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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 18th Jun 2022, 6:11 pm   #1
Michael Haardt
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Default HEXIO compatible to Elektor SC/MP system

Hello,

off and on, I am working on creating a new HEXIO board that is functionally compatible to that used in the Elektor SC/MP system, but can be used with other systems as well.

Most parts used originally are very hard to obtain these days, so I use current THT parts, which simplifies the card some. The keycaps will be 3D printed. I removed the CONT/HALT and reset circuit, because that is specific to the SC/MP, and I put the actual bus interface on a separate mini board, which reduces the connection between both to a 20 pin ribbon cable. Currently I built a mini board for the NDR Klein Computer, a modular system similar to the RC2014, except it predates that by 40 years. It will be fun to run Elbug on that eventually.

Currently the schematics are drawn and the output works on a bred board and is slowly transferred to vero board. Input it still untested.

Is there any interest in this?

Michael
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Old 18th Jun 2022, 6:43 pm   #2
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

I’d be interested to see more details and also what substitutions you made with more modern components.
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Old 18th Jun 2022, 7:23 pm   #3
Michael Haardt
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Default Re: HEXIO compatible to Elektor SC/MP system

I converted my notes to HTML, but remember this is still unfinished work in progress:

http://www.moria.de/~michael/hexio/

Michael
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Old 18th Jun 2022, 9:03 pm   #4
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

Interesting method of operation but I guess that is how the Elektor Hexio worked so you are forced to do something similar for compatibility. The use of three priority encoders for the keyboard input is odd, but I guess reduces software complexity at the cost of more hardware.

I quite like the display driver using memory scanned by hardware as that would stop the display from blanking or showing a single bright digit like the MK14 when a user program is executed.

You might consider putting the right hand decimal point displays upside down. That would give a left hand decimal point at the top instead of bottom of the display, which might look better if software is implementing a left hand decimal point. You just need to reassign the segment positions to show the correct characters when the display is inverted.

Its nice that you created a document to describe the circuit.

Note the 74172 was an 8x2 register file with three ports. The 74870 and 74871 are both dual 16x4 latches and also the dual port addressing is a bit strange.

Last edited by Mark1960; 18th Jun 2022 at 9:04 pm. Reason: Spelling correction
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Old 19th Jun 2022, 11:33 am   #5
Michael Haardt
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Default Re: HEXIO compatible to Elektor SC/MP system

Most likely it would be possible to convert the circuit from using register files to SRAM like a 6116, keeping /CS and /OE to ground and controlling it using /WR, which has precedence over /OE. But that wastes almost all of its bytes and 74LS189 are still well available as NOS. Indeed it is nice that all multiplexing is done in hardware without hard to obtain controller ICs. I still want to investigate if the remaining slight ghosting may be due to parasitic capacity of the LEDs.

The keypad encoding is optimized for a hex monitor like Elbug and needs very little software to be used. Remember Elbug used to be stored in 3x 512 Byte ROMs and space was precious. Scanning a matrix would need much more software without saving that much hardware. The whole Elektor HEXIO board shows a brilliant design, just the original documentation was quite poor.

I checked the available code and could not find any that uses the decimal point for anything meaningful, if at all. I guess it was quickly replaced by a serial terminal with NIBL.

I do have a Kicad Layout for a 140x100 mm PCB and will publish everything once finished, but this projects drags on for over a year now and will take more time. I use the layout as aid for the vero board bird nest. I printed a few keycaps and the embossed labels look nice, but the whole mechanical design is just an idea up to now.

Michael
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Old 19th Jun 2022, 5:03 pm   #6
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

For the LED ghosting you might experiment with the enable input of the 74HCT237 to give a slight delay from the clock of the counter and disable before the next clock.

The 74LS189 is probably a better choice than 6116 as separate data input and output makes the interface a bit easier. 5101 could be considered but are no easier to get than the 74LS189. If you were ok to use more modern components then a 1kx8 dual port ram would remove the multiplexer.
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Old 19th Jun 2022, 6:28 pm   #7
Michael Haardt
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Default Re: HEXIO compatible to Elektor SC/MP system

In this case the separate outputs provide no benefit. You get the wrong data to the display during a write anyway, but writes are so short that it does not matter.

I need to think about the enable input of the decoder. There is another half 393 available, but most likely it would need some glue logic to generate the needed signals. Then again it is not clear why I see that 10 us delay. The transistors should be faster than that, which is why I wonder if that may just be LED capacity discharging.

Michael
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Old 12th Jul 2022, 5:11 pm   #8
nlpe1goo
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Default Re: HEXIO compatible to Elektor SC/MP system

Hi,
I'm new to this forum, but I've also made a replacement of the hex I/O board of the SC/MP from Elektor. For the key-part I use the original design from Elektor but for the display's I use a SPI serial 8 7 segment displays with a max 7219. Unfortunately it is not compatible the Elektor version but with a little subroutine as usable as the original design. I have also the source code of elbug.
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Old 12th Jul 2022, 8:09 pm   #9
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

Hi nlpe1goo and welcome to the forum. It would be interesting to see some more details of your system, maybe you could start a new thread and add some pictures and schematic. I’d also like to see your code for the SPI interface, I have a couple of teletext display ICs to try and connect to my MK14 that I think use an SPI interface.
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Old 13th Jul 2022, 6:47 pm   #10
nlpe1goo
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Default Re: HEXIO compatible to Elektor SC/MP system

Hi Mark1960,
I don't have a working SC/MP system anymore. I use the HEXIO for other microprocessors. But I've a lot of knowledge of the SC/MP because this was my first computer. Do you know the 8070/8073? This is also known as SC/MPIII. The 8073 contains an improved NIBBLE basic interpreter. Bitbanging of SPI is fairly simple. You can use one of the two sense inputs and the three flag outputs generating data clock and cs. If you know Z80 or 6809 assembly also I can send you an example of a SPI routine.

