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Old 18th Mar 2020, 9:41 pm   #1
Slothie
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Default MK14 memory upgrade

I've been looking at designing a memory add on for my "rev 5.5" MK14 using a 6116 type 2k x 8 SRAM chip. These have the advantage that they interface directly to the signals on the SC/MP since OE = NRDS and WE = NWDS, you just need to generate a /CE signal for the RAM chip.
If you just want to add 1.5k after the PROM from 200-7FF then the CE decode is pretty simple:
Note: X = NRDS&NWDS
Code:
Truth Table

   X.  A11 A10 A9  A8  /CE
00 0.  0.  0.  0.  0.  1
01 0.  0.  0.  0.  1   1
02 0.  0.  0.  1.  0.  0
03 0.  0.  0.  1.  1.  0
04 0.  0.  1.  0.  0.  0
05 0.  0.  1.  0.  1.  0
06 0.  0.  1.  1.  0.  0
07 0.  0.  1.  1.  1.  0
08 0.  1.  0.  0.  0.  1
09 0.  1.  0.  0.  1.  1
10 0.  1.  0.  1.  0.  1
11 0.  1.  0.  1.  1.  1
12 0.  1.  1.  0.  0.  1
13 0.  1.  1.  0.  1.  1
14 0.  1.  1.  1.  0.  1
15 0.  1.  1.  1.  1.  1
16 1.  0.  0.  0.  0.  1
17 1.  0.  0.  0.  1.  1
18 1.  0.  0.  1.  0.  1
19 1.  0.  0.  1.  1.  1
20 1.  0.  1.  0.  0.  1
21 1.  0.  1.  0.  1.  1
22 1.  0.  1.  1.  0.  1
23 1.  0.  1.  1.  1.  1
24 1.  1.  0.  0.  0.  1
25 1.  1.  0.  0.  1.  1
26 1.  1.  0.  1.  0.  1
27 1.  1.  0.  1.  1.  1
28 1.  1.  1.  0.  0.  1
29 1.  1.  1.  0.  1.  1
30 1.  1.  1.  1.  0.  1
31 1.  1.  1.  1.  1.  1
/CE = NRDS&NWDS + A11 + /A10&/A9
MA10..0 = A10..0
(MA10..0 are the 11 address lines on the memory chip)

Its more complicated if we want to use the 0.5k "hidden" by the PROM to map to the standard and extended RAM on the main board, allowing the MK14 to be used with easy to get 6116 chips rather than the 2111 chips that when available are expensive. Leaving them off the board is sufficient since the decoding on the board doesnt affect any memory off-board.
Note: X = NRDS&NWDS
Code:
Truth Table

   X.  A11 A10 A9  A8  /CE
00 0.  0.  0.  0.  0.  1
01 0.  0.  0.  0.  1   1
02 0.  0.  0.  1.  0.  0
03 0.  0.  0.  1.  1.  0
04 0.  0.  1.  0.  0.  0
05 0.  0.  1.  0.  1.  0
06 0.  0.  1.  1.  0.  0
07 0.  0.  1.  1.  1.  0
08 0.  1.  0.  0.  0.  1
09 0.  1.  0.  0.  1.  1
10 0.  1.  0.  1.  0.  1
11 0.  1.  0.  1.  1.  0
12 0.  1.  1.  0.  0.  1
13 0.  1.  1.  0.  1.  1
14 0.  1.  1.  1.  0.  1
15 0.  1.  1.  1.  1.  0
16 1.  0.  0.  0.  0.  1
17 1.  0.  0.  0.  1.  1
18 1.  0.  0.  1.  0.  1
19 1.  0.  0.  1.  1.  1
20 1.  0.  1.  0.  0.  1
21 1.  0.  1.  0.  1.  1
22 1.  0.  1.  1.  0.  1
23 1.  0.  1.  1.  1.  1
24 1.  1.  0.  0.  0.  1
25 1.  1.  0.  0.  1.  1
26 1.  1.  0.  1.  0.  1
27 1.  1.  0.  1.  1.  1
28 1.  1.  1.  0.  0.  1
29 1.  1.  1.  0.  1.  1
30 1.  1.  1.  1.  0.  1
31 1.  1.  1.  1.  1.  1
/CE = NRDS&NWDS + /A10&/A9 + A11&/A9 + A11&/A8
MA10 = /A11&A10
MA9 = /A11&A9
MA8 = /A11&A8 + A10&A8
MA7..0 = A7..0
Note the top 3 bits of the RAM address are "rewritten" to move them to the expected Bxx and Fxx addresses.
This is obviously a fairly complex logic circuit a sketch of which is attached.

