C025914 vs C070713 vs C101608 -- the 3 shifters of the Mega ST

[sporniket]

Hello,

Studying the Mega ST schematics, like the first one that one can find on the great download thread, I found an answer that @mrbombermillzy and me were wondering about on this thread about combining 2 shifters to extends the colorspace of the STF palette (16 colors among 32K-65K colors instead of 512 colors).

So I will put it there.

So on the schematics of the Mega ST, I could see that there are 3 variants of the Shifter, with the following differences :

1. C025914, dubbed "Gate Array without DAC" :
   • pin 1 is named XTAL0 and is pulled down to ground with a 10K(or 10R?) resistor ;
   • From pins triplets {21,22,23} (B2,B1,B0) ; {24,25,26} (G2,G1,G0) ; and {27,28,29} (R2,R1,R0) ; there are 3 ladders of resistors to get the raw RED, GREEN and BLUE signals, that are then gated by the /BLANK signal through the respective diodes D9/D10/D11 and send to their respective amplifier through diodes D12/D13/D14 ; RED, GREEN and BLUE are also wired to VCC through R129/R130/R131.
2. C070713, dubbed "Full custom without DAC" :
   • pin 1 is named /BLANK, and is wired to the /BLANK signal
   • Like C025914, from pins triplets {21,22,23} (B2,B1,B0) ; {24,25,26} (G2,G1,G0) ; and {27,28,29} (R2,R1,R0) ; there are 3 ladders of resistors to get the raw RED, GREEN and BLUE signals ; RED, GREEN and BLUE are also wired to VCC through R129/R130/R131.
   • Unlike C025914, the are raw RED, GREEN and BLUE signals are not gated by the /BLANK signal (NO diodes D9/D10/D11) and are directly sent to their respective amplifier through diodes D12/D13/D14 .
3. C101608, dubbed "Full custom with DAC" :
   • Like C070713, pin 1 is named /BLANK, and is wired to the /BLANK signal
   • Unlike C025914 and C070713, the raw RED, GREEN and BLUE signals are directly output by pins 27, 24 and 21 respectively and directly sent to ther respective amplifier, meaning the resistors from the original ladder (R120, R123 and R126) and the diodes D12, D13 and D14 are replaced by a Zero-ohm resistors ; there are no R129/R130/R131 to wire RED, GREEN and BLUE to VCC.
   • Unlike C025914 and C070713, pin 28 is named VCC-A and is wired trough Zero-ohm resistor R146(?) to a filtered VCC (filtered by ferrite bead FB246?) .
   • Unlike C025914 and C070713, pin 29 is named 32/9 and is not wired anymore
   • Unlike C025914 and C070713, pin 22,23,25 and 26 are without names and are not wired anymore

[czietz]

Differences between C025914 and C070(7)13 shifters, concerning timing and particularly the BLANK input were discussed quite a while ago in Atari-Forum. In this thread: https://www.atari-forum.com/viewtopic.p ... 18#p303218 and others.

As for C101608, I doubt it actually exists in the wild. Note that the resistors making up the internal DAC are already present inside older Shifters, just not connected, i.e., not bonded. I presume Atari was not able to meet the expected performance. (Resistors on a chip are always hard.)