An Enhanced MK14 (MK14E)

[SiriusHardware]

Oops, where you said you 'still had this one and it still works' I thought you were referring to your MK14, sorry.

[DavidMS]

Quick question on the keyboard connector, is there a reason why it is so far from the board edge ? - I am thinking about use of a right angle connector, could it be positioned close to the PCB edge in the same way as the main edge connector ?

[ortek_service]

Originally Posted by DavidMS

Quick question on the keyboard connector, is there a reason why it is so far from the board edge ? - I am thinking about use of a right angle connector, could it be positioned close to the PCB edge in the same way as the main edge connector ?

It looks like the J2 Keyboard connector is in that position, for a neater more-symmetrical appearance, lining-up with reset switch etc.
It should still be possible to fit a right-angle type - Including an eject-latch type, as the polarising-notch is at the top - for mating with a cable IDC socket. But could be troublesome, if wanting to use a PCB-Mount socket, if it doesn't stick-out far enough to reach into a R/A header on the MK14E.

One other thing to check, is where pin 1 is on the MK14E J2, as it's not clear being as there's no square pad on this (unlike J1) - Maybe as Chris was thinking of using just an unpolarised header-pins strip, rather than a boxed type with a centre-polarising notch. Pin 1 is on the Right-side, looking into a standard Right-angle Eject-latch header etc.
So need to check where pin1 is on the MK14E J2, as if closest to board end-edge, then a R/A header would have to face inwards! (Or maybe fitted underneath, if pin1 isn't on the right row)

[ChrisOddy]

My intention was to use a right angle 3M type connector as in the photo ?

[DavidMS]

Originally Posted by ChrisOddy

My intention was to use a right angle 3M type connector as in the photo ?

My issue a RH connector is that an IDC terminated cable will come out at a right angle and have to go up and then 'fold' over. A vertical connector would allow a ribbon cable just to come out flat.

As has already been suggested it would be good understand the pin out you are planning ?

[ChrisOddy]

The schematic is here: http://theoddys.com/acorn/replica_bo...14e/mk14e.html

[DavidMS]

Originally Posted by ChrisOddy

The schematic is here: http://theoddys.com/acorn/replica_bo...14e/mk14e.html

Looking at the picture of the board am I correct in thinking the orientation (pin numbers) when viewed from above is

Reset push

12
34
56
78

etc

if so I think that will end up being in the order of the existing boards if you use an IDC connector and ribbon cable which is good

[SiriusHardware]

Not quite, as NRST and +5V are next to each other whereas most recent replicas which have reset available on the keypad edge connector have it next to 0V or more correctly, they have replaced one of the two original 0V fingers with a reset-in.

It's not a bad idea to have +5V available on that connector as another potential supply point for offboard interfaces etc but I would put it at the opposite end away from NRST.

[ortek_service]

Originally Posted by DavidMS

My issue a RH connector is that an IDC terminated cable will come out at a right angle and have to go up and then 'fold' over. A vertical connector would allow a ribbon cable just to come out flat.

As has already been suggested it would be good understand the pin out you are planning ?

Well you could always fit a vertical header instead, as pinout should work the same - But this is likely to be a bit higher than a R/A one, when mated.

However, with a Right-angle header, you can still choose whether to crimp the cable going up or down in the IDC socket. And many better IDC socket also come with a thin cable clip, so it can be folded-over to provide better strain relief. Standard stranded IDC ribbon cable is quite flexible, so can be bent to lay flat so will come away from the connector at pretty-well 90degrees to it.
- Many computers - like the BBC etc - used R/A PCB-headers and ribbon cables would lay flat OK from these.

[ortek_service]

Originally Posted by SiriusHardware

Not quite, as NRST and +5V are next to each other whereas most recent replicas which have reset available on the keypad edge connector have it next to 0V or more correctly, they have replaced one of the two original 0V fingers with a reset-in.

It's not a bad idea to have +5V available on that connector as another potential supply point for offboard interfaces etc but I would put it at the opposite end away from NRST.

Might have to decide which approach is best / most common - or have yet-more configuration links, to select mapping to connector pins (with care, as having +5V supply moving around on different pins sounds like it could be problematic in work out what's compatible with what - not that there's too much available, other than DIY speed-loader etc projects, that plug into Keyboard connector)

[SiriusHardware]

It is well established practice for the 0V connection to be at the end of the external keypad connector which is nearest to the reset switch, all the way back to the original issue II at least.

