Unable to get RGB image from MSX2

Supporter (7)

31-05-2023, 15:23

I have National FS-4700. It has a JP21 connector. I have been trying to get RGB picture out of it to my OSSC with no luck.

First I tried using JP21 to scart cable from videogameperfection (https://videogameperfection.com/products/jp21-to-scart-adapter/). I used male to male scart cable between my MSX and that adapter, since it expects JP21 signal on the female end. Didn't work.

Since I thought maybe the male to male scart cable in that setup messes things up somehow I made my own adapter going directly into it from JP21 using pinout listed here: https://www.msx.org/wiki/RGB21_connector
Tried my adapter with the stated capacitors installed and without them. Didn't work.
What is the meaning of those capacitors and what is the meaning of the resistor?
Also, is the pinout listed on that page 100% correct?
I checked using continuity meter and my Videogameperfection adapter uses the following pinout:
1 -> 6
3 -> 4
5 -> 2
7 -> 17
8 -> 18
9 -> 20
11 -> 8
13 -> 13
14 -> 14
15 -> 15
16 -> 16
17 -> 9
18 -> 5
19 -> 11
20 -> 7
21 -> 21

atleast audio pin (mono/left) is wired differently, not that I think that it matters since I can't get RGB video to display.

I checked inside my MSX and everything looks fine. No leaking or bulging on capacitors. I also checked capacitors near JP21 connector and Yamaha V9938 chip with ESR meter and every capacitor I tested seems to be working. I tested continuity in the actual JP21 connector and every pin had working continuity and appears to be attached properly.

My OSSC, monitor and Scart cable work with everything else so that isn't the problem either. MSX machine appears to be working perfectly apart from the JP21 connector.

Any ideas? I don't own an oscilloscope and even if I did, I don't really know how to start troubleshooting with it as that is a bit beyond my expertise. Should I just accept that I am going to have to play my MSX games via composite video only?

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Par Pentarou

Hero (559)

31-05-2023, 17:46

Liner wrote:

I used male to male scart cable between my MSX and that adapter

Liner wrote:

9 -> 20

You are connecting the JP21 C-Sync (Composite Video) input to the Scart C-Sync (Composite Video) input, the correct way should be JP21 pin 10 to SCART pin 20.
Male to male SCART cables aren't straight, I don't know if you have already considered that in the above.

Par Liner

Supporter (7)

31-05-2023, 21:12

Pentarou wrote:

You are connecting the JP21 C-Sync (Composite Video) input to the Scart C-Sync (Composite Video) input, the correct way should be JP21 pin 10 to SCART pin 20.

Yes, because of things like this I decided to make my own cable following exactly what was on the wiki, so that I would be absolutely sure how it was connected and what was in it. I now took a look at that Videogameperfection cable, and it is eviden't that at some point I did try to move a couple of wires around to get it to work, with no luck.

Pentarou wrote:

Male to male SCART cables aren't straight, I don't know if you have already considered that in the above.

2nd reason why I made my own adapter, with male plug at the msx end, so that I wouldn't have to use a separate male to male scart cable betwen MSX and adapter. That could have been additional problem with bought adapter.

Anyway, my own adapter is made exactly how the wiki describes it, but it didn't work either. What is the reason the wiki suggests putting capacitors and a resistor in the cable?

Par Repair-Bas

Paragon (1176)

31-05-2023, 22:11

www.bas-ditta.info
the adress for all msx cables

Par Pentarou

Hero (559)

31-05-2023, 22:57

The capacitors are needed on some machines and the 100 Ohm resistor is a safety measure (*), but AFAIK machines with SCART connectors don't need them as they're already installed inside.
.
The OSSC should give you some kind of feedback if it detects a video signal, did you get nothing?

Quote:

Anyway, my own adapter is made exactly how the wiki describes it, but it didn't work either

I don't know if the OSSC needs it, but any TV would need to have pin 16 wired too, on both sides RGB21#16 to SCART#16.
I'm actually surprised that the WIKI pinout leaves that out, as any TV would fail to switch to RGB mode without it. (*)
Could you post a picture with your cable where the wiring is visible?
(*)Edit.
Sorry I just noticed it's mentioned at the bottom. Yes, it is also possible to power pin 16 via pin 8 + resistor.

