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Cumana disk interface problem

Posted: Thu Jun 06, 2019 8:57 am
by Vyper68
I have built a Cumana disk interface and all the chips are in place as well as all the passives and 7905 Voltage regulator. So the interface is complete. The ROM is one that was supplied with the PCB's and is a 27c64 with the 2K Bootstrap and I have also used a 28c64B with the full 8K ROM image from the Oricutron. I have made some IDC connectors and have checked the pins so pin 1 goes to pin 1 on the expansion port of the Oric. So everything is where it should be and connected how it should. I have a power in connector fitted to the board and a power out for the Oric and I have a 10v PSU centre negative jack. So when powered it gives the correct voltages on the 5V side and Ground, so the power seems good.
Problem I have is when the Cumana board is connected to the Oric it will not get past the ROM initialisation. So Black/White vertical bars or horizontal bars. From repairing the Oric-1 you can tell the stage of the boot sequence by the garbage on the screen. So normally it seems to be the ROM, Clear the RAM then start the 6522 and then you get the welcome screen. I could be wrong so don't take it as fact, just from observations.
So I have a Oric-1 with a 6502A and my Atmos with the 65c02P4. I have a original V1.1 ROM and EPROM with V1.1 ROM and I know the EPROM cannot be used so I swapped the original ROM between machines.
So with the IDC connected and Powered from the Cumana the Oric-1 and Atmos will not start. With the IDC disconnected and Powered from the Cumana the Orics will not boot. With the IDC connected and powered from my normal Oric PSU the Orics will not boot.
Naturally with the Orics disconnected from the IDC and powered the from the normal Oric PSU they work fine. So it's something about the Cumana they don't like and given it's both machines it something to do with the Cumana board sadly. I had no Floppy drive connected during this as I was just trying to see if I could get to the Boot screen. The other thing is the IDC for the Cumana to Oric is about 3 Inches long if that makes a difference.

So does anyone have any ideas of what to try? Would an amplibus be something that would help I know that there are none available now and would we ( I would be up for making some ) be able to make some more if the Gerbers or .BRD or .SCH files were released to the Oric community

Re: Cumana disk interface problem

Posted: Thu Jun 06, 2019 10:44 am
by iss
First, 2 things in your post get my 'special' attention:
Vyper68 wrote:
Thu Jun 06, 2019 8:57 am
...and I have a 10v PSU centre negative jack.
Maybe it's just typo, but normally it should be center POSITIVE jack.
Vyper68 wrote:
Thu Jun 06, 2019 8:57 am
...With the IDC disconnected and Powered from the Cumana the Orics will not boot.
This means Orics mainboard are absolutely without power, so not booting is the only expected result :).

Anyway, the best is to power Cumana board with 12V/5V PSU AND Oric with its standard PSU.
After you turn on the system, it's state can be totally unpredictable, so let's consider that the "system start" is not power-on, but the moment when you press (and hold shortly 1-2 sec) and release the RESET button attached to the Cumana board!
So, what happens in this case? (BTW, some photos may help lot :)).

Things that can be easy checked:
- use voltmeter to measure the correct voltages on the usual power pins of the TTL chips on Cumana and on the mainboard;
- use voltmeter attached to the RESET pin of the CPU (pin 40) after power-on you should have high level (>4.5V), when press RESET button on Cumana, voltage should drop very low (<<1V) and when you release the button it should go high again.

Next, you can try to replace 2x'LS244 and 1x'LS245 on the Cumana (if you have available spare chips).
The amplibus is good option but Cumana should work without it - my board work fine with all Oric-1/Atmos I have.
The longer ribbon cable can be source of problems but the issues are different: sporadic freezes and random chars at random screen positions.

Do you have EPROM programmer? If so it would be easy to write kind of diagnostic ROM to be inserted in place of Cumana ROM.

Re: Cumana disk interface problem

Posted: Thu Jun 06, 2019 2:03 pm
by Chema
If you are facing the usual problems with Microdisc units (Cumana is just a clone, I think, so it may well be), you can end up with a non-booting Oric with the symptoms you describe.

You should have some activity when pressing the RESET button on the Cumana interface, but maybe just a RESET to the black/white bars.

I think the Cumana, as well as the Microdisc board, has a variable resistor labeled RV1. Tweaking this, changes a delay regarding the /MAP signal and the register read operations (or something along that line, anyway), and may help. Too short the time window, and the system will fail up to the point where the Oric won't boot... It is worth a try. In Silicebit's Microdisc clone a good starting value is 1450 ohms. If it does not work, try increasing the value. At some point the Oric may show a "normal" screen with a message on the top row asking the user to "adjust RV1", or it may just boot into "Insert system disk".

But, again, many original drives (usually Microdisk to be frank) refuse to work at all without the Amplibus.

Re: Cumana disk interface problem

Posted: Thu Jun 06, 2019 4:16 pm
by Vyper68
Yeah, somethings not right with the PCB.I get 4.9V on most IC's but the LS244's ( IC1,IC3,IC4 ) are showing -4.4 to -4.5V and the LS93 ( IC14 ) is showing -3.9V. I have checked them with my EPROM programmer and they pass the checks on the logic gates so it's something else on the PCB causing this but there is not many discrete components. I will start testing all the Logic chips but all the rest are showing 4.9v and the board itself is showing 4.9v on the power rails so it's not like something is pulling the power low like you would see if there was a faulty chip somewhere.

The PSU is a 10v 1A centre negative PSU maybe it's not enough for the PCB? and not enough to power the PCB and Oric together and I know the Cumana had has a transformer so what amps does that output, anyone know?

Re: Cumana disk interface problem

Posted: Thu Jun 06, 2019 8:24 pm
by Chema
+2A are recommended for Silicebit's microdisc clone, though that is intended to power the board and two FDDs, IIRC. Maybe the board and a kind of Gotek or HxC needs less. But 1A sounds too little for me, based on those recommendations.

If you plan to power the oric too, better go for 3A. The Oric itself gets ~600mA.

Re: Cumana disk interface problem

Posted: Thu Jun 06, 2019 8:43 pm
by Silicebit.
If you made the measurements between the pins 10 and 20 of LS244's, I think that 4.4V - 4.5V can be usual. For LS93 3.9V is strange. For LS224's, I can see each pin 10 (GROUND CHIP) go to resistors R1, R2 and R3 (GROUND rail). Note that for LS93 pin5 is VCC and pin10 is GND. You must adjust RV1 too.

Re: Cumana disk interface problem

Posted: Fri Jun 07, 2019 9:09 pm
by Vyper68
Yes Slicebit I was taking the readings from the wrong pins they all read 4.9v :shock: :oops:

Will look at RV1 and see what I get some time. So it looks like it’s probably the power or the ROM or something else