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Old 17th Feb 2015, 1:03 am   #1
jay_oldstuff
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Default Got me an Atom

Hi all.

I've just got what I consider one of the holy grails of British micro computer history, namely the forerunner to the BBC, the Acorn Atom.

I have been after one of these to go alongside my Research Machines 380Z and my Sinclair ZX80 for some time now. I even had the opportunity to buy a mint one in the late 90's but the guy wanted £25 which at the time I thought was ridiculous for an old micro computer, (at the time I was picking old micros up for a couple of quid each at boot sales). Oh if only id known

Anyway I have one now, it's not mint and cost me considerably more than £25.

For anyone unaware like the Sinclair ZX80 you could buy an Atom ready made or in kit form, I think mine is a kit version judging from some of the shall we say less than professional soldering of the components.

I have downloaded all reference data I could get my hands on and will be digesting that before even thinking about showing this beast some power.

see pics below

Jay
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Old 17th Feb 2015, 9:00 am   #2
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Default Re: Got me an Atom

I've still got one of these, built from a kit back in 1981. It's been much modified adding extra RAM to fill in the gap in the memory map plus I've added a UART and wrote a simple monitor programme. I've got some software for it on cassette tapes which I've put onto the PC and can burn an audio CD if you want it. I've also got all the manuals that came with it although these are available on line.

One thing you have to be wary of is the internal 5V regulators, the heatsink is hardly adequate for the basic Atom with the minimum of RAM and peripherals. Most people who had them and added more memory etc bypassed the internal regulators and used an external 5v power supply.

If you do need any information and/or a CD of software I'll be at Golborne in April.

Keith
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Old 17th Feb 2015, 12:55 pm   #3
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Thanks for the offer Keith, I think I've downloaded just about everything ever uploaded to the Internet about the atom. There is a thriving forum too unfortunately I'm still waiting for there administration to approve my membership so I can't post there for help yet.
My Atom seems to have had a number of modifications to it over the years. The seller mentioned additional RAM and it looks to be fitted in a piggy back fashion, see pics I posted.
Also note in the last pic mods to the power input, the connector has been replaced by flying leads terminating in a phone plug. This will all need proper investigation before I apply power.
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Old 17th Feb 2015, 1:25 pm   #4
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Default Re: Got me an Atom

That was one way of adding extra RAM. I made up an extra board with a header and socket for the processor and used a couple of 8k RAM ICs plus a couple of 2114s to add an extra 17k of RAM. I also fitted extra ROM sockets and an 8k EEPROM split into two 4k blocks so I could save working code to EEPROM and ultimately copy it to an EPROM. I had a switch to select which EPROM or EEPROM was in circuit.

Keith

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Old 17th Feb 2015, 3:09 pm   #5
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Cor, that brings back memories. The best things about it where (are) small enough for you to remember where all the memory was (as far as I remember variables where at the second half of page zero RAM) and fast enough to be useable. Great keyboard and inline assembly language built in to the BASIC. The book 'Atomic Theory and Practice' is a must.

One of the nicest early computers, the 6502 processor command set is very easy to learn and trips of the fingers when programming and the BASIC is very good too. I made a battery backed SRAM module for mine to save retyping programmes.

One thing to look out for is that the chips being upside down can fall out with repeated keyboard use, the trick was to solder two diagonal pins into the sockets.
 
Old 17th Feb 2015, 9:54 pm   #6
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Default Re: Got me an Atom

Oh wow, that has triggered the old nostalgia in me and sent me scurrying to find my trusty old Atom. It almost certainly hasn't been out of its box in well over 20 years but I've also un-earthed some associated bits and pieces.

It was bought in Bath - you couldn't get one in Bristol, at least not one that had the floating point rom fitted. 2K ram - 512 bytes for graphics, 1K for system and 512 bytes for your own program if memory serves. Even then, the 5v regulator got stinking hot ! I later got one of them new fangled external SMPS for it - it was nearly as big as the atom but was good for 10A if I recall correctly. It was then off to buy some 2114's, I went to Tandy and when they said how much it was I recall asking "oh, so how much are they _each_ ?" assuming it was a box of 10 or something. No deal so I went off to Target Electronics in Bristol and got a bundle for certainly less than a quid each (50p rings a bell?).

