T68KMB, Tiny68K with RC2014 expansion busses

[Bill Shen]

It took me a while to realize that instead of building SBC in 1.5X RC2014 format that plugs into a RC2014 backplane, I'm better off building the SBC in 102mm X 102mm format and add several RC2014 bus connector.  T68KRC is such case; it is a SBC 68000 in 1.5X RC2014 format (76mm x 50mm), I redesign it to 102mm X 102mm board with two RC2014 bus connector and an expansion connector and named it T68KMB.

Other than the added RC2014 connectors, T68KMB also added a I2C connector specifically for a 128x64 OLED display.  It is software compatible with T68KRC.

T68KMB homepage is under construction here: https://www.retrobrewcomputers.org/doku.php?id=builderpages:plasmo:t68kmb

[Colin MacArthur]

You come up with GREAT boards...

Love the OLED and Serial EEPROM...

CM

[Bill Shen]

The I2C is rather tricky to use because I'm sharing the same 2 data lines with serial EPROM which has different protocol than I2C.  In any case, it is working.  This is the 128x64 OLED displaying an ASCII table.  I'm going to port Conway's Game of Life.
  Bill

[Bill Shen]

I'm on a long trial to convert all my SBC designs with RC2014-like expansion to standalone computers.  Standalone means a computer with with its own keyboard and VGA display.  For some SBC, it is just a matter of plug in a VGARC and write the supporting software.  But for others there may be cuts and jumpers since 16-bit I/O was't a design consideration when some SBC were conceived.  T68KMB and its close cousin, T68KRC are example of SBC without 16-bit I/O support.  At end of this long trial is hopefully a common VGARC design for all my SBC as well as a list of hardware upgrades to existing SBC to make them 16-bit I/O capable.

T68KMB has 3 RC2014-compatible expansion that actually provided good emulation of Z80 timing where CPLD generated the correcting timing for /RD, /WR, and /IORQ.  The original CPLD design generated /IORQ for address range $FF8000-$FF8FFF.  It is a solid interface to other RC2014-compatible board, but there are several problems when it comes to 16-bit I/O.
1.  Only 8 lower addresses are on the connector--it was meant for 8-bit Z80 I/O only
2.  Only /IORQ is mapped to 68K memory region; /MREQ is pulled high.
3.  The memory mapped region is too small to support 4K dual port RAM and PS2 keyboard.  68K has 16-bit wide data bus but no dynamic bus sizing, so when mapped to 8-bit RC2014 bus, every other byte is discarded.  This means it needed 8K memory space in order to access 4K dual port RAM.

Some modifications to T68KMB is needed to get it working with VGARC:
1.  Add 4 more addresses, A9-A12, to the expansion connector
2.  Add /MREQ output to the expansion connection and assign addresses $FFA000-$FFBFFF for assertion of /MREQ
3.  Expand the memory space associated with /IORQ to $FF8000-$FF9FFF
4.  Since the nominal frequency of T68KMB is relatively slow 12MHz, there are no provision for wait state generation.  This is good news because the signal definitions of the RC2014-compatible connector are same as the standard connector definition.

Picture shows the modification to T68KMB to accommodate VGARC.

It is also necessary to modify VGARC by cutting out A12-A15 and patch in A4-A7 for the same reason as was done for CB030.  This is now a standard modifications for 68K and 6502 SBC.  New version of VGARC will have 4 jumpers to move between Z*80 address scheme and 68K/6502 address scheme.
  Bill