According to suggestions from Sirius another thread I invisage putting the memory expansion on a board with signal buffering and a couple of DIN 41612 a+b connectors to allow other boards like the VDU to be plugged in. if this wasnt required the not fitting them would be the answer

I would also put on a voltage regulator supplied from the raw 8v input on the MK14 as the 5v on the MK14 is already stressed enough!

Another possibility that was suggested was to allow an EPROM to be fittef to the RAM socket since the pin allocations are similar to the 2716 allowing custom firmware to be tried. Presumably this would map from 000 to 7FF. The various option links would pretty much require the use of a PROM or GAL unless we want the challenge of a board full of TTL!
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Old 19th Mar 2020, 12:43 am   #2
SiriusHardware
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Default Re: MK14 memory upgrade

I personally only imagined 0000-01FF of the EPROM (The original range of the PROM) being mapped, the main aim being to do away with the need to use those danged awkward bipolar PROMs.

You could provide jumper links or a DIP switch to set the upper address pins so different PROM images programmed into 200H chunks of the EPROM could be swapped in at will - New OS - Old OS - several demo programs - VDU demo program etc.

If including bus buffering remember that the address line buffers would have to be bidirectional as well, as the VDU needs to be able to drive the MK14's address bus when it takes over the buses.

Agree a secondary regulator powered from the same 8V rail as the main regulator would be a good idea.

One caution about the 6116, we briefly had a forum member called Gert who had all but completed his MK14 replica but for the RAM, and I remember him saying that he had tried a 6116 in the meantime but that it had not worked. Unfortunately he didn't explain why, perhaps he didn't get time to investigate before the proper RAM arrived.

As regards the addition of DIN connectors I don't really favour going vertical unless the cards are mounted in a proper rack, so I would ask that one of the pinouts on the memory expansion board be at the rear with the correct geometry for a right-angle DIN connector, and pinned specifically for the VDU, allowing the MK14 board, expansion board and VDU to be laid in a straight horizontal line backwards. The completed VDU board is actually quite a heavy PCB, especially if it has the modulator fitted at the far end.
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Old 19th Mar 2020, 1:28 am   #3
Slothie
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Default Re: MK14 memory upgrade

Thanks for mentioning the address lines. Id already realised the data lines would be hard to buffer as the ditection would depend on which side the addressed device was as well as.reaf/write strobes, but forgot the address lines are driven by the VDU. I suppose I could use NENOUT to reverse the buffers when the SC/MP surrenders the bus. The data bus is more problematic!

Do the DIN connectors on the VDU have 2 or 3 rows (i.e. are they 64 pins in a 96 way shell or the 64 way shell)?

I have over a dozen 6116 RAMs of different makes and I'd definitely be breadboarding the simpler 1.5k version first to make sure it works before considering more complex versions. Another possibility would be to battery backup tje RAM as some 6116 variants are specifucally designed for that...
If theres a lot of DIP switch/jumper options Id definitely need a GAL or CPLD to implement it which seems a bit over the top. Mind you it'd be interesting learning how - I've used CPLDs before but not GALs which would be a better option because 5v logic CPLDs are pretty hard to find.
Theres definite scope for feature creep in this project! Still, at the moment I have nothing but time!
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Old 19th Mar 2020, 9:59 am   #4
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Default Re: MK14 memory upgrade

Quote:
Originally Posted by Slothie View Post
Do the DIN connectors on the VDU have 2 or 3 rows (i.e. are they 64 pins in a 96 way shell or the 64 way shell)?
Good question. My gut feeling is it's the 64-way housing because the rows on the VDU PCB are screenprinted A and B, whereas if it used the two outer rows of a three-row housing I would expect that to be A and C.

If you trawl back miles through your main project thread I believe I posted several pictures of the edge of the VDU PCB with its DIN connector fitted, but I will have a look at it again when I get home to confirm.
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Old 19th Mar 2020, 12:01 pm   #5
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Default Re: MK14 memory upgrade

The images I posted before are here (in post #133 of the thread). Unfortunately I didn't show a view looking into the pins / housing of the VDU connector.

https://www.vintage-radio.net/forum/...=145663&page=7
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Old 19th Mar 2020, 7:12 pm   #6
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Default Re: MK14 memory upgrade

Confirmed, it is a right angled 2-row 32 * 2 (64 pin) a / b connector, and that is clearly what the VDU PCB was laid out to accept.
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Old 19th Mar 2020, 10:12 pm   #7
Slothie
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Default Re: MK14 memory upgrade

Good, theyre still easy to get. I agree with you about making the VDU lay down as its quite heavy. It just makes the combined assembly quite long!
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Old 19th Mar 2020, 10:48 pm   #8
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Default Re: MK14 memory upgrade

The long, narrow shape of the MK14 main PCB was a quirky choice, one for fans of perspective, posssibly. If you lay it out so the extra memory PCB and VDU extend horizontally away from the rear connector the modulator will almost be at vanishing point. I just think doing it any other way will leave the extra PCBs poorly supported.

