National FS-VG501, repair and usage problems

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By popolon_

Master (153)

popolon_'s picture

10-04-2021, 19:53

Now AD and DA analog parts in this PCB are drawn also.
https://github.com/popolonx/MSX_STUFF/blob/main/FS-VG501/Fra...

It seems registers are used to disable or enable some bits in digitized byte, and also to replace the bit with static value from register byte.
Register#1 = 1Q0..7
Register#2 = 2Q0..7

1DA/2DA/3DA bit:
ENABLED D5 (value from next table)
1Q0=0 D4 (value from next table)
1Q0=1 D4=0
1Q1=0 D3 (value from next table)
1Q1=1 D3=0
1Q2=0 D2 (value from next table)
1Q2=1 D2=0
1Q5=0 D1 (value from next table)
1Q5=1 D1=0
1Q4=0 D0 (value from next table)
1Q4=1 D0=0

1DA:RGB-R? 2DA:RGB-G? 3DA:RGB-B?
S1 S0 D0...D7 D0...D7 D0...D7
0 0 Original bits...........................
0 1 all from 2Q6 all from 2Q7 all from 2Q3
1 0 all from 2Q4 all from 2Q5 all from 2Q2
1 1 Original bits...........................

1Q7 1Q6 1Q3 S0_S1_ENA LS54_9/10 LS54_12
0 0 0 0 0 1
0 0 1 1 0 1
0 1 0 0 0 1
0 1 1 0 0 1
1 0 0 1 0 0
1 0 1 0 0 1
1 1 0 1 1 1
1 1 1 0 0 1

If S0_S1_ENA is 0, then it means
S1=0 and S0=0 (so using original bits for signal).
if S0_S1_ENA is 1, then
S0 = signal from IC30_pin2
S1 = signal from IC30_pin3

when reading data (using address 7FC0/7FC1/7FC2/7FC3), D7 bit goes to low or high when data ready?
BFC2/BFC3 addresses could be just to trigger the digitize (store one frame to buffer) ? Or that is handled somehow with
7FC0/7FC1/7FC2/7FC3 addresses as in those it could be read or write.

By popolon_

Master (153)

popolon_'s picture

17-04-2021, 10:49

It seems there is difference between FS-5500 MSX/FS-VG501 RBG DA conversion accuracy. FS-5500 uses 4 bit DA conversion, when this FS-VG501 unit uses 6-bit DA conversion.

By popolon_

Master (153)

popolon_'s picture

21-04-2021, 08:04

I replaced broken memory, inverters for address lines, and 74LS245/244. Ram was faulty, but those others were not. I think the board lights up again (from termal camera view point), but no picture. Scope is seeing rgb output from this pcb, so i have to now look what is broken in that other pcb:
P_20210421_083508_vHDR_Auto.jpg

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