I've got an Ampro Littleboard that I'm trying to bring back to life.
Sometime last year it quit transmitting data on Port A (the console port)
so I shelved it until I had time to look at it.
I've checked the Z-80 DART with a logic probe and I'm seeing data coming
out of Port A (Pin 15 on the DART) when the machine boots up. I check the
input pin on the 1488, and get the same sound (the logic probe chirps with
each state transition) as I get out of the DART. However, checking the
OK so far
[OK, on first reading I read 'DART' as 'LogicDart' -- HP's
'advnaced
logic probe'. I now of course realiuse you mean the Z80 serial chip of
that neam...]
output of the 1488, I don't get anything. Since I
don't want to blow my
probe apart on 12V, I used an LED to watch the output on the 1488. The
LED lights the instant I attach it to the output and doesn't flicker or go
out when I reboot the computer to make it send data out the port.
The Port should be running at 9600 baud, so I should see at least some
flicker activity on the LED if the 1488 is toggling the output pin.
Before I spend the time to replace the 1488, I'd like to hear from you
folks if the chip is actually bad. Is what I'm seeing indicative of a
dead 1488?
For the output to be correct, 4 things have to be right :
1) The 1488 gets the right input signal(s)
2) The 1488 gets power
3) THe 1488 is working correctly
4) The output must bot be shorted to something else
You have checked (1) -- or have you. The 1488 contains 4 drivrs. One is an
inverter ,the other 3 are NAND gates. _Often_ the inputs on the latter
sections ae strapped together turning them into inverters too. But is
that the caser here? Have you made sure there's not another signal being
fed into the other input?
Now as for (2), check there's +12V and -12V on the right pins of the
1488. And that the ground pin is, indeed, groudned. Remember a supply
might be misisng because osmething is overloading it. Maybe a 1488 or a
dcoupling capacitor has shorted.
Nwo, if this is, indeed, the TxD line, the output shoculd be about -10V
when the thing is idle. Check that with a meter. If it isn't, then either
the 1488 is playing up, or the output is shorted to something. The
voltage you get there might tell you what it could be shorted to. If in
dout, remove the chip (or lift the output pin) and see if the votlage on
the TxD trace on the PCB goes away -- is it sourced by the 1488 or not?
If you get this far, I'd then take one of those bicolour LEDs with 2
wires. The have a red and green LED in inverse parallel inside. COnnect
that in series with a suitable reisstor -- 3.3k would proaly do --
between the TxD output of the 1488 and ground. You should see one LED
glowing when you turn the machine on./ See if you get flickers from the
other one (you can often see the 2 dice inside the package, BTW) when the
machine tries to transmit something. Having used that sort of tester on
9600 buad lines in the past, I will say that the flicker is clearly visible.
At htis point I'd proably replave the 1488. Put the new on in a socket if
there's space for it. I've had them fail in the past...
-tony