New Project: PC Engine JAMMA Analysis

[NFG]

I've had this PC Engine JAMMA board for a long time.  It uses any PCE or Core/Super Grafx, and provides a JAMMA connector interface.  Only two cables connect to the PCE: the expansion bus (RGB and Audio from the PCE, and Power to the PCE) and a controller port cable.

It basically uses coins to start a timer, enabling the start button.  When the timer is low a loud beeping is heard over the speaker, and the screen flashes with a red (selectable, could be green or blue also) overlay.  When the time runs out the start button is disabled and the system resets.

Things I want to learn:

1. How the RGB amp works
2. How the coin counter flashes the screen when time is low
3. How the controller inputs are encoded (is it 1P or 2P?)
4. How the MegaDrive side of the PCB works (it has vacancies for ports etc for what I assume is a MD setup)
5. What's in the NEC D8749HC PROM code?

Let the adventure begin!

[NFG]

The first thing I did was figure out the RGB circuit.  It uses a 2-transistor amp and gives a very clear, stable output.

[NFG]

Regarding the colour-flashing when the timer runs low:

The effect is applied via a signal from the NEC microcontroller, which signals a 555 timer circuit to start pulsing.  The pulse effect is routed through a set of three jumpers (red, green, blue).  One jumper is hard-wired on this board (red) but it can be changed if you really want to.

The jumper then is routed directly to the red/green/blue output line, essentially maxing out that channel, but leaving the other two (blue, green) alone.

[NFG]

Here's a PDF link for the NEC D8749HC microcontroller.

[viletim]

Capacitve coupling on an arcade monitor video amp = dodgy design

[NFG]

Why, viletim?  And what would you suggest instead?

[viletim]

Why, viletim?

Well, to clarify, there's nothing inherently wrong with capacitive coupling, it just needs to be done properly. Coupling through a capacitor removes a some vital information from the video signal, the black level. This is a fixed voltage that is declared to be "black". Any voltage above this level is active video information. If this information is removed it must be put back a some stage before it gets to the CRT cathodes. This RGB amplifier circuit doesn't so the monitor must do this.

Most old monitors aren't prepared for AC coupled video and won't display it properly. At least that's what I believe. I downloaded 10 random monitor manuals from http://www.mikesarcade.com/arcade/monitors.html (when the links still worked) and two of them had internal video clamping circuits (which would restore the black level based on the knowledge that the video signal is "black" when the sync pulse is active). From what I've heard from various people on on the internet, it's clear that most new (less than 10 years old) monitor chassis don't care to much about this.

I leared from the other thread all about the PC Enginge's wacky video. When using a transistor amplifier there's simply no choice but to couple the signal with a cap. With the addition of a single transistor per amp (or just one cmos switch IC like the 4066) the video can be clamped to the sync pulse and everyone would be happy. Another benefit of this would be an increased output swing (bigger signal on output).

And finally there shouldn't be any need for pots on the output. The maximum voltage swing on the output of this amp is already less than a typical arcade board. Why would anyone want to reduce it? Just another thing to be mis-adjusted.

(you did ask)


My guess on the microcontroller is that it controls the coin timer related stuff (like switch debouncing -- very important for a coin input!) and very little else.

[NFG]

Yeah, the microcontroller has nothing else to do on this board but manage the timer and coins, and flash the screen when the timer's low.

The video pots are handy in a non-arcade sense, as I can use this device on my TV or other computer monitors with the appropriate adjustments...  Tho you're probably right that they're redundant overall.

Thanks for the video explanation.  My electronics theory knowledge is really poor, and your lesson explained a few things I didn't know before.  =)

[NeWmAn]

This is the 3rd kind of PCE-Jamma adapter that I've seen, was this one made in Japan?

[NFG]

Made in Japan or by a Japanese company called テクナート (technart).

[yocke]

Any new information on this PCB?
Just got one, but need help with DIP settings!

Any idea?

[NFG]

As luck would have it, mine came with an instruction sheet.  It took me a while, but I found it.  I'll get a proper image of it later, but the DIPs are:

1 & 2 - coins and credits.
3 - demo sound
4 - credit game or timer game

DIP 2 = 1-minute increments
DIP 3 = 10-minute increments

[yocke]

Cool, really appreciate your fast replay!

Did you get any understating over how the MegaDrive side work?

As luck would have it, mine came with an instruction sheet.  It took me a while, but I found it.  I'll get a proper image of it later, but the DIPs are:

1 & 2 - coins and credits.
3 - demo sound
4 - credit game or timer game

DIP 2 = 1-minute increments
DIP 3 = 10-minute increments

[NFG]

I never looked into it, the PCB's been in a box since I wrote the first post.  =/

[NFG]

I've added the entire 1-page manual to the wiki:

http://gamesx.com/wiki/doku.php?id=arcade:technart_pc_engine_jamma