Nick
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Old 14th Jul 2022, 6:00 pm   #11
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

Hi Nick,
I have a couple of INS8073 and have one running on protoboard. It would have been nice to get hold of an INS8070 to play more directly with the machine code but I have too many different projects occupying my time.

I was particularly interested in any optimisations you might have for SPI on an 8060, I don’t think SPI will be difficult to program.

I have a couple of z80 projects, loosely based on RC2014 and RomWBW. Attempting to get RomWBW running at 27MHz, but currently stalled due to work.

I have SPI running in hardware for z80, for sd cards and wiznet. An idea for a simpler hardware interface was to buffer A15-A8 to D7-D0 with one bit position shifted. Then an IN instruction can input one bit and also shift the value in the accumulator. Still nothing more than an idea and would need some work on clock timing.

Btw I should mention that as a new member of the forum your posts will be delayed until reviewed by a moderator, but that should pass as your post count increases.

Mark
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Old 15th Jul 2022, 10:08 am   #12
Phil__G
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Default Re: HEXIO compatible to Elektor SC/MP system

Its almost like the SIO instruction with SIN & SOUT was designed for SPI
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Old 16th Jul 2022, 6:33 pm   #13
Michael Haardt
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Default Re: HEXIO compatible to Elektor SC/MP system

The alternative is to build a SPI master controller using a few TTL chips. The description is all German, because this is a German modular computer, similar to the RC2014, just like 40 years older:

https://hschuetz.selfhost.eu/ndr/har...dio/index.html

Using OTIR SD Card access is about as fast as a RAM disk for the Z80.

Michael
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Old 16th Jul 2022, 6:54 pm   #14
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

This was my version using a ‘163 as a state machine to reduce the chip count.

https://groups.google.com/g/rc2014-z...m/gbm5jgtaBgAJ

Or similar to use featherwing wiznet module

https://groups.google.com/g/rc2014-z...m/LgeC8WiNAwAJ
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Old 16th Jul 2022, 9:07 pm   #15
Michael Haardt
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Default Re: HEXIO compatible to Elektor SC/MP system

That looks smart and saves the extra flipflop I needed. I miss the clock generation with low and high speed, though. I have seen cards that required it. Are there no level shifters for card select?

Michael
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Old 16th Jul 2022, 11:07 pm   #16
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

I didn’t include slow clock as it doesn’t seem to be needed for most cards.

Chip select is driven from 74HCT175 via schottky diodes so no level shift needed. The pull up within the micro sd card can be used as a card presence detect, though the sockets I’ve used also include a switch.

I did have some reliability issue at 20MHz clock speed, possibly timing between clock and data due to gate delays. I didn’t have a scope to investigate in detail, and still haven’t got round to trying to improve this now that I have a scope as it works fine at 18MHz.
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Old 17th Aug 2022, 5:43 pm   #17
Michael Haardt
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Default Re: HEXIO compatible to Elektor SC/MP system

Coming back to the original topic, I just finished the new HEXIO card:

http://www.moria.de/tech/scmp/hexio/

Running Elbug on the NDR Klein Computer with the SC/MP CPU card is fun.

Michael
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Old 17th Aug 2022, 11:40 pm   #18
Silvester
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Default Re: HEXIO compatible to Elektor SC/MP system

I noticed that you do not use 2K2 pull up resistors on the outputs of the 74LS189, as shown in the original Elektor design. I don't get a ghosting problem on my LEDs using similar circuit to Elektor using 7489 chips with 2K2 pull ups on output. Though I didn't invert inputs and outputs, just buffered outputs with 7417.
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Old 18th Aug 2022, 12:12 am   #19
Silvester
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Default Re: HEXIO compatible to Elektor SC/MP system

Quote:
Originally Posted by Phil__G View Post
Its almost like the SIO instruction with SIN & SOUT was designed for SPI
Perfect for MIDI 31250 baud, which it can send faultlessly at full rate. Also 9600 Baud Rx/Tx fileserving is possible (albeit with 2 stop bits)
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Old 18th Aug 2022, 6:21 am   #20
Mark1960
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Default Re: HEXIO compatible to Elektor SC/MP system

Quote:
Originally Posted by Silvester View Post
I noticed that you do not use 2K2 pull up resistors on the outputs of the 74LS189, as shown in the original Elektor design. I don't get a ghosting problem on my LEDs using similar circuit to Elektor using 7489 chips with 2K2 pull ups on output. Though I didn't invert inputs and outputs, just buffered outputs with 7417.
7489 has open collector outputs so it would need pullup resistors. 74LS189 has tristate outputs so shouldn’t need pull up resistors.

The ghosting might be due to modern LEDs being more sensitive to low current or short duration drive currents.
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