M200 interfacing continues...

From: Sellam Ismail <foo_at_siconic.com>
Date: Thu Jan 2 18:08:01 2003

On Thu, 2 Jan 2003, Tony Duell wrote:

> > The main problem I'm having is reading the control lines of the 6522.
> > According to the spec sheet, CB1, CB2 and CA2 are all supposed to be TTL
> > level outputs when programmed for such. So they are, but I'm having
> Do you mean output or input here? If you're reading a pin, then I would
> have assumed you were using it as an input.

Sorry for confusing the hell out of anyone. I meant input. From the 6522

"CA1 is a high impedance input, while CA2 represents one standard TTL load
in the input mode."

...and regarding CB1 and CB2...

"Each control line represents one standard TTL load in the input mode and
can drive one TTL load in the output mode."

> Try a pull-up instead (TTL sinks a lot better than it sources). In other
> words change the end of the resistor from ground to +5V
> make this +5V
> |
> V
> > READER SIGNAL----+----/\/\/\-------GND
> > |
> > |
> > 6522 CB2

Wouldn't this give me a default state of logical true on the input pin?

> If you still have problems, then maybe the driver chip in the M200 has
> died. It happens. I've had a couple of HP9810 calculators across my bench
> where there were 10 or so dead TTL chips (74Hxxx TTL chips especially)
> that appeared to work, but which couldn't give a proper output signal,
> so the chip it was connected to didn't always get the right logic level.

Well, it works, just not all the time. So from what John has told me, it
seems like the signal is just being lost half the time.

Sellam Ismail Vintage Computer Festival
International Man of Intrigue and Danger http://www.vintage.org

 * Old computing resources for business and academia at www.VintageTech.com *
Received on Thu Jan 02 2003 - 18:08:01 GMT

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