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Old 5th Mar 2023, 4:33 pm   #41
ChrisOddy
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Default Re: An Enhanced MK14 (MK14E)

At last finished updating the design, the schematic changes were straight forward but the layout not so, it was at least a 50% re-layout !

I've updated the website notes here: https://theoddys.com/acorn/replica_b...14e/mk14e.html

and attached the revised Schematic and PCB layout.

Chris
Attached Files
File Type: pdf MK14E Board Schematic.pdf (193.4 KB, 39 views)
File Type: pdf MK14E Board PCB.pdf (258.6 KB, 47 views)
File Type: pdf MK14E Issues.pdf (54.8 KB, 32 views)
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Old 6th Mar 2023, 12:41 am   #42
ortek_service
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Default Re: An Enhanced MK14 (MK14E)

I presume you've also fixed the Single-Step not working due to clash with data outputs from EPROM, as was still active when upper 4 address bits +Status were being put on the databus, so needed to gate its select input with nRDS
- Even though SoC didn't seem to do this, and wondered if anyone had actually had their Single-Step working properly
As I didn't spot that issue on the list (Even if it was one you'd found yourself & fixed, whereas display rotation one was on this list).
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Old 6th Mar 2023, 8:59 am   #43
SiriusHardware
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Default Re: An Enhanced MK14 (MK14E)

I certainly had the single step feature working on my original issue II for many years - I probably still have the little hand drawn, etched and drilled PCB which I made for it. No problems that I can recall but then that machine never had more than 640 bytes of RAM.

I believe a once working single-step was also one of the many mods which were originally present on Buzby's 'Micky' as well.
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Old 6th Mar 2023, 9:10 am   #44
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Default Re: An Enhanced MK14 (MK14E)

I've just noticed it has provision for TWO crystals to be fitted, link selectable - very versatile.
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Old 6th Mar 2023, 9:43 am   #45
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Default Re: An Enhanced MK14 (MK14E)

Maybe the reason the single step circuit worked without nrds qualified by the proms on earlier versions of the MK14 is due to the D7 content of the proms for the small number of instructions between the halt instruction and xppc p3. It might prevent single stepping of code in the proms, but then maybe nobody expected that to work.

I remember the first pcb I drilled at home using a 1mm drill bit in my Dad’s black and decker drill on a block of wood on the kitchen table. When I was about three quarters done the drill was jamming, it took a few attempts for me to make the connection between the drill not turning and the tugging on my hair and the drill tight against my forehead. The next fifteen minutes was spent dismantling the drill while still tight on my forehead to get my hair out of the vents on the back of the drill.
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Old 6th Mar 2023, 10:27 am   #46
ChrisOddy
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Default Re: An Enhanced MK14 (MK14E)

Quote:
Originally Posted by ortek_service View Post
I presume you've also fixed the Single-Step not working due to clash with data outputs from EPROM, as was still active when upper 4 address bits +Status were being put on the databus, so needed to gate its select input with nRDS
- Even though SoC didn't seem to do this, and wondered if anyone had actually had their Single-Step working properly
As I didn't spot that issue on the list (Even if it was one you'd found yourself & fixed, whereas display rotation one was on this list).
My mistake, the original MK14 does use RDS to gate the PROMs - I briefly looked at an original circuit which is not easy to read and missed that line !
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Old 6th Mar 2023, 10:56 am   #47
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Default Re: An Enhanced MK14 (MK14E)

Just added another mod - an additional link (LK13) that enables RAM when no 8154 is fitted.
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File Type: pdf MK14E Board Schematic.pdf (194.4 KB, 31 views)
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Old 6th Mar 2023, 9:35 pm   #48
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Default Re: An Enhanced MK14 (MK14E)

Is there a spare pin on the extension that could be used for nHold? This would be useful for dual port memory vdus to use the busy signal.
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Old 6th Mar 2023, 10:36 pm   #49
ChrisOddy
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Default Re: An Enhanced MK14 (MK14E)

At the moment the backplane is fully occupied which was a concern for me as there is nothing left for future options ? I've followed the original MK14 and VDU signal allocations plus the extra signals I think on Ian Rolfe's replica ?
Let me have a review of the backplane so I can see where we are.

Chris
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Old 6th Mar 2023, 10:48 pm   #50
SiriusHardware
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Default Re: An Enhanced MK14 (MK14E)

I think Slothie did intend, on next revision, to take the NENOUT signal to the rear connector - there is a gap next to NENIN on the issue VI where it should logically be.