I learnt a lot on that machine - no internet and few people had a computer, so if you got stuck you were on your own and had to work it out yourself, I beleive that was a good thing.

I've not found the tapes yet, I don't think I had any commercial tapes. I did write a PacMan and it worked quite well - primative logic, I simply set half the monsters to always try to turn 180 degrees (with a slight randomness) to the way you were travelling. If that tape turns up then I'll see if it can be digitised, blimey, wont have ran it for over 30 years.

It travelled with me back and forth to Manchester when I was a student, accompanied by a monitor . Took its toll as you can see as some of the keytops broke off (they weren't particularly strong in the first place). Whilst at uni I also acquired a Watford utility ROM.

Part of the trick in writing programs to fit in very little RAM was in making calls to the existing ROM code - so, being a nerd even back then I dissasembled the ROM. No printer in those days, so it is all hand written. Again, doing this taught me a hell of a lot, including high precision arithmetic routines, knowledge that came in handy when I started work and wrote signal processing routines on, off all things, 8048/8051 processors.

Computer magazines at the time - such as Personal Computer World - were far more technical than the thinly disguised advertising pulp that became computer magz from the 90's onwards. In the back pages you'd often find some assembler routines for 6502 and Z80 (and even dead posh 6809) where various nerds rejoiced in shaving a byte or a microsecond off of code that had come before.

I finished uni with £20 in the bank account. I then started work which plunged me into debt for over a year (everything needed paying in advance, salery paid in arrears!) but once the debt was gone I celebrated by upgrading to an Acorn Electron, even splashing out on a few games - I liked puzzles more than shoot-em-ups and XOR and Repton have to be my favourites.

The 6502 has basically 3 8-bit registers that are of any real use, but the instruction set had addressing modes that meant it was actually more powerful than Z80's with its fist full of 8/16-bit registers. It also helped that page 0 memory (aka the first 256 bytes) could also be used as complex pointers.

Happy days.

TTFN,
Jon
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Old 26th Feb 2015, 10:23 am   #7
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Very quick update on this wee beasty. up to now I have removed the bodged wiring for the power connector. I've removed the voltage regulator as one of the legs broke off when I touched it and have ordered a new PCB mount power connector I'm wondering if the 5V 2amp regulated SMPSU I put together (ok salvaged) to run my master compact and amstrad CPC computers would be ok with the Atom too, would 2amp be enough for the Atom?

Jay
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Old 26th Feb 2015, 10:32 am   #8
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Default Re: Got me an Atom

If it's a fully loaded Atom I think it would be close but would probably work. If the PSU has remote sensing connections I'd use those to make sure there is 5V at the board rather than at the output of the PSU.

If I remember correctly there are two 5v rails in the Atom each running from one of the two regulators. There is a link to short these together for use with an external 5V supply

I actually built a 5v 3A linear supply for mine but that had an extra 17k of RAM fitted. It ran warm but I never had a problem with it.

Keith
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Old 26th Feb 2015, 11:29 am   #9
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Default Re: Got me an Atom

This PSU Isn't that fancy it was originally supplied with a fairly expensive free-view box. the output is a steady 5V even with a quite substantial load on it.
Interesting about it originally using two voltage regs mine has only one fitted and the legs literally fell off when touched.
I think my PSU should be fine for initial testing at least.

Jay
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Old 26th Feb 2015, 11:39 am   #10
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Sounds as if the link has been fitted and one regulator used. That would almost certainly overload it especially with the microscopic heatsink originally fitted. I'm sure your external PSU would be fine.

Keith
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Old 26th Feb 2015, 1:02 pm   #11
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Default Re: Got me an Atom

what heatsink? non on mine the reg was just floating in air. no wonder the poor thing failed it must of been cooked.

Jay
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Old 26th Feb 2015, 1:17 pm   #12
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Default Re: Got me an Atom

I think with the extra RAM in your Atom the heatsink would have been as much use as the proverbial chocolate teapot. You're doing the right thing by using an external power supply.

I also modified mine so I could take a video output direct from the buffer amp rather than use the modulator.

