Chuck(G)
25k Member
Part 1 of this blog post can be found here.
So, basically, the problem boils down to the problem of having a black box with 10 inputs and 8 outputs and having to figure out what's inside. We do know that it's simple combinatorial logic and there's no memory inside. Furthermore, none of the outputs are fed back internally to the inputs, all of which greatly simplifies our task.
All we have to do is present every possible combination of the 10 inputs (all 1,024 of them) and record each of the 8 outputs as we change the inputs.
To do this, I threw together a simple circuit (PDF attached below) consisting of a few simple LSTTL ICs: A 16 bit ripple counter and an 8-bit multiplexer that can be controlled by a couple of output bits on a PC parallel port and serialize data to be read by looking at a single parallel port status bit. Note that this circuit allows for additional counters to be attached in addition to another multiplexer to allow up to 16 outputs to be read. The circuit can be thrown together in an hour or so on a hunk of perfboard--be sure to leave room for a socket for the PAL. Add a +5 volt supply and you're good to go.
Attached, you can see what mine looks like. The white IC socket is for the PAL.
On to the programming of the reader...
Click here to go to part 3...
So, basically, the problem boils down to the problem of having a black box with 10 inputs and 8 outputs and having to figure out what's inside. We do know that it's simple combinatorial logic and there's no memory inside. Furthermore, none of the outputs are fed back internally to the inputs, all of which greatly simplifies our task.
All we have to do is present every possible combination of the 10 inputs (all 1,024 of them) and record each of the 8 outputs as we change the inputs.
To do this, I threw together a simple circuit (PDF attached below) consisting of a few simple LSTTL ICs: A 16 bit ripple counter and an 8-bit multiplexer that can be controlled by a couple of output bits on a PC parallel port and serialize data to be read by looking at a single parallel port status bit. Note that this circuit allows for additional counters to be attached in addition to another multiplexer to allow up to 16 outputs to be read. The circuit can be thrown together in an hour or so on a hunk of perfboard--be sure to leave room for a socket for the PAL. Add a +5 volt supply and you're good to go.
Attached, you can see what mine looks like. The white IC socket is for the PAL.
On to the programming of the reader...
Click here to go to part 3...