Quote:
Just added another mod - an additional link (LK13) that enables RAM when no 8154 is fitted
Good idea, does it enable RAM across the whole 0800-08FF range or just the 0880-08FF range originally occupied by the RAM of the 8154? If the lower part of the range is not occupied by the 8154 I/O registers I see no reason why 0800-087F can not be RAM as well.
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Old 7th Mar 2023, 11:10 am   #51
ChrisOddy
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Default Re: An Enhanced MK14 (MK14E)

Quote:
Originally Posted by SiriusHardware View Post

Quote:
Just added another mod - an additional link (LK13) that enables RAM when no 8154 is fitted
Good idea, does it enable RAM across the whole 0800-08FF range or just the 0880-08FF range originally occupied by the RAM of the 8154? If the lower part of the range is not occupied by the 8154 I/O registers I see no reason why 0800-087F can not be RAM as well.
It fills the whole 512 bytes occupied by the 8154 i.e. RAM and I/O.
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Old 7th Mar 2023, 11:32 am   #52
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Default Re: An Enhanced MK14 (MK14E)

OK I have had a review of the MK14 expansion connector/backplane.

I've added this page to the website that shows the original connections on the MK14 and VDU: https://theoddys.com/acorn/acorn_sys...backplane.html

That leaves just 4 spare connections on side A (26,27,29,30), side B is full !

XOUT for the VDU on B27 clashes with SIN so has to move, Ian R has moved it to A27 which is logical.
He also added NWDS to A29 which makes sense as NRDS is on A28, he's also added NRST to A26 an essential signal for any off board devices.

So that leaves A30 which I have used for my EXT I/O signal which is required to be able to add external I/O devices ?

I've also used B7 for the backup battery supply, that could go if backup is not required on the mainboard ?

So that leaves NENOUT and NHOLD that have been requested ?

Its a shame so many pins have been used for power - 8 ?

Could some of the lines used for Cassette and Serial be dropped (FLAG0, SENSE B, SIN) ?

Is the INTR line useful ?

thoughts ?
Chris
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Old 7th Mar 2023, 1:29 pm   #53
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Default Re: An Enhanced MK14 (MK14E)

Quote:
Could some of the lines used for Cassette and Serial be dropped (FLAG0, SENSE B, SIN) ?
I have to say no, you can't lose any of those, they are part of the fundamental I/O capability of the SC/MP and programs which were not even specifically written for the MK14 but can run on it may use them because they are the only I/O pins that every SC/MP based system will definitely have.

I don't think battery backup is necessary on the mainboard and as mentioned earlier there are self contained NVRAM devices with 6116 pinouts which can be put into a 6116 socket if anyone really needs that, similar to what Phil__G has done on his NIBL computer (although he used a 28-pin device there).

For any lines / signals to which access is desirable but which can't comfortably be accommodated on the main rear connnector, just provide those signals on an extra pin row connector so that they can at least be picked up and jumped across onto the next PCB with a Dupont female-female jumper wire or a ribbon cable with similar female connectors on the ends.

Last edited by SiriusHardware; 7th Mar 2023 at 1:35 pm.
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Old 7th Mar 2023, 4:17 pm   #54
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Default Re: An Enhanced MK14 (MK14E)

In a more general sense, all of the connections which were available on the edge on the original MK14 should be available wherever they originally were so the new revision remains 'connection compatible' with anything which someone may have done in the past.

Slothie's issue VI kept the historic original connections where they were although the temptation to untangle the order of the 8154 Port A / Port B bits must have been strong: He left them as documented, which was the right thing to do. To this he added the system buses and control lines in a 'best fit' fashion so that most of them ended up facing their SOC VDU counterpart on a one-for-one basis, although there were some anomalies such as the Clock-In on the VDU which would end up shorted to 0V if you carried it straight across, so that had to be moved to the same pin on the other row.

I would prefer that this revision keeps the same order of connections in the same places as the issue VI with any 'new' connections added onto the few pins left over. If there are not enough pins left over for all the signals which people want, route those to their own small pin row header - let's say 6-way, and have that same 6-way connector on the backplane / expansion board so those lesser used signals can be distributed there by plugging in a 6-way female to female ribbon jumper if they are required there.
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Old 7th Mar 2023, 7:43 pm   #55
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Default Re: An Enhanced MK14 (MK14E)

I’m not sure if Ext IO is needed to add external devices as these could be mapped into a separate 4k page. I was planning to use F000-FFFF for this.

Nenout might not have much value on the connector without also including nBusrq.

nHold and Cont could share the same pin connection, as the function would depend on when the pin is asserted. Before a memory cycle it would halt the processor and during a memory cycle it would extend the access.

Although A15 to A12 are defined by Slothie’s schematic there is nothing connected. There might be no need for these to be provided by the main board as they could also be latched on any board that needed these. If they are generated by the main board then maybe they should be tristate outputs.

