6502.org

For supporting a project which aims at building a TTL implementation of the 6526,
I dug out my old schematics for an experimental 6522 TTL implementation, the X22.

Because the most recent version of Eagle can't open Eagle 3.55 files anymore, I converted them to *.png.

Edit:
The thread became a bit longer than expected, also featuring:
6521, 6523, 6532, 6545, 6551\6850.

;---

The design was experimental and partially untested,
I'm not sure if the timers were cycle exact,
and if you try to make use of these echematics,
please take them with a grain of salt and try to build things a bit different from them.

It seemed to have worked with a Rockwell R65C02-2 running at 4MHz.



Test setup:

Bottom: my DRC2 SBC, which is supposed to be compatible to the SBC-2.

2B22 bus interface is plugged into DRC2.
X22 backplane is plugged into the 2B22 bus interface.

Plugged into the X22 backplane, we have:
1* 1PA22, Port A module
1* 1PB22, Port B module
2* 1H22, handshake module
2* 1T22, timer module
1* 1TC22, timer control module
1* 1SR22, shift register module
1* 1I22, interrupt module

First the bus interface 2B22, which also had been used for other projects (don't get confused).
The bus interface was plugged between the computer and the X22 backplane.



It assumes that the address bus is stable when PHI2 goes HIGH,
same thing for the data bus during write cycles...
what, of course, isn't the case in a C64 when CPU and VIC-II are sharing the bus.

For a TTL implementation of the 6526, please build things different, sorry.

Now for the X22 backplane:

Now for port A, which has open collector outputs: 1pa22

Port B has three state outputs: 1pb22

Handshake PCB: 1h22, two such PCBs required.

One PCB for port A, another (identical) PCB for port B, but jumper settings for both PCBs are different.

16 Bit timer modules: 1t22, two (nearly) identical PCBs required.



Some chips are not used on one of the PCBs, but having all of the chips plugged into both PCBs does no harm.

Since getting hands on down_counters turned out to be difficult,
I just had used up_counters while puttting 7404 inverters into the counter inputs and outputs...
to make them up_counters look like down_counters to the CPU.

Again: I'm not sure if timer overflow etc. is cycle exact, you better check !

Since we have two timers,
we also have a timer control PCB for the two timers: 1tc22

Now for that shift register PCB with all these odd operating modes, which gave me quite a headache: 1sr22

Now for the last part, the interrupts: 1i22



The 100nF capacitors are hidden below the IC sockets.
Again I'm not sure if things are cycle exact, and you better check before trying to rebuild it.

...and I think that's the whole set of schematics.

Again: it was experimental, not everything was tested, you probably would have to build things a bit different,
all the hardware went scrapped years ago, and I don't remember much of all the details after all these years.

Good luck.

(Muckraking around in a dusty drawer)
But wait: there is more.

Years ago I also did some paperwork for a 6522 TTL implementation, aiming at 20MHz.

Untested stuff, never built, a mere paper design, contains some odd tricks,
better analyze them schematics carefully before trying to put any of that circuitry into use.
...and better check, if timers and interrupts are cycle exact.

Here we go.
Address has to be stable before the _falling_ edge of PHI2,
same thing for data during write cycles.
IIRC there also is a trick for preventing ghost interrupts.

Again: good luck... and now: get off my lawn.

Great work ttlworks!

Your construction is always amazing. Quite the eye candy!