Motorola 68000 ram-memory mapper-decoder schematic help please

Holering · 05-08-2019, 01:19 PM

Motorola 68000
4 megabytes memory space

Helping friend assemble RAM-memory mapper-decoder with 74LS612. 74LS612 seems to be best supported ram mapper-decoder with some guides available. Unfortunately these guides target 8-bit machines. Motorola 68000 is 16-bit with 24-bit address (24 address lines), and makes things complicated. Target only has 4 megabytes of memory space left after all other assembly (i/o, expansion, accessories, etc).

We're thinking we might need four 74LS612 controllers to extend ram to at least 32-64 megabytes. Preferably 64 megabytes.

Mostly want something simple like schematic going to available 24 address lines and 16 data lines of Motorola 68000.

Thanks for any help!

EDIT 0:

Here's pinout of 74LS612

Code:

Pin outs
          +---------------------+
   RS2   -+  1               40 +-   +5V
          |                     |
   MA3   -+  2               39 +-   MA2
          |                     |
   RS3   -+  3               38 +-   RS1
          |                     |
  /CS    -+  4               37 +-   MA1
          |                     |
/STROBE  -+  5               36 +-   RS0
          |                     |
   R/W   -+  6               35 +-   MA0
          |                     |
   D0    -+  7               34 +-   D11
          |                     |
   D1    -+  8               33 +-   D10
          |                     |
   D2    -+  9               32 +-   D9
          |                     |
   D3    -+ 10               31 +-   D8
          |      74LS612        |
   D4    -+ 11               30 +-   D7
          |                     |
   D5    -+ 12               29 +-   D6
          |                     |
   MM    -+ 13               28 +-   NC / C (see text)
          |                     |
   MO0   -+ 14               27 +-   MO11
          |                     |
   MO1   -+ 15               26 +-   MO10
          |                     |
   MO2   -+ 16               25 +-   MO9
          |                     |
   MO3   -+ 17               24 +-   MO8
          |                     |
   MO4   -+ 18               23 +-   MO7
          |                     |
   MO5   -+ 19               22 +-   MO6
          |                     |
   GND   -+ 20               21 +-   /ME
          |                     |
          +---------------------+

EDIT 2:

Found some stuff here http://www.unige.ch/medecine/nouspik...9/superams.htm and here http://www.unige.ch/medecine/nouspikel/ti99/hams.htm but still would be nice to receive some ideas. Thank you and kind regards.

business_kid · 05-09-2019, 05:00 AM

An 8-bit memory mapper is not what you want for a 16 bit PC.That limits your page size to 256 bytes.Motorola, Intel, probably Zilog and many others made all the support chips for their 16 bit CPUs, and that should surely be your first port of call. Obsolete parts can often be sourced from a broker who will be only too happy to part with through hole stuff.

That design might theoretically work but will always be the achilles heel of your thing, because of fab size and propagation times, which will depend on manufacture date. Personally, I feel your friend is going down in flames. I'm sorry to be harsh, but if he started with 4G of ram and committed all the high lines to I/O, he's cruising for a bruising. Drink pints, watch football & chase girls (or whatever you're into) and let him fail on his own. There's 3-->8 & 4-->16 decoders out there for thinning down data bus use.

The key thing for you is carry out. The easy bit is triggering upper memory lines but the memory 'page' you want imho = 16 bits/64k pages. You need a carry-out line. That way you can increment upper memory lines with some cheesy counter or set some latch.

If you're into SMT, why use a 68K which is no small chip (68 pins @2.54mm lead spacing). The 68000 was good for it's time (16 bits, 15 Mhz), but it's a bit of a steam engine today.

Holering · 05-09-2019, 04:31 PM

Here's some more info about the Motorola 68000 memory https://hackaday.io/project/5-mc6800...nd-rom-modules. Please let me know of a common 16-bit or 24-bit, or suitable mapper-decoder commonly available and well supported. Thanks.

business_kid · 05-10-2019, 03:58 AM

To put it briefly, No I won't come up with components for you.

I'm retired, and a stroke survivor. It's your design, and therefore I would expect you to do the grunt work, which is over 50% of the design process these days. I'm giving the benefit of my experience; lat me know if you don't want it and I'' unsubscribe from your thread. I don't see what this has to do with linux anyhow, and this is a linux forum. What I found sourcing discrete components recently(2013) was that most are gone. Small glue logic & opamps are alive & prospering, but bus logic seems unavailable. Component suppliers are vanishing, too, as everything has transferred to fpga. I mentioned that Lattice (as of 2013) do the smallest fpgas I am aware of, and suggested component brokers for obsolete parts. Your use of a 68000 suggests this is a private one-off design. If it's in any way commercial, ditch the 68k, ditch the memory mapper and install an fpga. You can buy cores and put in any devices you can buy cores for. That's a lot more than you'll get off the shelf. VHDL is not a scary language to learn.