2 Jan
2003
2 Jan
'03
10:24 a.m.
From: "Sellam Ismail"
<foo(a)siconic.com>
On Thu, 2 Jan 2003, Tony Duell wrote:
Sorry for confusing the hell out of anyone. I meant input. From the 6522
datasheet:
"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."
Hi
For a TTL, one normally has an open pin set to one. One pulls
it to zero to be active. Ever notice that all of the strobes and
such for things like parallel ports on a PC are always negative.
For those pins that you are using as inputs, make sure that
you've set the direction register correctly.
Dwight
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. 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
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