Working on the CPU of a Star Wars DE. Have it set up on the bench with +5v and GND and I get all three LEDs (Blanking, 5v & PIA) lit and staying lit.
I've read somewhere that this means one of the PIAs is bad.

What I don't understand is whether I use Leon's Test EPROM, the Star Wars EPROM or just no EPROM at all, there's no difference over the board. This is why I think the CPU isn't loading the EPROM. I've checked all the connections from the schematic between the CPU, the address buffers and the EPROM and there's continuity everywhere it should be.

I'm guessing one of the smaller chips might be the cause. Is there a way to check if a chip is malfunctioning without removing it from the board?

Hey thanks for the reply.

I just pulled the address line buffer chips (6C & 6D) and the 5E chip and popped them into a TL866 tester. They passed the logic test in that :/

The chip socket has been replaced with a new one and theres continuity between every pin in it and where each needs to connect to. The jumpers are over J5 and J3 (which i believe is correct for the 27C512 EPROM. The CPU was not working originally, but have since replaced it before I got to this poin and according to Leon's guide, all the pins are giving the correct readings.

Ben

Clock pin 39 pulses low on 3.4v , pin 37 pulses with 2.5v and pin 5 pulses with 4.3v

I pulled and socketed some chips earlier today and since then the blanking light now does not turn on at all, which I think is progress?
So now it's just the PIA and 5v LEDs which light and stay lit, blanking LED is completely unlit.

The ones I put in the tester which i mentioned earlier, 6C, 6D and also 5E

okay since my last reply, blanking LED has started coming on again :/

Pin 40 of the CPU reads low with the logic probe and about 4v with the DMM

well with the DMM it's 4.5v and with the logic probe it just says Low (which I'm guessing means its a GND pin). I was trying to work out what's up with the blanking issue and noticed there is a short between pins 28 and 29 on all the PIAs and pins 16 and 17 on the EPROM and RAM and also pins 6 and 7 on chip 5E. That's because in the circuit diagram PIA pin 28, EPROM/RAM pin 16 and 5E pin 6 are supposed to be connected and the same is true for pins 29, 17 and 7 on the PIAs, EPROM/RAM and 5E chip, respectively. I've been checking the traces and cant see any stray solder. The short is still there when I take out the EPROM, RAM and 5E chip so it must be one of the PIAs. Is it possible for a PIA to short two legs internally?

Ben

hmm the logic probe has 3 lights: red 'HI', green 'LOW' and a yellow 'PULSE'. I connect it to the +5v and GND then touch the pin and the green light is lit.

I'm pretty sure the CPU is fine as its brand new. The last one wasn't working. Is there a specific PIA which would stop the code from the EPROM from running? Would be nice to have one to start with. I didn't really understand how to test the legs of the PIA from Leon's guide. All of my input legs are stuck on High except for the lowest PIA are stuck on low.

Okay fixed the short. Still three lit LEDs :/

Oops! Pin 40 actually reads High, you are right

Turns out the 555 chip wasn't in properly so now the Blanking LED never lights.

Oops! Pin 40 actually reads High, you are right

Yes pins 28 and 29 had continuity, but that was a stray solder splash which has been fixed. Still hasn't solved the 2 red lights.

pins 2, 6 and 40 of the CPU are 5v not 4v, is that a problem?

pin 39 is less than 2 volts (1.88v), pin 5 is 4.3v, which is different to what Leon's guide suggests.

The data Lines (pins 26 - 33) are 5 volts (2 volts higher than they should be). With pins 33 and 31 at 4v. If I take out the chip at 5E (or put in the EPROM), pins 29 and 32 remain at 5 volts with the rest returning to 4volts.

Pin 20 on the EPROM is almost 3v (not sure if that's correct)

Pin 23 on all PIAs are about 4 volts except for the PIA 5F which is almost 5v. Changed the PIA, no difference.

aw that's good to hear especially after losing my mind over this slowly haha

Yes they all pulse with the probe. I have an oscilloscope but had difficulty using as I have no idea what to do

Also I noticed that the CPU gets a little bit hot after some time. Is that normal?

Am I right in thinking that the problem with the board is that the CPU isn't loading the program?

Hey Cactus Jack thanks for the input!
So are you saying that in my case, something else could be corrupting the data bus which is causing the blanking circuit to kick in and prevent the CPU from reading bytes? Would this cause the blanking LED to light? (my blanking LED does not light)

From Leon's guide, I've learned the address pins are pins 9 to 25, except pin 21 and on my board these are all about 2.5v.
So does the fact that the voltages for the data pins (26 to 33) are 4v-5v mean that something else is pulling up the voltage and thus causing this corruption?

Thanks again!

Do i just do this with the probe and check if the pulse light goes?

i'll look into working with the scope.

Not back in the workshop until later this week.
Seems like I won't be able to get much done without working out how to use the scope.

I will see what the pins on the CPU socket say with the CPU and the EPROM pulled.

I'm tempted to try creating the "NOP" CPU that CactusJack has mentioned. Though I understand the science behind it, I'm yet to know how I would use it's operation to fault find on the board.

Okay, so if I put a NOP CPU in, how do I go about identifying whatever might be loading down the address or data bus lines?