The CSYNC on Genesis

Simple question :

What the difference between CSYNC from cxa1145p (pin 11) and CSYNC directly from VDP ?

 

Simple answer: CSYNC on MEGA DRIVE consoles come straight from the VDP chip and are open collector signals.

What SEGA did was put a 2.2K resistor on the line to pull up the signal (give the line +5v bias and the VDP chip latches it to GND when it want to tell the TV the sync is low) so the line has proper voltage. That certainly cause problems with some monitors which sink too much current from the CSYNC line. (voltage might be lower than what is needed for the TV to figure out what is high or low in such cases and that results on loss of sync)

The CXA1145 chip has a high current buffer for the sync pin, but it operates on the "SCART" standard. I mean it outputs a 1v-ish sync pulse which has to be coupled with a 75ohm resistor and a capacitor (same circuit as the composite video and RGB signal outputs) and that signal is more "proper" for TVs than what SEGA designed. The point was probably save some pennies as they never expected all people to be using RGB cables anyway.

 

OK thanks. That's what I was thinking .

Why this question :

On french "Megadrive RGB" the "comp" signal was disabled (missing components) and the sync signal from VDP pin 42 replaced by the sync signal from CXA1145's pin 11 (trace from VDP is cut and replaced by a yellow wire under the motherboard).

But I notice that on PAL and NTSC Megadrive, the sync signal delivers a non standard sync signal. On a scart TV, PAL and NTSC Megadrive gives a black screen, with sound.
There is an image, but only if I add a 75 ohm resistance with a 220µf capacitor in line with sync signal.

But with a 32x things are differents. On same PAL or NTSC Megadrive with sync signal (without mod) there is no image for Megadrive, but there still one with the 32x. In this configuration (PAL or NTSC Megadrive + 32x using sync signal) the 32x uses the synch signal from the Megadrive and pass it through its video-in/out connector.

How the 32x can display its graphics with sync from Megadrive (VDP pin 42) whereas the Megadrive still has no image ?

 

Your answer is simple. The SYNC signal which comes from the VDP is not suitable for direct connection to your TV. Add a CXA1145 and build a proper sync circuit to it's output and then you will have proper 1vpp sync pulse which is what the TV expect. The 32X probably uses a digital input without termination, which means it doesn't draw current from the SYNC. Of course the TV needs termination load as it expects to be connected to long video cables. The 32X doesn't expect long cables on the connection from it to the MD so no need to put terminating resistors (load).

 

Thanks oliveira

I don't understand. How the 32x can display its graphics with the sync from the Megadrive, and not the Megadrive itself ? I checked the traces from video-in to video-out on the 32x minidin, the sync pin on video in has a direct connection to 32x's video out. Only comp signal go to 315-5788 video mixer. The RGB cable from sega (the one with white connector bundled with 32x) carry only sync, no comp. Normaly I should have no image at all. Am I missing something here ?

 

I explained. The TV HAS TO HAVE a 75 ohm resistor on the sync line so it is properly terminated (to eliminate ghosting from the signal, an effect called "ringing") and the 32X which is going to use a short cable don't need to do such a thing. That termination resistor overpowers the circuit which supplies power to the sync line and the voltage drops below 1v that the TV uses as 1/0 reference. The CXA1145 has a high current driver which will solve your problem.

 

Using c-sync from the CXA1145 via a 75ohm+220uF cap works wonders, I use this trick on my pal md1 and sms1.

Speaking of French megadrives and RGB cables, what's the deal with the 3085 cable? Why does it have a pcb?

 

That box will have the proper RGB/CSYNC amplifier circuitry. Look how the side that connects to the MD console has a very short cable. That's intentional... So termination is not necessary.

 

^ so , this is a special cable for specific French consoles, right? It always bugged me, I never got a straight answer about this cable

 

Probably SEGA decided to save a buck by not using the CXA1145 chip inside the console so they bypassed the RGB completely and made an RGB only amp for France. You know that CXA1145 can't generate SECAM video so point in having one on the MD was so moot. CXA1145 was an expensive chip back then.

 

got it! thanks for the clarification l_oliveira appreciated! I've been wondering for quite some time about that cable, then forgot it, then remembered it again

 

I had a similar issue - CSYNC modded a mega drive 1 (with cap and resistor), but pal 32x wouldn't display images correctly. I had to revert it for that console.

Am I understanding correctly that it's simply that 32x won't work with that signal? Seems a bit odd, since whatever comes out of a MD2 should be equivalent to the CSYNC mod + cap + resistor? Perhaps I'm not reading closely enough.

 

All Megadrives use the CXA1145. The NTSC and PAL ones use pin 20 for composite video that goes to pin 3 of video out DIN8. VDP's pin 42 csync goes directly to pin 7 of video out. However, it's also used by CXA1145, the csync signal from VDP's pin 42 goes through pin 10 of CXA1145 before going to pin 7 of video out (to make a composite video signal).

