Master motherboard without the rest of the machine...

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myelin
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Master motherboard without the rest of the machine...

Postby myelin » Fri Sep 29, 2017 3:19 am

Thanks to Hywel, I now have a Master 128 motherboard... time to see if I can get it running without its usual power supply, keyboard, etc.

It looks like it should be able to run without the -5V line connected, so I soldered up a few alligator clips (I don't have any spade terminals) and hooked up 5V/GND to a 10A 5V plugpack I had handy.

master_powered_with_alligator_clips_sm.jpeg

It seems to work -- boots up and shows Acorn MOS / Acorn 1770 DFS, then hangs, presumably because it's trying to boot off the nonexistent disk. I haven't connected anything to the battery connector, but once I have something it recognizes as a keyboard (either one of Prime's MT8816-based boards, or an idea I have involving a CPLD), I should hopefully be able to reset the configuration memory and get it to do something useful :)
SW/EE from New Zealand, now in San Francisco: http://myelin.nz/
Having fun making hardware projects for the Electron!
So far: 32k flash cart, USB cart interface, 3-cart expansion, Elk PiTubeDirect. Later: Dual ported ram cart.

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1024MAK
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Re: Master motherboard without the rest of the machine...

Postby 1024MAK » Fri Sep 29, 2017 5:47 am

All the logic runs from the +5V supply. The -5V supply is used for the audio tape circuitry and some of the I/O systems.

It is possible to use a BBC Micro keyboard. I can't remember where the details are, so try searching the forum :wink:
The service manual may also be useful.

The boot device (if any) is determined by the configuration settings that are normally stored in the CMOS SRAM that is part of the real time clock (RTC) chip. But without a battery, this will take up random values at power up. Without a working keyboard, to escape from the system looking for the 'boot' file, you are stuck.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
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myelin
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Re: Master motherboard without the rest of the machine...

Postby myelin » Fri Sep 29, 2017 5:59 am

That's interesting! I'd spotted the two keyboard options on the motherboard, but from the schematic, it looks like I'd have to remove IC16, the keyboard encoder, to make it work with a BBC Micro keyboard (plugged in to PL7). I'd assumed the scan matrix would be different between the different keyboards, but if not, that could be an interesting project :)

I've started on some VHDL that'll let me connect a CPLD to the keyboard connector and emulate a keyboard... will post more here if I get it working!
SW/EE from New Zealand, now in San Francisco: http://myelin.nz/
Having fun making hardware projects for the Electron!
So far: 32k flash cart, USB cart interface, 3-cart expansion, Elk PiTubeDirect. Later: Dual ported ram cart.

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myelin
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Location: San Francisco, CA, USA
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Re: Master motherboard without the rest of the machine...

Postby myelin » Fri Sep 29, 2017 6:39 am

I realized I can also just short pins together on PL11 and PL24...

PL24 pin 3 (C3) and 6 (R4) = 'R'
PL24 pin 13 (KBD SW) and 14 (GND) = BREAK

Powering on and hitting R-BREAK using this technique gives "Acorn MOS" / "Searching" / "This is not a language".

Powering on with R pressed gives the CMOS RAM reset message, after which hitting BREAK gives "Acorn MOS" / "Acorn ADFS" (in MODE 6).

So it looks like it's working as expected :)
SW/EE from New Zealand, now in San Francisco: http://myelin.nz/
Having fun making hardware projects for the Electron!
So far: 32k flash cart, USB cart interface, 3-cart expansion, Elk PiTubeDirect. Later: Dual ported ram cart.

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myelin
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Re: Master motherboard without the rest of the machine...

Postby myelin » Fri Oct 06, 2017 6:12 am

Success -- I can now use my laptop as a keyboard for the barebones Master 128! The multi-key rollover is a little odd, but that's probably a bug in the Python code that runs on the laptop. Otherwise it's working pretty well; I can hold R on startup to reset the CMOS, enter all the *CONFIGURE commands, *UNPLUG and *INSERT ROMs, and so on. It doesn't have a -5V line, so the cassette input probably doesn't work, but the basics seem OK :)

As always, open source:

https://github.com/google/myelin-acorn- ... d_keyboard

I haven't documented it awfully well, so if anyone wants to try this at home, reply here and I'll walk you through it.
SW/EE from New Zealand, now in San Francisco: http://myelin.nz/
Having fun making hardware projects for the Electron!
So far: 32k flash cart, USB cart interface, 3-cart expansion, Elk PiTubeDirect. Later: Dual ported ram cart.


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