Any vdu board would need some way to set A15 to A12, otherwise the vdu will read the ram from whatever page was last accessed by the 8060. If nenin is used to clear the latch this reintroduces the problem of memory corruption in page 0 if a write is in process when nenin is raised. It might be possible to use nenout or nbusrq, though I haven’t checked the timing to see if they are raised before nwds is inactive. A safer method might be to gate nwds with nenin to terminate any write as soon as nenin is raised, in combination with clearing the address latch.
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Old 8th Mar 2023, 9:06 pm   #56
ChrisOddy
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Default Re: An Enhanced MK14 (MK14E)

Quote:
Originally Posted by Mark1960 View Post
Although A15 to A12 are defined by Slothie’s schematic there is nothing connected. There might be no need for these to be provided by the main board as they could also be latched on any board that needed these. If they are generated by the main board then maybe they should be tristate outputs.
I completely missed that those address lines need to be tri-state so thanks for spotting that ! If I were to add tri-state buffers I'm not sure what to control them with ? Would enabling it during NRDS or NWDS (low) work ? I wonder if NWDS is to short ? Or should control be by NBREQ ?
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Old 8th Mar 2023, 9:34 pm   #57
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Default Re: An Enhanced MK14 (MK14E)

I don’t think nBREQ would work, nBREQ would still be asserted if NENIN is raised to kick the 8060 off the bus. Also any device requesting the bus should assert nBREQ and that would enable the tristate buffers when you don’t want them enabled.

Then again if the vdu kicks the 8060 off the bus you still need A12-A15 driven by something to address the memory for the vdu.

Maybe NENOUT high is the best choice to enable the buffers, assuming the MK14 8060 is always the highest priority controller, second only to the vdu.

You might be able to reset the latch if NENOUT and NENIN are both high, so the vdu always accesses 0000-0FFF. Probably first need to confirm the timing of NENOUT when an 8060 write cycle is interupted by NENIN. As I have mentioned in other posts, you really don’t want the address lines changing while NWDS is still asserted.

Edit: you could use 74LS173 for the address latch, but then you need inverters or use 74LS260 to detect page zero.
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Old 9th Mar 2023, 12:25 pm   #58
ChrisOddy
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Default Re: An Enhanced MK14 (MK14E)

I've been looking at some of the old Elektor SC/MP articles.

Their BASIC microcomputer (May 79) just uses NRDS and NWDS to control the buffers ?

A more complex solution is used in their 'experimenting with the SC/MP part 2' (Dec 77) project. Here they use a flip flop controlled by NADS and BREQ to enable the address buffers, looks like the buffers are enabled from when NADS goes low to BREQ returning low.
Its a slightly confusing article because the circuit is for an SC/MP I but then they talk about SC/MP II, several control lines are inverted between the two devices including N/BREQ. Though as you say BREQ is not the one to use.

I have a replica VDU card that I haven't hooked up to a MK14 yet so perhaps I will so I can see how NENIN, NENOUT etc function in a real system.
My prototype MK14E is awkward to use with the display being upside down !, I was hoping to move on to a second revision before hooking it up to anything else but its still a bit too early to go for a respin of the PCB. Though I probably only need to hook up the control lines to the VDU to see how things work and not bother with the address and data bus for the moment.
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Old 9th Mar 2023, 12:52 pm   #59
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Default Re: An Enhanced MK14 (MK14E)

I bought a Mk14 when it first came out as a kit. I remember spending a while typing in the example programmes, my favorite being the Lunar Lander. It was just sad to power it down and loose all your hex entered machine code. It was amazing what you could do in 256 bytes of ram. Nowdays you need a higher spec laptop every time Microsoft release a new version of their Windows Bloatware. I wish I'd kept it but sold it to a work colleague who wanted to learn machine code programming. I later bought a 16k ZX Spectrum which I upgraded to 48k by plugging in the extra RAM chips. I still have this one and it still works. Have the occasional blast on Manic Miner, Moon Cresta etc when I feel the need for a retro gaming fix after getting my arse kicked in GTA V online on my Xbox😆
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Old 9th Mar 2023, 3:16 pm   #60
SiriusHardware
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Default Re: An Enhanced MK14 (MK14E)

Hi Astonman - welcome - have a look around for other MK14 threads here, there are lots of them. You will find there are a number of much faster ways of getting code into your MK14 than typing the code in by hand.

This is one example which is just under development at the moment:-

https://www.vintage-radio.net/forum/...d.php?t=197465

There are some other methods as well but for an original MK14 where you won't want to change anything on the machine itself a 'keypad injection' loader - one which works by 'typing' the code in at very high speed via the keypad edge connector - is probably best. (The project mentioned above is one example of that type of loader).
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