Need help with my oric 1

If you want to ask questions about how the machine works, peculiar details, the differences between models, here it is !
How to program the oric hardware (VIA, FDC, ...) is also welcome.
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Symoon
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Re: Need help with my oric 1

Post by Symoon »

Stupid question on your 1st attepmpts: do you 1st plug the power supply to you wall plug, and then plug it to the Oric ?
By doing it the other way (plug Oric 1st, then power supply) my Orics remain in this garbage boot screen.

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ibisum
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Re: Need help with my oric 1

Post by ibisum »

Whenever I see garbage at boot like this with one of my Orics, I get a different power supply. Works every time - Oric needs the amps!

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protek
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Re: Need help with my oric 1

Post by protek »

Symoon wrote:
Thu Aug 27, 2020 8:45 am
Stupid question on your 1st attepmpts: do you 1st plug the power supply to you wall plug, and then plug it to the Oric ?
By doing it the other way (plug Oric 1st, then power supply) my Orics remain in this garbage boot screen.
I’ve got best results with having PSU connected to both Oric and a mains splitter with an on/off switch. When I flick the switch, the Oric boots every time.

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mikeb
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Re: Need help with my oric 1

Post by mikeb »

Tips above about power supply types and order-of-switch on are correct, but all that comes back to the power on reset not working well enough.

If the POR worked, a sluggish power application wouldn't matter, Oric would start correctly. As it is, (especially true with the original reset capacitor + age), if the 6502 comes out of reset while the supply is still stabilizing, it tends to either not-work at all, or crash.

I mentioned shorting Pin 1 to Pin 40 (on the CPU) after power up, which was tried and didn't work. So hopefully it's not that ...

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Chema
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Re: Need help with my oric 1

Post by Chema »

One stupid idea... if you have an oscilloscope at hand, can't you check for activity in, for instance, address line A0? If the CPU works it will start getting data at least the RESET vector address and the first instruction.

Not sure if the ULA would interfere, though, while reading the screen memory, but maybe worth a try...

Or maybe there is another signal you could check to make sure the cpu is booting.

One thing I saw many times in repair videos is checking for plausible signals in the clocks, RAS, CAS, RESET and Irq and the address and data buses.

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mikeb
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Re: Need help with my oric 1

Post by mikeb »

Chema wrote:
Thu Aug 27, 2020 11:22 pm
One stupid idea..
Not a stupid idea!

Address lines A0-A15 (as on the CPU and expansion port) are not affected by the ULA (ULA only reads A15-A8, and doesn't connect to A7-A0). So activity on any of the address lines indicates running CPU. Although, if the CPU reads "bad code" from memory, it will quickly crash and stop doing that. (Hence the Diag rom staying in ROM until sure that the RAM works, and *definitely* staying in ROM twiddling thumbs when RAM failed!)

6502 crashes the moment it reads any memory location holding a byte ending in 2 (#02, #12 ...) apart from #A2 (a legit opcode). You'll hit one eventually.

ULA reads then screen memory "in secret" directly by hijacking the Row/Column addresses while the CPU is shut out. CPU has no idea this happens.

You could check for scope (or logic probe at a push) activity at every one of: ULA Pins 38,36,37,4,3,2,40,39 (which should be all active all the time with Row and Col addresses, even with a dead CPU), and also IC8 pins 7,4,12,9 and IC 20 pins 7,4,12,9 (output from the multiplexer to DRAM). A mysterious loss of "activity" here indicates the multiplexer isn't happy any more.

The CPU, when running, pulses the SYNC pin (pin 7 of CPU, not shown on Oric schematic, as it's not used). Again, that will stop quickly if the CPU crashes or never starts :)

A scope is very handy if available. Clock going into the CPU (p37)/out of the CPU (pin 3, 39) all at 1MHz. Data bus/Address bus -- if any one line within a bus looks very different to the others, something may be holding it down or up. Or just letting it float about.

Dumb stuff: like pin 40 of the CPU shorted to ground (never ending reset ...) would spoil your day too.

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