But, french Megadrive uses a slightly different configuration. The csync from VDP's pin 42 is disabled at video out (trace cut) and replaced by csync from CXA1145's pin 11. French Megadrive uses the csync from pin 11 of CXA1145 while all others Megadrives use pin 42 from VDP. French Megadrive has a yellow cable under the board to link CXA1145's pin 11 to din8's pin 7.

In a nutshell :

- NTSC and PAL Megadrive : csync from VDP's pin 42 | comp from CXA1145's pin 20
- French "Megadrive RGB" : csync from CXA1145's pin 11 | no composite video

In RGB mode, the csync from VDP's pin 42 does not work and give black screen, TV detects a signal but can't understand it, OK. But the strange thing, 32x needs and uses the same csync from Megadrive to display its image. Without its AV bridge cable between MD's AV out and 32x's AV in, 32x can't display its image, normal, there is no sync from MD. Image is out of sync.

However, in RGB when AV bridge cable is used, MD gives a blank screen and 32x displays its image properly. Usually, we should not have any image at all, nor 32x or MD.

In a nutshell :

In RGB :

- MD1 PAL or NTSC + 32x + AV bridge cable + official 32x's RGB cable from Sega (that uses VDP's csync) == black screen for MD with 32x graphics displayed correctly
- MD1 PAL or NTSC + 32x + AV bridge cable + official MD2's RGB cable from Sega (that uses comp) == MD's graphics displayed correctly and 32x graphics displayed correctly
- French MD1 + 32x + AV bridge cable + official 32x's RGB cable from Sega (that uses CXA1145 csync) == MD's graphics displayed correctly and 32x graphics displayed correctly
- French MD1 + 32x + AV bridge cable + official MD2's RGB cable from Sega (that uses no sync) == 32x's image only out of sync
- All MD1 + 32x without AV bridge cable + official MD2's or 32x' RGB cable from Sega == 32x's image only out of sync

My initial question : how the 32x can display its graphics using the non standard csync from MD that normally give black screen ?

 

As I said first time, it's the default CSYNC on normal MDs that are out of standard. Tapping the CSYNC from an CXA1145 output will actually give you proper sync.

Hence why I don't understand your doubt.

Also I mentioned that because the 32X don't have a load on the CSYNC input, it won't overpower the MD output but it won't make it incompatible with a "proper" CSYNC input.

The 32X does have an properly amplified CSYNC output so it can deal with terminated CSYNC inputs on TV sets just fine which explains the behavior you just described.

 

But the 32x does not have any sync signal at all and does not perform any "transformation" on it (on csync, not on comp). It uses MD's sync that is directly connected to the 32x's video out. Really I am missing something...

 

You realize you're now writing something stupid ?

The 32X is a genlock. It doesn't generate it's own sync but it use the sync which comes from MD as reference for it's own internal video clock source (an PLL to generate a reference voltage plus an VCO which varies it's frequency slightly based on the input sync so it runs synchronous with the input so the two video signals can be properly combined) and outputs an amplified signal at it's own connector.

All I said was that the 32X has an amplifier at the sync output, which the original MD lacked.

 

I need to write stupid things to get accurate answers ! ^^

I know this, but only with composite signal, the sync signal goes directly to video out without goes through any chip. AFAIK only comp goes to 315-5788, no csync. And the "RGB box" uses only csync, not composite video.

I don't understand how the 32x can outputs an amplified signals from an other that it never recieve (csync).

(or... I have badly follow the traces between csync-in and the 315-5788...)

 

By Composite you mean composite video ?

Composite video has "composite sync" as one of it's signals. It's just Y (luminance plus composite sync, which is the black and white video signal) and C (color information used to create the RGB signals from the Y signal). So yes you can always get any component of the video signal out of composite video. But there's a caveat. Because all information is being "squeezed" on the same wire we have a very noticeable quality drop on the signal.

But then Vsync and Hsync are digital signals, we won't lose much on them. That's why it's acceptable to use CVBS as CSYNC source on PlayStation consoles.

 

Yes CVBS. The 32x's CVBS-in goes directly to 32x's 315-5788 chip, it's OK to use it as reference for 32x video clock. But not csync, csync goes directly to... video out. And french RGB cable takes only csync to synchronise image, not CVBS (there is no pin for CVBS). That's why I am wondering how is possible to obtain an image from 32x with PAL MDs, when at the same time MD displays nothing.

I have to search deeper and look if the csync received by the 32x is linked to the 315-5788 or not.

 

But how about the cable with the grey connector which is supposed to be used with Mega Drive I only and not Mega Drive II when you want to plug a 32X into your Mega Drive 1 ?

Is this a different problem or is there a connection between what you're saying and the SCART cable dedicated to Mega Drive I in use with a 32X ?

What is the difference between this SCART cable for 32X to use with a Mega Drive I and the simple RGB cord delivered with Mega Drive II bundles in France ?