The Case of the Missing Pixels
By David Wilson
Copyright (c) 1995 Apple Users' Group, Sydney
Republished from Applecations, a publication of the Apple Users' Group, Sydney, Australia.
david@cs.uow.edu.au
With versions 4 and 5 of the popular AppleWorks integrated package requiring 256k of RAM, many Apple //e owners replaced their 64k extended 80 column cards with a combination 1MB RAM and 80 column card from Sequential Systems called the Meg80z. Any owner of a PAL //e immediately noticed that the top left hand corner of their screen just wasn't being displayed correctly. David Wilson has found out why.
The Case of the 7 missing pixels or Meg80z + PAL //e = trouble
The Problem:
When used with a PAL Apple //e, the Sequential Systems Meg80z 1MB Auxiliary Slot RAMcard suffers a minor but annoying problem: the first 7 pixels in 80 column or Double Hi-Res mode are not displayed properly.
How to demonstrate the problem:
The following commands will quickly show the problem on a 50Hz Enhanced Apple //e equipped with a Meg80z:
] PR#3
] INVERSE
] HOME
The screen should be a white (or green or amber depending on your monitor) rectangle with a ] on the second line. Instead, the rectangle will have a small notch taken out of it in the top left corner. To illuminate the missing pixels type:
] CALL-151
* C073:3 I C055:0 I 400:20 I C073:0
If you now type control-C <RETURN> to re-enter Applesoft and then type
] NORMAL
] HOME
You will find that the missing pixels are now white while the rest of the screen has returned to black.
The Explanation:
The Apple // series of computers are designed to display their video output on a colour TV set. This would be fine except that there are at least 3 different colour TV standards in use in the world: NTSC (used in USA & Japan),
PAL (used in Australia and most of Europe) and SECAM (used in France).
As the original Apple ][ was designed in the USA it used NTSC to generate the colours and it output a video signal compatible with the TV sets used in America (using a vertical refresh of 60Hz). When the Apple ][+ was introduced Apple decided to support users in Europe (and Australia) by selling the Europlus. This was an Apple ][+ with jumpers changed on the motherboard to select 50Hz vertical refresh, a new main crystal to more closely match the 15625Hz horizontal refresh and a PAL colour card to encode the colour information using the PAL standard.
When the Apple //e was introduced, Europe and Australia got an entirely different motherboard to the American version. This //e had the PAL colour card integrated into it so that slot 7 was not wasted for the colour card.
Also, rather than having jumpers to select the vertical refresh frequency, the IOU chip was built in two different versions - one for 60Hz and one for 50Hz.
Now comes the problem - the Meg80z was designed using an American //e and was apparently never tested on a PAL //e before being released. So why does the video refresh frequency affect the Meg80z? The answer again has its roots in the past.
In order to display a picture on a TV screen the video display must be continuously refreshed. Dynamic RAM chips also need to be refreshed to prevent the information they hold from fading away as the tiny capacitors that make up each memory cell discharge.
The Apple ][ utilized the video refresh to also refresh the dynamic RAM. By carefully arranging the address lines used to access the video memory it was possible to get the dynamic RAM refresh for free. This explains the seemly bizarre non linear screen memory map.
The //e automatically refreshes both the main 64K and the auxiliary 64K as it creates the video display. This is however insufficient to refresh the full 1MB so the Meg80z must do some refreshing of its own. The only time available for this to occur is during the vertical blanking interval.
Auxiliary slot memory cards treat their memory as a number of banks of 64KB (16 for a 1MB card like the Meg80z). To access a different bank the program must select it by writing the bank number to location $C071 (or $C073 depending on the card). The only tricky part is that the video display data must always come from bank 0 otherwise the 80 column display would be messed up each time the bank was changed.
The problem occurs because the NTSC and the PAL standards have their vertical blanking interval at different stages. It appears that the Meg80z logic is confused by the extra 100 lines that a PAL //e has per screen.
The Solution:
To fix the problem, you'll need to install the IOU used in the NTSC Apple //e (part number 344-0020). If your Apple //e has a socketed IOU chip, you can simply purchase the new chip and replace the old one.
Platinum //e owners will find that their IOU is soldered. The process of removing the old chip is better left to the experts which unfortunately means that the labour cost will probably make the solution unfeasible for you.
The part numbers in question are:
344-0020 //e 60Hz NTSC IOU
344-0022 //e 50Hz PAL IOU
Do not confuse them with the Apple //c IOUs - they are different again.
344-0021 //c 60Hz NTSC IOU
344-0023 //c 50Hz PAL IOU
These chips are listed on the Apple Parts list. So any Authorised Apple Service Centre should be able to help you. The chip will cost approximately $45. The labour charge for replacing a soldered chip will be approximately $80.
Replacing the IOU chip with the 60Hz version does have some disadvantages. The video output will no longer be able to be recorded on standard PAL VCRs and some colour TVs may not be able to lock onto the picture giving a rolling display.
The IOU chip can be obtained from an Authorised Apple Service Centre. One such service centre is:
IC Technologies
124 Forest Road
Hurstville NSW 2220
For those who are interested in upgrading their Apple //e's and who don't mind missing seven pixels, Sequential Systems' Meg80z is a 1 megabyte, auxiliary slot memory expansion and 80 column card for Apple //e computers. The Meg80z is designed to be fully compatible with software programs that are designed to recognise large extended memory additions.
Sequential Systems
1200 Diamond Circle
Lafayette, CO 80026
303-666-4549

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