Atari 8-bit Computers: Differences between NTSC and PAL Models
Introduction
Once I wanted to repair an Atari 800. It was broken and some chips were missing. It was a PAL model for european marked (PAL-A is used in Germany, PAL-B is used in the UK). Repairing the computer was not easy since most descriptions one can find on the internet refer to the NTSC (US) model.
By using parts from an NTSC model I was able to find out the diferences and repair the old computer. Maybe someone else has a similar problem and might find the information I offer on this page of value.
Components
The following components are different between NTSC and PAL models:
- The crystal. At the time the Atari computers were designed and build, hardware components were much more expensive than today. So the same crytsal was used do clock the CPU and all other devices. The draphics device needed to be clocked with a certain clock rate in order to produce the proper TV signals, i.e. screen refresh rate (59.94 Hz on NTSC Ataris, 49.86 Hz on PAL machines). This leads to flashy CPU clock speeds: the NTSC model is running on 1.7897725 MHz while the PAL system runs on 1.773447 MHz, even though the CPU could run at 2 MHz.
- The modulator. The modulator processes the pure video and sound information comming from the graphics/sound device in a way that a TV set can display the picture and reproduce the sound.
- The operating system ROMs. They also differ from NTSC to PAL. Since both machines are running on different CPU clock speeds, compensation for that is required in order to produce proper Baud rates, timing etc. in both cases. OS ROM "F" has PAL code C015299 and NTSC code C014599. OS ROM "E" hat PAL code C015199 and NTSC code C012499 (see table below).
- The graphics chips, ANTIC and GTIA (CTIA), are also specially designed for either PAL or NTSC. By the way, there seems to bee a colour bug regarding PAL machines! I have only one game which is problematic. It's Electronic Atrs' Hard Hat Mack. This game shows proper colours on my NTSC computer only. The PAL picture is looking very fancy. Please see http://www.steverd.com/faqs/atari-8Bit.htm for more details on the colour bug. ANTIC has device code C014887 (PAL) and C012296 (NTSC). CTIA has NTSC code C012295, it's PAL code is not known to me. GTIA has PAL code C014889 and NTSC code C014805 (see table below).
CTIA or GTIA?
The following short BASIC program helps you determine, whether you have an older CTIA or a newer GTIA chip running in your ATARI 400/800 without having to open it. If the screen is blue, after running the program, you have a CTIA. If the screen is black, then you have an GTIA.
10 GRAPHICS 9
20 GOTO 10
Device Codes
| Place |
Name |
PAL Code |
NTSC Code |
| A301 |
CTIA |
? |
C012295 |
| - |
GTIA |
C014889 |
C014805 |
| A302 |
ANTIC |
C014887 |
C012296 |
Table 1: Graphics Devices
| Place |
Name |
PAL Code |
NTSC Code |
| A401 |
OS ROM "F" |
C015299 |
C014599 |
| A403 |
OS ROM "E" |
C015199 |
C012499 |
Table 2: ROM Chips
Pictures
Please find below some pictures illustratinge the things said by showing the internals of an Atari 800 (NTSC version).
Figure 1: The TV Modulator
Figure 2: The Crystal
Figure 3: The CPU Board, C015500 Revision 6
Figure 4: The OS Board, C012989 Revision E
Related Sites
Atari 8-bit FAQ - http://www.steverd.com/faqs/atari-8Bit.htm
Suppliers
Best-Electronics California - www.best-electronics-ca.com/
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this page was updated: 09.05.02 14:10:30
holger@zahnleiter.org
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