500 Board

falter
Posts: 31
Joined: Sun Dec 27, 2015 8:15 pm

500 Board

Post by falter »

Hi there,

Thanks to Dave and others for inviting me here.

I have an OSI 500 board. I believe it may have been used previously in a system -- maybe just a Superkit or perhaps a Challenger. What I've been trying to do is get it working on serial, but there's no signs of life. I have -9V and 5V+ confirmed to the proper pins on the 6502 and 1702 chips. Originally when I hooked it up I goofed and had the serial connections one pin over from where they should be (into the 20ma current loop). On correction, I now get a local echo back at the terminal if I type something, but nothing at all coming from the computer. The local echo occurs if I use a null modem cable. If I use a straight serial cable, there is no echo at all.

I am a bit confused about pinouts and such with serial. From what I read, null modem pin 1 is ground, pin 2 is TXD and pin 3 is RXD. If I understand correctly, I want the 'out' pin on my 500 board going to the RXD pin on the cable, and the IN connected to TXD? Not sure. I've tried it both ways but still no joy.

There are two pairs of black and red wires that I'm not sure of the purpose of. They are soldered in on the bottom left holes where the current loop/rs232 selector is, and also under a 7404. They go to a single, male 8 pin molex connector. There is also two ribbon cables (think they are PIA) soldered in on the left side of the board that go to connectors that would plug into an IC socket. In pictures of Challenger systems it appears the red and black pairs go to what I think is a RAM board (can't make out the model number in pics) or maybe video board.

Also, where the baud selection area is, there are some, I think, resistors of some kind that have been soldered in.

Anyway, any advice would be most welcome. Would love to see this thing come to life.
dave
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Re: 500 Board

Post by dave »

Hi, and welcome to the forum.

Yes, you'll be using a Null Modem Cable.

Regarding board function and serial port config, do you happen to have access to even a very simple oscilloscope? It will be a big help.

Can you post pictures of the red/black wires you're referring to?

It looks like the ACIA clock is set by a 555 timer, and the bit rates are derived from that (/1, /16, /64). The usual mode is /16, so the clock rate will by 16 times the bit rate. You can calculate the clock frequency based on the capacitor and resistor values, but much better to just scope the baud clock signal and be sure.

Good luck,

Dave
falter
Posts: 31
Joined: Sun Dec 27, 2015 8:15 pm

Re: 500 Board

Post by falter »

Here are a few pics. You can see where the connector and where they solder. Two of them solder in just below where the RS232/20ma loop selection is. Based on docs it looks like I'm set up for serial. But I don't know what to make of those little caps/resistors/? that are soldered in where the baud rate select should be.

I don't have a scope, but I'm thinking this might be the time to give in and get one. I have enough projects I think to justify it. :) Is there a scope model you'd recommend?

http://s1381.photobucket.com/user/uncle ... deshow/OSI
[Moderator edit (with permission): attached photos from photobucket link]
Attachments
500-connector.jpg
500-connector.jpg (566.76 KiB) Viewed 6741 times
500-mod.jpg
500-mod.jpg (937.31 KiB) Viewed 6741 times
500-ROM.jpg
500-ROM.jpg (776.87 KiB) Viewed 6741 times
falter
Posts: 31
Joined: Sun Dec 27, 2015 8:15 pm

Re: 500 Board

Post by falter »

Reading the manual a bit more carefully in the baud selection section.

If I'm understanding right, you jumper from one of the four holes on the left to the single hole on the right using a capacitor with a certain rating. I'm assuming the four holes on the left provide the ability to handle caps of different lengths.

The manual says for 1200 baud, you put a cap in the third row, with a max rating of .0110uf. The cap (I'm assuming that's what it is, the red thing there), says .01+-10%. I'm thinking that's the .0110, and therefore the board is set to 1200 baud?

The only thing I don't understand is the yellow cap directly beneath the red one. It's soldered in on the line that is for 300 baud.
john
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Location: New Brunswick, Canada
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Re: 500 Board

Post by john »

I use RS232 serial communication on my 510 CPU. It is a bit different as the 510 has a clock signal not a 555 timer.

