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From: <bogus_at_does.not.exist.com>
Date: Sun Feb 27 13:22:22 2005

1 : Gnd
2 : Rst/ (grounded by Break key)
3 : 1MHz cloxk
4 : KB EN/ (Low for software scanning)
5 : PA4 )
6 : PA5 ) Inputs to row mux from VIA on mainboard
7 : PA6 )
8 : PA0 )
9 : PA1 ) Inputs to load inputs of scan counter (and thence to column
10 : PA2 ) selector) from VIA on mainboard
11 : PA3 )
12 : PA7 (Output from row mux to VIA to indicate key down in software scan)
13 : LED 3 (Low to turn on LED)
14 : CA2 ( Interupt output from ROW NAND gate to VIA)
15 : +5V
16 : LED 1
17 : LED 2

OK, how it should work :

The keyboard is scanned in hardware by the '163 counter IC1. It's clocked
by the 1MHz clock, parallel inputs from PA0--PA3, load input to KB EN/.
When the last signal is high (hardware scan), the counter runs and
selects each column in turn using the 7445 IC3. KB EN/ goes low to strobe
a particular column address into the counter from the VIA, which is then
held by the counter (used as a latch) as long as KB EN/ is low.

The outputs of the '45 go to the columns of the key matrix. The rows are
pulled up by 10k resistors and go to both the inputs of the '30 NAND gate
IC4 and the inputs of the '251 multiplexer IC2.

Let's consider the NAND gate first. The output of that goes to the CA2
input on the VIA. Normally all the row lines are high, so the output (and
CA2) is low). If a key is pressed, then when the counter/decoder selects
that column, the row line also goes low, so CA2 goes high. This causes an
interrupt.

The '251 mux is used when the keyboard is scanned in software to select a
particular row. It's enabled by KB EN/, and the select inputs go to
PA4-PA6. The output goes to PA7 on the VIA. For software scanning, the
VIA generates a 7 bit address on PA0--PA6. KB EN/ goes low, loading the
low part of the addess into the counter, causing the counter to hold that
value, and enabling the mux. The machine then checks the state of the PA7
line. If it's low, then that's the key that was pressed.

To debug it, I'd start by looking at the state of CA2 on the 'system; VIA
(IC17 on the US board). If it's anything but a steady TTL low, you've got
keyboard or cable problems.

Now for some more hints from the US version schematics :

The ROMs are IC71 to IC75. They are identified as :

IC71 OS ROM
IC72 View ROM (so it looks as if View was standard on the US machines)
IC73 DIsc & Net ROM (DNFS I guess)
IC74 BAISC ROM
IC75 Aux ROM

It sounds like you have the standard configuration.

IC52 is, indeed, the video ULA. IC92 is the serial ULA (the other custom
chip).

it appears that the Econet, Disk, and Speech 'options' are all standard.
Or at least, the only components identified as 'not fitted' are some
small passives in the audio circuitry.

The fact that the speech chip (IC21, TMS5220) is fitted would explain a
12 way ribbon cable to the keyboard. It carries serial ROM signals for
expansion ROMs for that chip.

One last bit of trivial. Link S37 selects between NTSC and PAL video.
Well, it will enable the PAL switching circuit. Of course you'd have to
change the colour encoder Xtal, and the OS ROM to get UK video out of the
machine.

-tony
Received on Sun Feb 27 2005 - 13:22:22 GMT