It always made it very awkward to put into any sort of enclosure, especially if you wanted to connect at least one additional PCB to the rear connector, as in the case we are considering here.

To be fair, with its PCB-mounted display and keypad, putting in a box was the last thing the design catered for. It was really only meant to be used as it came, on your tabletop.
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Old 19th Mar 2020, 10:58 pm   #9
Slothie
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Default Re: MK14 memory upgrade

I suppose the motherboard/memory board could have the connectors on the same edge so the computer is laid out like an inverted "U" with the vdu beside the MK14 hopefully not obscuring the keyboard connector.
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Old 20th Mar 2020, 12:08 am   #10
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Default Re: MK14 memory upgrade

The VDU is (for the benefit of others who have not seen one) approximately the same width as the MK14 main board but much shorter. Milan, the person who made the VDU videos linked to in another thread, appears to have used a vertically mounted link PCB between the MK14 main board and the VDU, so that the VDU PCB ended up underneath and parallel to the MK14 main board.

That certainly saves space but creates a requirement for some kind of custom chassis / bracket / support system.

If the boards are just laid flat out in a straight horizontal line, the only 'mounting arrangement' needed is for them all to have identical stick-on rubber feet so they all sit at the same height above whatever surface they are resting on.

With respect to the MK14 generally, one of the problems with mounting it on any sort of base is the lack of mounting holes - the only large holes through the PCB are those for the fasteners which hold the keypad frame on. If you want to mount the PCB on some sort of baseplate or in an enclosure then really it needs to be supported by the edges. One way to do it might be to use two pieces of trunking, cut to the same length as the long edges of the PCB * and fixed as shown in the attached sketch. The screw in the illustration is flat-head but could be countersunk-head if the baseplate material was thick enough.

* If you mean to attach anything to the keypad connector, the right hand piece will have to be shorter.
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Old 22nd Mar 2020, 8:10 pm   #11
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Default Re: MK14 memory upgrade

I had considered a rear board that turned the VDU to the right side and had space on it to plug in two or three small vertical cards for Memory, The Audio Addon, Single Stepper, Tape Interface etc... I always intended to build the unreleased 40 key keyboard as well which would clip in nice there - a baseboard and a nice clear perspex top section and you have a fairly robust machine that looks Good for display as well.

With support for Paging we could run NIBL BASIC - also prepared but, unreleased. Probably the modified one for the Elektor which RAN in Page 1 so had only upto 6 pages of RAM it could use.

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Old 25th Mar 2020, 5:49 pm   #12
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Default Re: MK14 memory upgrade

Quote:
Originally Posted by Timbucus View Post
With support for Paging we could run NIBL BASIC - also prepared but, unreleased. Probably the modified one for the Elektor which RAN in Page 1 so had only upto 6 pages of RAM it could use.
Support for paging would be simple in principle as it means latching the data bus when NADS goes low->high complicated only by having to mod the MK14 to only use page 0 for its devices. There's room saved on the bus for the upper 4 address bits. The best way I've seen for this is intercept the MK14 NRDS and NWDS signals so the devices on the MK14 don't see them when page isn't 0. This would require a signal back from the memory board but there are enough free pins. I think there was also a couple of spare gates that could be pressed into service.... It all depends if the effort is worth it
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Old 1st Jun 2020, 6:07 pm   #13
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Default Re: MK14 memory upgrade

Simple 1.5k memory upgrade for MK14. Can't verify it in hardware but might be worth knocking up on breadboard or Vero for those with VDU cards?
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Old 1st Jun 2020, 6:25 pm   #14
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Default Re: MK14 memory upgrade

Mind reader! That's almost simple enough to try on a breadboard/plugboard if the leads are are kept short, just to try the concept of a plug on 'Ram Pack' for the MK14.

I was thinking of something similar but using a single PROM as the address decoder - I have quite a few Philips 82S185 (1024 * 4-bit) PROMs looking for something to do. Obviously yours, using discrete TTL, would be more build-friendly and would avoid the question: Would a PROM be responsive enough in this application?

However I do remember occasional forum user gertk64(?) mentioning that he had tried a 6116 mapped into his MK14 replica while he was waiting for the proper RAMs to arrive, but it didn't work for him for some reason - unfortunately I don't think he came back with a theory about that.

