Translating schematics

[rthorntn]

Hi,

I'm new here and pretty new to Amiga, please be gentle

I'm working through repairing an A2000 4.3 it has what looks to be mild battery damage, the old battery is gone. Around 5 of the 68000 legs and their respective sockets are a little corroded. I cleaned everything up as best I could. Cleaned and reseated all the socketed chips.

I have it powered using a big box adapter and ATX PSU. I haven't got a floppy yet but I do have a Buddha IDE.

I replaced the power LED, it stays bright.

It's connected to the monochrome RCA and its showing a single grey colour but no Kickstart. I have ordered a DB23 to HD15 to get colour.

I have a Lyra, PS/2 keyboard powers on OK.

I also ordered an A520, CPU socket, CPU and a DiagROM.

I want to start to figure out the schematic so I can do continuity checks from the CPU, I have an Amiga A2000_R6 Rev.1.38 (05.09.2012) pdf but it's all split up with lines with symbols linking to other pages (attached), what is the difference between the symbols?

I'm somewhat familiar with the schematic symbols, resistor networks and all that.

Has anyone put together a list of the 64 CPU pins and what they connect to:

1 - U105 P6 & U106 P12
2 - U105 P5 & U106 P9
3 - U105 P4 & U106 P6
4 - U105 P3 & U106 P5
5 - U105 P2 & U106 P2
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29 - U500 P9
30 - U500 P8
31 - U500 P7
32 - U500 P6
33 - U500 P5
34 - U500 P4
35 - U500 P3
36 - U500 P2
37 - U500 P40
38 - U500 P39
39 - U500 P38 & U300 P36 & U301 P36
40 - U500 P37
41 - U500 P36
42 - U500 P35
43 - U500 P34
44 - U500 P33
45 - U500 P32
46 - U500 P1
47
48
49 - N/C
50
51
52 23
53 - N/C
54 - U103 P9 & U104 P19
55 - U103 P8 & U104 P16
56 - U103 P7 & U104 P15
57 - U103 P6 & U104 P12
58 - U103 P5 & U104 P9
59 - U103 P4 & U104 P6
60 - U103 P3 & U104 P5
61 - U103 P2 & U104 P2
62 - U105 P9 & U106 P19
63 - U105 P8 & U106 P16
64 - U105 P7 & U106 P15

I gave up but you probably get the idea.

I guess if the CPU goes to U103 (octal transceiver) I test that and then from the U103 output pin to U200?

Trying to understand pins like _RST on P18, it's connected to +5v through 2.7K resistor network RP101 and its also connected to U200, CN605, U800, U102, U607 and CN600

Any traps, good tutorials, youtube videos?

Thanks!
Richard

Equipment: hakko iron, hakko desoldering gun, eevblog multimeter, rigol oscilliscope, hot air, hot plate, nikon stereo microscope and adjustable benchtop power.

[Chucky]

Symbols is direction of signal..

[rthorntn]

Thanks Chucky!

[Hewitson]

Yep. Not familiar with the diamond symbol though. Bi-directional pin?

[rthorntn]

Thanks Hewitson, it must be.

[project23]

You might find this pretty informative:

https://www.nxp.com/docs/en/referenc.../MC68000UM.pdf

It's long, i know, but it's for reference so what the hey.

So for example on your schematic you have that pin 6 goes to _AS in parallel with RP101 which is a pack of 2.7k pull up resistors as I'm sure you know.

The _ in the _AS name can be read as AS 'bar' (a bar written above AS) and indicates that it is (logically) true when it is (electrically) low and (logically) false when it is (electrically) high. In other words it is the other way around from the normal convention of 5v true 0v false etc.

Thus by default it is false, because of RP101 bringing it up to 5v.

_AS is the address strobe... From the link above:

Address Strobe (_AS).
This three-state signal indicates that the information on the address bus is a valid
address.

So as far as I understand it _AS will be pulled low by the 68k itself if the address on the address bus is a valid address. I.e. the 68k will drain current through RP101, and anything trying to 'read' pin 6 will read an electrical zero, which can be regarded as a logical 'true' - we have a valid address on the address pins. Another chip might want to know this before trying to read/write to that address or whatever. This is why the triangle points outward - it is giving data (in this case information) out.

I am very new to this myself, though finding it incredibly fascinating. With the schematics and that 68k document (Amongst other great ones out there) you should be able to connect the other dots.

I was introduced to the following site a few days ago and it truly is amazing. While sadly it does not have an A2000 board in the collection, the A500 is similar in so many ways that at least it might prove useful:

http://www.amigapcb.org/

It lets you follow traces and identify bits and bobs - from the 68k itself it may help with the schematics, even if you're looking at the A500 board - i'm not sure how similar they are.

Anyway, good luck

John

EDIT: As a next step, look at the 68k link provided and the _RESET and _HALT lines, referred to as _RST and _HLT in the schematics. The fact that they're bidirectional is explained, but the fact that they're connected to so many other chips should start to look obvious - other chips may need to assert a full reset of the CPU, and some chips may need to know if a reset of the CPU is occuring (or indeed if it has been halted).

[rthorntn]

Awesome, thanks project23, the PCB Explorer is very cool.