OSI 400 Board, 6800 and MOS 6501

[glitch]

We built up the original OSI 400 board blank that the reproductions were made from, but we built it up for the Motorola 6800, not the 6502. Why would you ever do that, right? Because we wanted to test some MOS 6501s!







It does in fact work:



We ran into a few problems using the official OSI docs for building up the 400 board for the Motorola 6800, which I'll document in a full writeup on glitchwrks.com at some point. There were major problems with the onboard RAM.

[musovern]

Great work on the 6800 and 6501, Looking forward to seeing your write up at some point on this board. I still have not been able to get it to work with the on board RAM.
Are you using your 68A2 Motorola 6800 serial monitor.? What baud rate have you set.? with your signal generator.

Never seen a MOS 6501 in person.

[glitch]

Yes, I'm using the 68A2 out of the OSI 510 board (though we made a little adapter to take a 28C64 EEPROM to make getting test code into the 6501 easier, so it's running out of that).

We were running at 300 bps (4.8 kHz on the signal generator), 50% duty cycle, 5V peak. I'd bodged a connection to a little MAX232 breakout board on the back for testing, I will build up OSI's recommended transistor based RS-232 converters soon. I am also going to calculate resistor/capacitor values for common bitrates and make that available -- for example, you change the 555's timing cap to 82pF and the 1K resistor above the trimpot to 47K to get 9600 bps (153.6 kHz).

I have not given up on the onboard RAM yet, but it's encouraging to hear someone else has also had problems with it!

[musovern]

That's a great idea for the little adapter board with the 28C64.

May I ask what RAM board you are using I'm guessing its the other board in the test setup. I've been looking at buying 1 of you Reproduction OSI 495 Prototyping Boards and making 32K RAM board design from your website. I kicking myself not getting one when I ordered my other boards from you. I did get the GW-OSI-RAM1 so might try that over the week again.

I'm now thinking i might try using a MC6800 and the 68A2 monitor as you have shown its working.

[glitch]

The RAM board is this one:

http://www.glitchwrks.com/2016/04/23/32k-ram-for-osi

It's one of the prototypes used in the design of the GW-OSI-RAM1. We also used the GW-OSI-RAM1 with it, but thought the prototype was more fitting for pictures/videos

The prototype is indeed built on a reproduction 495 board. The linked article includes schematics for doing it yourself!

[glitch]

I cleaned up the 400 board some:



Built the RS-232 level shifters from the manual, adjusted the values for the 555 bitrate oscillator to 153.6 kHz (9600 bps). Tacked down all the blue wires with epoxy.

Backside current state:



Level shifters/bitrate clock detail:



I used some old TO-106 package transistors for the level shifters, the one on the left is NPN, one on the right is PNP. To get 9600 bps, use a 4.7K as the manual says, then a 47K right above the capacitor. Use a 82pF capacitor -- the ceramic on there is unacceptable as it has a horrible tempco, just putting your finger on it will swing the frequency around 4 kHz in around 15 seconds! I've got some polystyrene caps on order for that, but it does work.

I got onboard RAM working, turned out to be a cracked pin on one of the 74LS03s. I am going to port Martin Eberhard's Altair 680 RAM tester to the 400 board to fully check it out.