'/'chain link' the ground in the scart cable so the circut is closed.

[venkman]

further answers:

"I guess grounding is the only thing left a little up in the air. You have a ground line coming in with each bundle and in the case of the coin door bundle, you can loop it to every terminal that requires a ground, e.g., you can go to coin switch 1 com, loop up to coin 1 lamp (either side, with 12 volts on the opposing side), then over to coin 2 lamp, down to coin 2 switch com, and if no other grounds are needed, you can close the loop by running back to coin switch 1 com. It'll now take 2 broken wires to kill your coin door ground, but you can omit this last closing lead, if you want."


SO the answer is (CORRECT ME IF I'M WRONG PLEASE!) .. you don't need to loop these but if you do, it would take take 2 broken wires to break the ground, as opposed to the one.

 

[Nipedley]

Reading through your posts I think I finally understand what you're getting at, I think you're thinking things through a bit too much. I went back and looked at the specs of the one that I built to refresh my memory and it's practically identical to the schematics off the website you linked.

What you need to think is that, say you used scart red ground, if you connect that to the ground pin on your jamma harness, and then with a seperate wire connect scart blue ground to your jamma ground pin, they will be common to each other through the ground pin on the jamma harness. Even if you add a wire loop between them (I can't see why you would want to?), they will still both be 'common' or ground to both each other and the jamma ground pin of course.

Options here are you can either wire each one seperately to the jamma ground pin if you like, or the best option is to use a connector block or something to connect them all to one wire which you can hook up to the jamma ground pin. No need to loop them since they will all be common to each other through jamma ground whichever route you go.

Hope I've understood what you meant and if not let me know, basically follow those plans and you shouldn't get in too much trouble anyway

[venkman]

the best option is to use a connector block or something to connect them all to one wire which you can hook up to the jamma ground pin. No need to loop them since they will all be common to each other through jamma ground whichever route you go.

OK !

i understand 

i'll do it that way - no need to chain them at the scart plug end because they are common through the board. thanks for your help clearing this up!

[Zebidee]

Use 75 ohm resistors (or similar) on the R-G-B signals otherwise they will probably be too bright for your TV. Arcade PCB video signals are normally higher voltage than TV RGB SCART input levels (which are 75 ohm terminated, expecting ~ 0.0v - 1.0v). Even a JPAC outputs these video signals at 1.5v+ so you need the resistors. The most obvious impact of not having the resistors is on the brightness level on the TV, which will probably suffer brighteness "blooming" and/or the colour will be "washed out".  In the med-long term, it might cause damage to your TV.

[venkman]

ok, thanks very much for the tip and explanation! - so it'll work so much as i should still get an image on screen without resistors, but it'll look too bright and washed out .. so as soon a s possible i should drop those in line on the G,R,B cables.

( if i ever get the monitor to work first! haha )

[SirPeale]

Don't toss the old monitor - either sell it or give it away.