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+--- Thread: Upgrading the original ROMs in a 32KB Microbee (/thread-260.html)
RE: Upgrading the original ROMs in a 32KB Microbee - Ernest - 08-01-2021
Yep all doable, just depends on how neat you want it.
Very challenging on the 1632 as only 24 pin chips, I have a similar set up on a 1732 board Basic, EDASM, Wordbee, Pascal & Disassembler a little easier with three 28 pin positions being available. This setup also has the ability to switch in an additional 24K RAM in place of Basic and Pak (EDASM) locations and switch the NET ROM out with a boot ROM and use the 50 WAY to run as a 56K Disk system.
Anyway back to the new board design.
The plan was to do a full emulation of the 8328 (as used in the Microbee PC85) with a one chip 62256 SRAM, expanded Basic in one ROM (instead of the three) and all four decodes of NET 0,1,2,3 in a second ROM, this also would include both the Shell and Help in ROM. The original plan was to combine all these into one ROM, but opted to do it across two as I was not confident with how they interacted so stuck with the two.
Finally would include the expanded decoding for all 16 PAC ROMs at C000 (0 - F) all in the last ROM, as this is the one that is most likely to change if expanding the system, keeped it in a separate ROM from the rest of the operating system to make updating less of a hassle. If you only wanted to stay with the 28 Pin EPROM instead of the 32 pin then decoding of only 8 PAC's would be available, which is equivalent to the Microbee PC85 anyway.
Ernest
RE: Upgrading the original ROMs in a 32KB Microbee - ChickenMan - 08-01-2021
This is a pic of the coreboard, looks pretty neat to me. There are a couple extra wires underneath.
RE: Upgrading the original ROMs in a 32KB Microbee - Ernest - 08-01-2021
Yep a couple of daughter boards would certainly give a neat solution.
I assume you are using a PCF layout style in the EPROM?
I'm sure Brett would be very interested in the layout of the daughter boards so he could achieve the same with his system.
Ernest
RE: Upgrading the original ROMs in a 32KB Microbee - someone - 08-01-2021
(08-01-2021, 10:42 AM)Ernest Wrote: ... I assume you are using a PCF layout style in the EPROM? ...Please enlighten someone.
Is PCF similar to BCF? (Boating Camping Fishing?)
RE: Upgrading the original ROMs in a 32KB Microbee - g67wjjk - 08-01-2021
I've dug out my copies of the Technical Manual and Hardware Handbook so I can see those changes referred to earlier - putting in a new crystal, etc.
One thing I am puzzled by - one of the 'Bees had a speaker glued upside-down (so the cone was facing up) to the speaker grill (it was larger than 50mm cutout), and so the little speaker attachment parts were missing.
Any ideas what I would call them when searching for replacements? I'm getting some new speakers and don't want to glue it in.
Brett.
RE: Upgrading the original ROMs in a 32KB Microbee - Ernest - 08-01-2021
(08-01-2021, 01:15 PM)g67wjjk Wrote: I've dug out my copies of the Technical Manual and Hardware Handbook so I can see those changes referred to earlier - putting in a new crystal, etc.Hi Brett,
One thing I am puzzled by - one of the 'Bees had a speaker glued upside-down (so the cone was facing up) to the speaker grill (it was larger than 50mm cutout), and so the little speaker attachment parts were missing.
Any ideas what I would call them when searching for replacements? I'm getting some new speakers and don't want to glue it in.
Brett.
First up the correct speaker is 50 Ohm not the usual 8 Ohm, I believe they are available from RS Components (not 100% sure on that one).
If an 8 Ohm has been fitted it is highly likely the driver transistor will be fried.
I doubt you will be able to buy the brackets anymore, shouldn't be too difficult to make a couple up, they only need to hold the speaker in place so it dont move.
The 12 Mhz to 13.5Mhz change is changing the divide by six to a divide by four hence the 2 Mhz CPU clock changes to 3.375 Mhz,
however as a result a change needs to be made to the version of Basic and also the Character ROM.
Ernest
RE: Upgrading the original ROMs in a 32KB Microbee - g67wjjk - 08-01-2021
Ahh, good to know. This is what Element 14 have - only 50Ω, 50mm speaker they carry:
https://au.element14.com/visaton/2916/speaker-k-50-wp-50-ohms/dp/1683896?ost=1683896
I like Element 14 because they have free overnight shipping from Sydney to Port (if you spend over $50) ... and they part-ship regardless. Doesn't make sense to me ... the courier will sometimes hand me a package containing a single capacitor. But I'm an engineer so what would I know.
Checking the speakers I've got - indeed, the one with the brackets is 50Ω but has a small break in the cone, so I'll replace. And the one glued in is unknown, because of all the glue. I can only assume it was 50Ω, and they used it because it was 50Ω and so they had to jam it in somehow (the days before Internet shopping).
I had a thought I could make some bracket that went over the speaker and was secured by the two screws. Let's see.
Brett.
BTW, I'm not allowed to click on images it seems, even ones I've uploaded myself - I get this response:
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RE: Upgrading the original ROMs in a 32KB Microbee - someone - 08-01-2021
Any 50mm speaker of appropriate depth (17mm) can be used but you'll have to increase the series resistor to a higher value to limit the output current.
Altronics has a 50mm one that is marginal on its depth (18.2mm) and Jaycar has 40mm 0.25W both are 8 ohms impedance.
Ideally as not to cook the driver transistor this should be around 120 ohms for 8 ohm and 75 ohm for 50 ohm speakers when running your microbee from +12V.
When running on lower voltages you can use appropriately lower values. Doing this may affect its volume.
You can also install a flyback diode on the back of the speaker to control the back emf generated and also install a serial high pass filter capacitor to preventing the constant DC from flowing through the speaker.
Those speaker clips were are a standard item back in the early 1980's.
Many microbees were assembled with the speaker upside down and glued in when the clips were out of stock (or couldn't afford them).
You can make your own clips from your 3D printer or just fashion your own after you get the replacement speaker so you can alter it appropriately.
You can actually remove the 74LS92 for the conversion between 2MHz and 3.375MHz.
The change of Font ROM is not because of the 13.5MHz crystal but to provide a smaller font for the additional 80x24 mode.
However, the Font ROM used must have a sufficiently quick memory cycle time to run at the higher 3.375MHz.
Even some of the custom masked Dual Font ROMs are not fast enough and require upgrading. (That's an embarrassing faux pas.)
Someone also gets the same error message for clicking photos.
RE: Upgrading the original ROMs in a 32KB Microbee - ChickenMan - 08-01-2021
Microbee Technology has 50 ohm speakers in stock and ask about the mounting brackets also. Also the early Kit Bees and I suspect also the early 2khz models used an 8 ohm speaker.
@Ernest, that coreboard was not made by me, was made but a old friend who unfortunately past away a couple of years back. But when he gave it to me, he wasnt able to find any paperwork with its full details nor the extra pcbs needed.
RE: Upgrading the original ROMs in a 32KB Microbee - Ernest - 08-01-2021
Hi someone,
Check out the PCF emulation in the uBee512 documentation.
It is a compilation all in one 256K ROM, able to emulate the PC85 as well as being able to boot from an IDE drive.
Leaving out the boot portion, a full PC85 should run all from the one ROM.
I'm guessing that this is what CM has on his Coreboard.
Ernest