Slothie certainly avoided having +5V on the keypad edge connector on the issue VI due to the danger that it might end up connected to something it should not, given the tendency of those original type edge connectors to skid around or get plugged in one step out.

Of course the IDC connectors on the proposed design would be more or less self-locating especially if the IDC male on the PCB will be a 'box' type with an index cutout so the female cable mounted plug can only be inserted one way around. In that sense it should be safe to add +5V to the already established connections, but I really think it should go at the lower end of the connector.

[ChrisOddy]

I've been away a bit lately and was then diverted by a PICL build but have now completed the changes to the Schematic, see attached. I've also updated the website info here: https://theoddys.com/acorn/replica_b...14e/mk14e.html.
Just got to finish implementing the PCB layout changes.

[ChrisOddy]

I've picked up the MK14E project whilst waiting for some other stuff.

I've made one small change to the Page 0 - EXT I/O circuit - it previously didn't work unless LK6 was fitted and for 'normal' MK14 mode it wouldn't be !

Hopefully that's it - comments ?

Chris

[ChrisOddy]

This morning I've finally got around to hooking up a VDU to the MK14E (a replica of the original) and it seems to be working. Only in graphics mode as no character generator and no modulator just video out to a monitor (using the mods that Timbucus posted).

It is fitted with a 74LS86 rather than 74L86 and seems to be fine, it was suspected that there might be a problem making this change - what do I need to look for ?

[SiriusHardware]

The 74L86 isn't pin compatible with the 7486 / 74LS86 - at least I'm fairly sure it isn't - so I guess you have rearranged the pinout in order to try this.

The theory about the use of the 74L86 was that it had been chosen intentionally due to its unusually slow propagation delay in order to make sure that things happened in a particular order. Maybe this is something which only affects character mode, we have never really looked into it. If you are having difficulty obtaining a char gen, forum member Realtime has a little daughterboard available which hardware emulates the original char gen and just plugs into the char gen socket.

If you are finding that a 74LS86 works OK then maybe that is something Tim or myself (or anyone else with an original or replica SOC VDU) should also try to see what happens, although the pinout of the 74LS would have to be rearranged into the right layout before fitting into an original VDU PCB.

[ChrisOddy]

Yes the board was laid out for an LS part. I'll have a look at the signals around the 86's and see if anything looks on the edge ?

My next step is to knock up a character generator on a breadboard and hook it up so will see how that goes.

[Phil__G]

Just wondering, whats the logic behind the two 6116s rather than a 6264 Chris, is it to facilitate the separate MK14 memory areas?

[ChrisOddy]

Simply because I didn't need 8K and it made the address decoding easier (and I have plenty of 6116's !)

[Phil__G]

Fair enough Chris

[ChrisOddy]

I now have the VDU working with an EPROM based character generator design based on that in the Nat Semi Databook. I get a good image, still no obvious issue with the 74LS86. I haven't yet got around to probing around the circuit to see if anything is 'on the edge'.

So I'm now looking at the design for the MK14E version of the VDU which will have the EPROM character generator in place of the DM8678CAB. Because there is plenty of spare space in the EPROM I have also been able to add a full ASCII character set, selectable by a link.

I've dropped the modulator as I'm guessing most will use a monitor for a better picture. I will also look at adding an onboard 4MHz clock to obviate the need for changing the crystal on the mainboard (although the MK14E has two selectable crystals), not sure if the CPU and VDU need to be synchronised so will try it and find out ?

So next its what to do with the b9-b17 connections ? I've had a look through previous threads and come up with the following:
b9-b12 (Page Select) need to be configurable but via links/DIL switches rather than via the 8154 ? and b9 with link/DIL switch to b17 (Top Page)
b13 (Enable) linkable to either FLAG1 or a switch ?
b14 (Graph/Char) linkable to either FLAG2 or Top Page (b17)
b15 (Reverse Page) pulled low (resistor) with the option of connecting to an 8154 pin ?
b16 (Invert Video) tied high (as the picture is rubbish)

I'll probably leave pads for the original 8154 port to retain backwards compatibility so they can be linked in if required.

I will also need to extend the address outputs to cover all 16 lines, the top 4 being selectable via another set of links/DIL switches like Page Select (usually set to all low but required for 64K operation).