General Category > Hardware related

Bryces MegaRom

(1/4) > >>

geebus:
Hey folks, I'm learning electronics and thought i'd have a go at making Bryces MegaRom board. After going over every single line and matching it up to the circuit image that was on the how to, I'm getting nothing.the cpc464 turns on to a normal screen even though i've programmed (and verified) Protext to 04000 and maxam 1.5 to 08000.
I'm using a Winbond W27c01-70 EEPROM. is that not compatible?
Driving myself nuts here trying to find an issue and I just can't find any.
I did have a few to start with as the schematic and the circuit image don't match up (assuming the schematic is aimed at experienced people as the chips didn't have any gnd lines attached).
I've noticed that pin 4 (A14) from the amstrad isn't connected to anything on the schematic or circuit image. Is that required?

Bryce:
Hi geebus,

cool idea. Here's a few answers and tips:

- A14 isn't used. The MegaROM only does partial decoding using A13 and A15.

- The 27C0170 should work fine.


- Troubleshooting. Start with the enable signal (CE) of the EPROM. This pin should be normally high, but will drop to zero every time the MegaROM is being accessed. If it doesn't go low a few times during startup then there's a problem in the decoding section. If it does go low like it should, then...

- Either the MegaROM doesn't think it should answer or is unable to answer. The "Answer" comes on pin 43 (the cathode of the diode) D1. This should go high when the MegaROM wants to activate itself.

- If CE isn't going low on boot, then you'll have to work backwards through the logic, starting with the 74LS20. For CE to go low, four inputs need to go high: Pins 1,2, 4 and 5. Pin 1 is monitoring A15, so it should be ok. Pin 2 is the ROM 7 select switch, this should always be high if the jumper (J1) is set correctly. Pins 4 is the complicated one. Pin 4 goes high if the ROM being selected is within the range of ROMs on the MegaROM (ie: ROM 0-7 or 0-15) depending on the position of J3. Pin 5 is like Pin 2, but checking whether ROM 0 on the MegaROM is to be used, this should also be permanently high if the jumper (J2) has be set correctly.

After you have found out which of the four signals is missing, the problem will be narrowed down significantly.

Bryce.

geebus:
Thanks Bryce. I'll have a poke about with my scope later.Only issue I can foresee is that I only have an old analogue scope so won't be able to store the signal as the computer turns on.
I'll record the scope screen so I can get some pictures.Will post them up and you can see how badly I have my scope set. I have no idea what I'm doing with that either. Every day is a school day!

geebus:
Ok. Got a quick moment to check this out.

I used the scope and seen that the CE pin was going "high" with fussy activity (can't get it clear on the scope screen), then the fuzzyness stops for a moment, then goes again.

I also checked the 74ls20 pins 1,2,4 and 5. all of which were showing an increase when i checked them with the scope.
The issue I think is in fact with the voltage that is showing on them. it seems that it is only around 1.5v which wont be a high signal at all will it? it should be around 4.6v and up or so?
Edit: I just checked the voltage from pin 27 and it's showing ~1.4v.Something must be shorting somewhere right?

Bryce:
Shorting would also cause the CPC not to boot, it might be a bad solder joint or really dirty contacts on the expansion port.

Bryce.

Navigation

[0] Message Index

[#] Next page

Go to full version
Powered by SMFPacks Media Embedder
Powered by SMFPacks Alerts Pro Mod
Powered by SMFPacks Mentions Pro Mod