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pacoarcade

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I'm looking to buffer CSYNC coming from arcade boards with a Schmitt trigger. I'm doing some experiments with a 74AHCT1G17 (single Schmitt trigger buffer) followed by a voltage divider.

I've tried different boards and I get a stable ~350mV output. They work perfectly, that's very nice... but unfortunately it doesn't work with Neo Geo MVS. This is the only board that doesn't sync, getting no signal at the output of the buffer. I've read about Neo Geo having an ugly clock signal but I'm not sure if that's the cause, or what should I look for to make this work properly.

Any suggestion or idea is most welcome.
 
I connect CSYNC directly from JAMMA edge to input pin (pin 2) of 74AHCT1G17 and after the output (pin 4) I use a voltage divider consisting in 470R and 75R resistors.
 
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Speaking from experience, you need a scope when dealing with stuff - doesn't have to be a fancy one. For fun, you can try to put a 10k resistor to gnd on csync before the schmitt to see if it makes a difference.
 
Speaking from experience, you need a scope when dealing with stuff - doesn't have to be a fancy one. For fun, you can try to put a 10k resistor to gnd on csync before the schmitt to see if it makes a difference.
Very true! In my opinion, me and anyone that really loves arcade stuff :) a really good reason is detecting ill signals from RAMs, TTLs etc... I'm looking to buy some cheap old analog scope, meanwhile I have a DSO150.
 
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So what's special about the MVS input signal? I think one of the MVS's have a bit of DC bias
 
I've connected the scope to the CSYNC coming from the MVS and I see a DC offset of 2V. Adding a 10K resistor to ground did not change anything I can appreciate, but adding a 100nF ceramic capacitor in series did the trick and the image is stable!

With a Double Dragon 2, though, the image doesn't sync except when I connect the 10k resistor to ground. I'll continue my tests using 74HC04 and 74HC14 instead.
 
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I've done some successful tests with a 74HC14 used as RGB explained; all the boards tested work fine :thumbup:

Accoding to my notes and if I'm not wrong, since the output impedance of a voltage divider is the equivalent of both resistors in parallel and 74HC devices have a typical output impedance of 50Ω, using a voltage divider consisting in R1 = 470Ω and R2 = 87Ω (1% tolerance) you get a signal of ~400mV and ~75Ω impedance.
 
I still need to tests more boards, but it seems to work fine. Since it's only 4k I attach the file to the post:

74hc14-csync-buffer.png
 
May I enquire as to the point of it? I've experimented with a similar circuit and found that it didn't "fix" any of the main known troublesome boards and broke a few boards that otherwise worked fine.

Is there a particular board it's a mandatory requirement for?
 
May I enquire as to the point of it? I've experimented with a similar circuit and found that it didn't "fix" any of the main known troublesome boards and broke a few boards that otherwise worked fine.

Is there a particular board it's a mandatory requirement for?
The main advantage I see is obtaining always the same signal amplitude and being able to select 75Ω or TTL using a jumper. Apart from that it seems that the Schmitt trigger turns the signal into clean pulses.
 
May I enquire as to the point of it? I've experimented with a similar circuit and found that it didn't "fix" any of the main known troublesome boards and broke a few boards that otherwise worked fine.

Is there a particular board it's a mandatory requirement for?
Pretty much what rsignal said, but the 87R would be a non-standard resistor, so 91R can be used instead.

It's not mandatory, it's just how I buffer the CSync in the HAS.
 
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