I think where there is the intention for this to go between the MK14 and the VDU (and all sort of interesting things besides) you would probably end up being obliged to buffer the buses and control signals to whatever extent possible without making it too complicated.

However, it may be worth building it as simply as possible and just turning it on with crossed fingers. I'm sure a lot of people mapped 1.5K onto their Issue Vs back in the day using the minimum possible hardware.
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Old 1st Jun 2020, 6:45 pm   #15
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Default Re: MK14 memory upgrade

Yes, I've missed out connecting NRDS to the ~OE pin on the 6116. I'll recommend that as write only memory is of limited use!
A PROM would probably work as NRDS & NWDS are active for ~750ns at 4mhz which given the 74S571 has a 35ns access time (presumably others would be similar) and with a reasonably fast RAM is plenty of time. Would be neater too.
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Old 1st Jun 2020, 7:07 pm   #16
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Default Re: MK14 memory upgrade

Thanks for pointing out the little problem with NRDS /OE, I probably would have overlooked that.

If I did set about programming a PROM as the address decoder would it be worth trying to map the 'lost' 0.5K somewhere else? I don't think there are any other handy 'holes' in the memory map to drop RAM into, are there - if there were, nobody would go to all the trouble to free up 0200-07FF, they'd just use a hole that was already there?

The PROM isn't the only thing which originally has images but I think some of the original software may depend on the the images existing so that it can reach into two areas with a pointer placed on the boundary.

Despite that, I think forum user 'circuitryboy' did go the extra mile and added mods to remove the other images as well so that RAM could be mapped into those areas.
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Old 1st Jun 2020, 7:19 pm   #17
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Default Re: MK14 memory upgrade

Corrected diagram.
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Old 1st Jun 2020, 7:37 pm   #18
Slothie
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Default Re: MK14 memory upgrade

Quote:
Originally Posted by SiriusHardware View Post
Thanks for pointing out the little problem with NRDS /OE, I probably would have overlooked that.

If I did set about programming a PROM as the address decoder would it be worth trying to map the 'lost' 0.5K somewhere else? I don't think there are any other handy 'holes' in the memory map to drop RAM into, are there - if there were, nobody would go to all the trouble to free up 0200-07FF, they'd just use a hole that was already there?

The PROM isn't the only thing which originally has images but I think some of the original software may depend on the the images existing so that it can reach into two areas with a pointer placed on the boundary.

Despite that, I think forum user 'circuitryboy' did go the extra mile and added mods to remove the other images as well so that RAM could be mapped into those areas.
Technically yes, the difficulty is in disabling the unwanted images really. It would probably be better to create a PROM to decode for all the devices and remap the top 4 address lines. You could possibly do this by pulling the TTL that does the decode and jumpering in the PROM? But it sounds like a new spin of the board to me!

On my ROM RAM.buffer board I was thinking about I was thinking about using a GAL for the same thing, that is complicated by needing to control the direction of the buffers while NENIN is low.
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Old 1st Jun 2020, 8:19 pm   #19
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Default Re: MK14 memory upgrade

One good reason to stick with your TTL decoder and avoid PROMs (apart from the difficulty they present for most people programming them) is that they are quite thirsty devices, easily over 100mA drawn by one chip. I don't think we need more excess current drain than we already have. So yes, if we must use a one chip address decoder then a GAL would presumably be better in that respect. I have wondered, without looking into it, whether an appropriately programmed GAL with its power pins in the right place could be used as a low power, lower cost substitute for the 74S157s. To hold the OS I mean, not as an address decoder.

I will try your 'simple' memory upgrade, but I'll do it in a phased way, first of all just with an edge connector and DIN connector joined one to one by 'double sided' stripboard and I'll make the minor mods required for basic connection to the VDU on that. Unlike Tim I'll probably use the 8154 to control the VDU initially because all the required connections are already in place. I'll get that working first, with the VDU scanning existing RAM.

Then on the copper underside of that interconnect board I will fix a smaller square of stripboard with its blank side 'up' against the underside of the interconnect PCB and its copper side 'down', facing the table, and on that smaller square I will build your RAM upgrade 'ugly' style by folding the pins of the ICs flat out sideways along the copper tracks and soldering them to the tracks - then I'll use wire-wrap type wire to pick up the data, address and control lines (NRDS, NWDS, etc) from the tracks on the underside of the interconnect PCB.

Hopefully, that will all 'Just Work'.

Last edited by SiriusHardware; 1st Jun 2020 at 8:26 pm.
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Old 1st Jun 2020, 8:35 pm   #20
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Default Re: MK14 memory upgrade

Cool - when you get it working I will build one as well I seem to have all those components waiting for me to make an expansion RAM... including a pile of 74LS00 with only three working gates...
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