I'll have to dig it out at some point and try it out.

Keith
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Old 26th Feb 2015, 4:43 pm   #13
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Default Re: Got me an Atom

a nice big box of ex equipment psu's I bought on eBay arrived today. one of the first I pulled out the box is a nice regulated 5V 3A supply made by Samsung, should be even better than what I was planning to use.

Jay
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Old 5th Mar 2015, 11:10 pm   #14
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Default Re: Got me an Atom

I got a few minutes to play today. My Atom had link 6 made, link 7 was made at some point but had been cut. The 5v reg to the left is missing the one to the right is still there.
As of yet I have been unable to find a power connector with pin spacing suitable for the Atoms PCB so for now I'm using flying leads. for initial tests I remade link 7 so as to be able to feed in a true stabilized 5V.
This I got from my trusty Ferranti bench PSU set to 5V output. With the current limit set to 1500ma I applied power, the power light a previous owner has fitted to the case lit but apart from that nothing so far nothing on screen from composite or UHF outputs and no beeps or any other sounds from the speaker. But a quick check shows a healthy 900ma draw from the PSU so something is taking some power. that's all the time I have to play tonight, next ill try re'seating all the IC's then its out with the scope multimeter and logic probe. I must say I'm quite looking forward to getting my teeth into troubleshooting this wee Atom

Jay
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Old 6th Mar 2015, 12:07 am   #15
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Default Re: Got me an Atom

That's correct links 6 & 7 should be made to feed an external 5V supply. Link 6 links the two 5V supplies and link 7 bypasses the 5V regulator. According to the manual a fully loaded Atom with the additional RAM and 6522 should take about 1.6A although this may be different if lower power RAM is fitted..

Check the clock oscillators, the main 4MHz and the 3.58MHz on the 6847 are running.

Keith
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Old 6th Mar 2015, 9:08 pm   #16
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I've done a little more on this today, after some reading on the web I decided it would be a good idea to replace all the 22uf electrolytic caps, indeed all tested bad for ESR and where duly replaced. I have also removed the remaining voltage reg and remade the links as the originals where a bit of a hatched job. I’ve also added a power connector by gluing the connector upside down to the PCB, not ideal but ok for now. still nothing on screen apart from a few wavy lines when power is applied. I seem to be getting 5v to all the places I should and both crystals seem to be oscillating. all chips seem to get equally warm nothing gets overly hot. can anyone tell me which ic's generate the video output and ill start looking round there.

Jay
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Old 6th Mar 2015, 11:22 pm   #17
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Default Re: Got me an Atom

The 6847 generates the video but it's NTSC (525line 60Hz) rather than 625line 50Hz. There is a block of memory, 6k I think, connected to the processor.

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Old 8th Mar 2015, 2:50 pm   #18
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Hmm I’ve been at it again and can confirm I have a clock signal to the MC6847 and can see active data lines going into the IC but nothing out. I’ve pulled Q1 and Q2 both BC107's and tested them both fine. at least that meant I could refit them neatly. I'm starting to suspect the MC6847 I may have a tip in the attic as there's a dragon 32 up there that I may be able to borrow one from.

Jay
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Old 9th Mar 2015, 12:20 am   #19
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Ok think I'm running up a blind ally with the MC6847 I pulled one from a Dragon 32 and tried that in the Atom, still the same. I then tried the Atoms MC6847 in the Dragon 32 and it worked fine. So out with the scope and logic probe again, I am definitely getting two clock signals 3.5mhz for the video output and 4mhz for the processor I can trace these right through the circuit, what I'm not getting is any activity on any data lines from the processor or anywhere else to be honest. all data lines I've tested are ether held high or low non seem to be switching.

Jay
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Old 9th Mar 2015, 1:49 am   #20
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Default Re: Got me an Atom

I've looked back through what you've said so far and I don't think you've mentioned checking the status / operation of the reset circuit yet.

If your clocks are running, supplies are OK but you don't have any activity on the data or address lines one possible cause (certainly the most obvious cause I can think of) is that the machine is being held in reset.

When you press / release the reset switch or reset key combination, do you see a corresponding change on the reset pin of the processor?
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