U1=LS133
U2=LS02
U3=LS00 (horizontal)
U4=LS00 (vertical)

In general, it's about logic at the gates.

1. Is U4 gate not used (pins 4,5,6) ?
2. What is the output U1 (pin 9) connected to ?
3. What are the pins 5 and 6 of the optocoupler connected to ?
    From the photos I get that pin 5 is connected to RxDATA (pin 2 ACIA) and to the U3 C gate (pin 9 and 10).
    If this is the case, where is the output of U3 C (pin 8 ) connected ?
4. The U2 A output (pin 1) is connected to CS0 and CS1 ACIA.
    My guess is that the inputs of the U2 A gate (pin 2 and 13) are A14 and A15, but I can not see these connections (you will find A14 on pin 11 U1 and A15 on pin 4 U1).
5. Finally, the biggest puzzle - U3 A (in 1,2, out 3), U3 B (in 4,5, out 6) and U3 D (in 12,13, out 11).
   The U3 A output seems to be connected to the CE ROM input, but it's kind of weird.
    I do not know where the pins 14 (Enable), 13 (R/W) and 5 (RTS) of ACIA are connected.

It's possible that in the end I would have guessed the right combination of connections (it's a bit like Sudoku ), but it's easy to make a mistake.

The attachment contains the current state of the schematic.

Thank You Very Much

However, I found one unconnected path.
Probably one end lead to a pull-up resistor.
Where's the other end ?

Thanks

Quote from: pear on 2019.March.12. 19:19:40

Probably one end lead to a pull-up resistor.
Where's the other end ?

Yes.

OK, fine, but the second end ?

Hm, I wonder why ?
These are not open collector gates (U3).

Nice!
The J2 how numbered?
And also the resistors/capacitors?

Here you are.
Connector J2 looks like the contacts for the DB9 socket.

That was on the original PCB.
On the connector there are signals for the third MIDI THRU socket and an IRQ signal from the ACIA system.