BABY-PACMAN Communication error _ solved

[vernimark]

I had some troubles with both Vidiot and MPU, now the last problem.
No vieo and if I activate TEST I can see a communication error but it is not into the cable, it seems 3 bits are not trasmitted. From where to where? MPU to VIDIOT or vv? Which pins? I couldn't understand.

Instead of:

01 01
02 02
04 04
08 08
10 10
20 20
40 40
80 80

I have

00 00
00 00
00 00
08 08
10 10
20 20
40 40
80 80

please see attached pic

thank you VERY VERY much

vernimark

[vernimark]

I knew signals pass trough VIDIOT and return to MPU; in detail I knew signal "08" returns into MPU at J1, so I back tracked all signals:
     |MPU| VIDIOT                                            | MPU
sig  |j2   |j1  | R|U1       |U2      |U3       |R  |J1  |J1|R    |U20
01  | 8   |26 |R1|2  -> 3 |2  -> 3|2        |R10|14 |20|R84|5 ->3     FAULTY
02  | 9   |24 |R3|5  -> 4 |5  -> 4|4        |R11|13 |21|R85|7 ->6     FAULTY
04  |10   |20 |R5|6  -> 7|6  -> 7|6        |R12|12 |22|R86 |2->1     FAULTY
08  |11   |17 |R7|9  -> 8|9  -> 8|12       |R13|11|23|R87 |9 ->10   OK
10  |12   |16 |R8|12->13|12->13|10      |R14|10 |25|R80 | ?
20  |13   |19 |R6|15->14|15->14|2(U4)  |R15| 9 |26|R81 | ?
40  |14   |23 |R4|16->17|16->17|15(U4) |R16| 8 |27|R82| ?
80  |15   |25 |R2|19->18|19->18|4(U4)  |R17| 7 |28 |R83| ?

So I changed U20 (MC14502B) on MPU and now Baby Pac works again