However, it appear someone soldered one cap (red) for 1200 baud and one cap (yellow) for 300 baud. You should only jumper one
of them across to the 555 timer. Its hard to tell from the picture but it looks like the 300 baud one is already jumpered.

If you change to the 1200 baud cap, you may have to adjust that trim pot to get the clock rate correct.
A oscilloscope or logic analyser (saleae have a nice little one) would really help. But worst case connect to a PC serial port and adjust until it works.

Here is a diagram of my wiring:
rs232_cpu510.png
rs232_cpu510.png (17.7 KiB) Viewed 16711 times
C3A-8P 510CPU, 470Disk Controller, 527 memory (3x=48k), dual 8" floppy
dave
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Re: 500 Board

Post by dave »

@john: Nice diagram.

@falter: Given the number of projects you manage to take on, I think you will not regret picking up a scope.

At the moment the best bang for the buck is probably the Rigol ds1054z. For $399, you get a 4-channel 50MHz digital scope with decent usability. Check out the extensive discussion of this scope at the EEVBLOG. If you are so inclined, you can find simple instructions to hack the scope to enable the 100MHz bandwidth and all options (such as UART decoding, memory search, etc.)

If you aren't interested in hacking the scope or paying for upgrades, then also consider the new $386 Goodwill GDS1054B. It looks to have similar basic specs to the Rigol, but has separate controls for each channel, which is nicer and easier to use. However, it's not really upgradeable or hackable. I am considering getting one of these as a second, portable scope.

Those scopes will be great for retrocomputing projects. I'm assuming if you had a need for a high-end expensive scope, you would know it. And if you develop the need, and have to shell out for a more expensive unit later, there is not much penalty for having had one of the above--you'll still have a nice inexpensive second scope, which is not likely a liability.

The ability to upgrade/hack the DS1054Z to get the UART protocol decoder is nice, but not entirely critical, since the serial decode is software-based and slow, and only works on the onscreen display. If you need UART decoding, consider a cheap USB logic analyzer such as the Zeroplus logic cube (16 channels, $119). The Intronix Logicport is really nice, is rock solid, and handles 34 channels, for $389, as much as these scopes. The Saleae logic series looks very slick and uses USB3.0, but is even pricier.

Let us know if you do decide on one :-)

Dave
falter
Posts: 31
Joined: Sun Dec 27, 2015 8:15 pm

Re: 500 Board

Post by falter »

I tried wiring things up as per that great diagram but still no joy. At 300 baud there is no reaction on my terminal screen when I fire it up or reset.
Klyball
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Joined: Tue Dec 09, 2014 12:53 am

Re: 500 Board

Post by Klyball »

Your really shooting in the dark, without knowing what signals you have , Do you know if the cpu is even running?

Things that can be bad , the ram chips, the rom code , 8t26 buffers, a scope is pretty much a must to work on this old gear

the serial signal could be so outa spec you won't get any action on your terminal,
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john
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Re: 500 Board

Post by john »

You could try putting a LED/resister from the tx line to ground. At 300 baud you should see the LED flicker.
With 9-12 volts on the tx line I would try it with a 2200-3300 ohm resister. That would give the LED 2-3ma.
That should b just enough to see it without loading down the signal too much.

You could also try with a multimeter on DC voltage but many of them don't react fast enough to see
the voltage change when the characters are sent. I'm assuming you are looking for the H/D/M prompt so that
only gives you 5 yo 6 characters to see the voltage changing.
C3A-8P 510CPU, 470Disk Controller, 527 memory (3x=48k), dual 8" floppy
falter
Posts: 31
Joined: Sun Dec 27, 2015 8:15 pm

Re: 500 Board

Post by falter »

Yeah until I have a scope obviously not much I can do here. I always just try a power up test anyway on 'new' equipment. I did have a few things hooked up wrong on the first attempts. But yeah.. think this is as far as we get for now.

I wonder.. could I strip the RAM down to a base level just for testing purposes? I have tons of 6502 chips so I can swap those real quick just in case.
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