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C070243 Rev C (520ST+, 1MB, quite botched with extensions)

Posted: Sat Jan 13, 2018 4:21 pm
by Gunstick
Hi

this is the original 520ST+ used to create Dark Side of the Spoon
Gunstick520STplus.jpg
Gunstick 520ST+ used for Dark Side of the Spoon
Gunstick520STplus.jpg (1.24 MiB) Viewed 1225 times
So what are you seeing here?
On the bottom is the 512K RAM, with piggyback soldered on a second 512K bank. This soldering job was originally done by Atari, and so the + in name was created.

From the resistors on the left of the RAM, and from the shifter, runs cables to the 2MB expansion board by GE Soft. Plugging this expansion in will deactivate the 512K expansion from Atari.
Gunstick520STplus-RAMshifter.jpg
GE soft shifter connection for RAM extension
Gunstick520STplus-RAMshifter.jpg (375.86 KiB) Viewed 1225 times
Unused RAM is sad, so here comes the 3MB RAM configuration using a little array of chips. Don't ask me how or what it does. If I remember correctly they put the 512K after the 2MB. So you end up with the following ram configuration: 0.5M + 2MB + 0.5M = 3MB
Gunstick520STplus-3MBcircuit.jpg
3MB expansion
Gunstick520STplus-3MBcircuit.jpg (336.57 KiB) Viewed 1225 times
So far for the RAM.
On bottom left there is a switch which was used to switch TOS, but I plugged the switchable EPROMS into another ST so you can't see them on this picture.

Under one of the ROM chips is a realtime clock with the battery pack near the ROM port. The RTC takes also some signals directly from the CPU pins.
Gunstick520STplus-RTC.jpg
RTC under ROM
Gunstick520STplus-RTC.jpg (486.49 KiB) Viewed 1225 times
And finally we have the second switch on the bottom right. It switches on the hardware overscan. So you could even have overscan in high rez. Very cool. It needs just a 7400 with diodes and resistor which you can see where the RF modulator would usually sit.
Gunstick520STplus-overscan.jpg
hardware overscan
Gunstick520STplus-overscan.jpg (574.79 KiB) Viewed 1225 times

Re: C070243 Rev C (520ST+, 1MB, quite botched with extensions)

Posted: Thu Sep 19, 2019 11:24 pm
by Gunstick
Gunstick wrote:
Sat Jan 13, 2018 4:21 pm
Gunstick520STplus-3MBcircuit.jpg
I found some paper scribblings when I built the 2.5MB to 3MB converter. It has the following chips:
7400 quad 2-input NAND gate
74375 quad bistable latch
74139 dual 2 to 4-line decoder/demultiplexer, inverting outputs

As input it uses CAS-L & CAS-H , RAS, A9
I should be able to reverse engineer this when I get more time. Will write this below when done.