Kaypro 2X starts up with silly display

From: Robert Maxwell <RMaxwell_at_atlantissi.com>
Date: Tue Nov 4 14:41:35 2003

Tobias:

  Sorry I've fallen behind on the CCtech output, so this is a little behind
schedule.

  First, I have schematics of your 2X (also the 2, 4, 10, II, Robie - most
of them except the KII use the same motherboard with different BIOSes and
varying levels of IC population). I can scan them, and send them to you in
a day or three.

  Second: your display... unlike terminals where characters arrive serially,
and unlike PCs (and the KII) which have video RAM in the CPU's memory map,
the later Kaypros use a 6845 CRT controller and a custom 40-pin LSI chip.
They form a separate video data/address bus, so any characters incoming have
to be sent to a specified location using registers holding the target video
address. Since the BIOS has to keep on updating the character location to
write to, there's a lot of CPU cycles tied up in the transaction. This is
why the Kaypro video is glacially slow on full-screen updates.

  I would suspect that you've got a bad contact on the LSI that bridges the
CPU to the video. Try pulling (gently!) all the 40-pin ICs (6845 and Z80
too) from their sockets, and re-insert them. (Open inputs tend to read as
logic 1, and the worst of your errors occur in the upper screen, where logic
0 is needed for the address.) If enough corrosion built up to mess up the
keyboard, there's a good chance the IC sockets are a little wobbly too. You
might also rub down the IC pins using a pencil eraser if they look
exceptionally cruddy. *Standard ESD precautions apply*

Hope this is useful.

Regards, Bob (and his stable full of orphaned hardware).

Part of original message:
> Here are a few more details regarding the problem:
>
> I didn't start my Kaypro 2X for several years. Some weeks ago I gave it
> a short try and everything seemed to be okay except for the keyboard.
> Due to corrosion many keys didn't react. So I gave it to a friend, who
> first tested all the keys to see which were in need to be cleaned. The
> display was still okay at that time. Then he opened the keyboard and
> cleaned the contacts inside the keyswitches (he had to solder them out
> in order to do that). When he had finished the cleaning he assembled and
> reconnected the keyboard and started the computer. From that time the
> display went silly:
>
> - The powerup message shows up at random coordinates, however some of
> the coordinates are okay (see
> http://home.arcor.de/toa/tmp/kaypro-defekt.jpg). There are a lot of
> additional characters, most of them are '?' (n with ~ above). A few of
> the characters are blinking or show up with low intensity. Effect is the
> same with or without keyboard connected. Everytime I hit the reset
> button I see another random distribution.
>
> - When you type something on the keyboard (at the CP/M prompt) the
> characters show up at random coordinates but the blinking cursor moves
> in the way one would expect (one step to the right for each character,
> 3rd position of new line when you hit return). W/o any keyboard input
> the display is stable except for the blinking chars.
>
> - When you type in CP/M commands (I still have all the disks) they get
> executed but the output characters show up at random coordinates (see
> http://home.arcor.de/toa/tmp/kaypro-dir.jpg for an output of "dir").
>
> Has anyone schematics for the Kaypro 2X?
> Any hints what to do next?
>
> Tobias
Received on Tue Nov 04 2003 - 14:41:35 GMT

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