Microbee mods ?

[zonared]

Hi there everyone, I have managed to score a free Microbee, my first one (it works, except for keys not working, I've ordered new key kit).

I have a question, does anyone know why this chip 74LS123 (Multivibrator/pulse timer) is soldered to the I/O port?

DB15 pins    to   74LS123
pin1            ->   VCC (pin14) & B1 (pin3)
pin6            ->   B2 (pin4)
pin7            ->   A2 (pin2)
pin8            ->   A1 (pin1) and D23 (between printer port and I/O port)

green cap is 2A103 (0.01uF) and resistor is (brown/black/orange) 10kOhm

https://www.ti.com/lit/ds/symlink/sn74ls123.pdf says tw = 0.28 * Rt * Cext = 28ns

What's is for??? Rapid fire trigger for games? 

Thanks in advance...

[MbeeTech]

It looks to me to be the same circuit used to convert the parallel port to make it work with centronics interface printers.
This circuit (or equivalent) is what is inside the standard microbee parallel printer cable.
Why someone would put it inside a Premium Bee is somewhat of a curiosity.

[zonared]

@mbeetech, thanks for your reply. This is very interesting indeed. I guess whoever owned it before me didn't have the Microbee external adapter/cable and figured they would build it in. Do you have any info this this adapter or circuit?

Also, do you have any details on the pinouts of all the connectors on the back of the Microbee, the DB15 parallel port included?

thanks again.

[ChickenMan]

Here is the pin out of the Parallel port and connection to a Centronics port

   

[MbeeTech]

 do you have any details on the pinouts of all the connectors on the back of the Microbee, the DB15 parallel port included?

You can apply for access to the Microbee Technology File Repository :
check out this thread for how to do that.
https://microbeetechnology.com.au/forum/thread-15.html

You can then download the Microbee Technical manual which has all the details you need.

[zonared]

Hi there, well, that definitely explains it, thanks ChickenMan. I'll also checkout the file repository.

I've started the keyboard repair too, I've unsoldered 15keys do far 👍