Par gdx

Enlighted (6429)

01-06-2023, 03:13

Liner wrote:

I checked using continuity meter and my Videogameperfection adapter uses the following pinout:
1 -> 6
3 -> 4
5 -> 2
7 -> 17
8 -> 18
9 -> 20
11 -> 8
13 -> 13
14 -> 14
15 -> 15
16 -> 16
17 -> 9
18 -> 5
19 -> 11
20 -> 7
21 -> 21

Why don't you do as indicated in the wiki?
The wiki seems correct. Here is another description which is the same as the one on the wiki.

Maybe it's clearer.

Par CASDuino

Champion (358)

01-06-2023, 04:53

Liner wrote:

1 -> 6
3 -> 4
5 -> 2
7 -> 17
8 -> 18
9 -> 20
11 -> 8
13 -> 13
14 -> 14
15 -> 15
16 -> 16
17 -> 9
18 -> 5
19 -> 11
20 -> 7
21 -> 21

Looking at the wiki picture I think 7->18 seems more likely the correct connection.

Par cjs

Master (143)

01-06-2023, 06:15

CASDuino wrote:

Liner wrote:

7 -> 17

Looking at the wiki picture I think 7->18 seems more likely the correct connection.

Pins 17 and 18 on SCART are both ground, so it makes no difference.

Multiple grounds are not separate signals. If you look at the PCB traces at the connector, you'll find that almost invariably all of the ground pins are shorted together very close to the connector.

The ground is provided on multiple pins for convenience in building cables. For example, you should have separate grounded shields around each of the R, G and B signals (i.e., coaxial cable internal to the SCART cable); it's much easier to build a cable that connects each of these three shields to its own pin than one that combines all three of these shields (and any other grounds) into one in the connector and then connects to the one pin.

Par sdsnatcher73

Enlighted (4291)

01-06-2023, 07:15

Well I think multiple grounds are also there to create separate wire bundles in the bigger cable. You can route audio with ground through a seperate bundle with shielding and do the same with video signals. Also you can seperate output wires from return wires. To prevent cross talk. This would make a higher quality cable and is especially useful with longer cables.

Par cjs

Master (143)

01-06-2023, 07:32

Liner wrote:

...I made my own adapter going directly into it from JP21 using pinout listed here: https://www.msx.org/wiki/RGB21_connector
Also, is the pinout listed on that page 100% correct?

Mostly. I just updated my notes on this from the English Wikipedia SCART page and other sources (listed at the end of that section), and pins in the MSX Wiki table are correct.

However, they leave out pin 16 from the table, and suggest that you may need a resistor from JP-21 pin 8 (GND) to SCART pin 16 (Blanking signal/RGB select). But according to Japanese Wikipedia, pin 16 at ≤0.4 V indicates to the remote end that you're sending a composite signal, and you need to set this to 1-3V to have the display read the RGB signal instead. This seems confirmed by the output of my Fujitsu FM77AV, which has 3.4V output (unloaded) on this pin.

So I would suggest checking pin 16 on your MSX with your voltmeter and, if it's also around 1-3 V relative to ground, putting that 100 Ω resistor between pins 16 on both ends. The resistor will simply reduce the chance of any damage if both ends try to drive the line, so if the cable still doesn't work, check the voltage on that line while it's operating; you may need to remove the resistor.

Quote:

I checked using continuity meter and my Videogameperfection adapter uses the following pinout...

That cable looks rather bogus. 5→2 connects an audio input to an audio input, and 9→20 connects the TV tuner's output back to the computer's input but there seems no connection in the other direction for sync. I suspect that this worked for something using sync-on-green and just the left audio channel, and they never tested it with anything else.

Par cjs

Master (143)

01-06-2023, 07:42

sdsnatcher73 wrote:

Yes, but you could do that just as well with a single ground pin. As I said, all the ground pins are (pretty much invariably, in my experience) connected together on the PCB right next to the connector, so if you ignored all but one of the ground pins, and soldered all of the separate grounds in your cable to that one ground pin, you'd get exactly the same effect.

You are correct that having ground lines separating signal lines is an excellent idea when building a cable, but that does not require separate ground pins on the connector. (At least, not at sub-GHz frequencies we're talking about here.)

(Also, in common terminology, the "return wires" are the ground wires; they provide a return path for the current flowing through the nearest signal wire, whether that signal is considered an "output" or an "input" for a device on either end of the cable. Since in an AC connection above a few thousand kHz the signal is actually carried not in the signal wire itself but in the electromagnetic field between the signal wire and whatever path it uses to return the current, this is why having a ground return right next to the signal wire reduces interference. Put another wire between the signal wire and its return and the EMI carrying the signal wire's information will induce a current in that other wire.)