Building a 68000 Single Board Computer - Possible Future Enhancements

I've learned a lot with this TS2 project and done much more with that than I had originally hoped. While I have some more things to work on, I think it is time to put it on the back burner for a while and get back to some other projects.

Looking forward, it might be interesting to do a new revision of the board, which I could call revision 2.x (I called my version 2.0 since it revised the original Teesside design).

Here are some thoughts on some possible changes and features in a new version.

Printed Circuit Board Layout

A printed circuit board would make it much easier and faster to assemble than the wirewrap prototype. Professionally manufactured PCBs that are double-sided (or more), silkscreened, and solder masked can be obtained very inexpensively now from a number of vendors. Since the design is now in a CAD system (kicad) which has a PCB layout facility, it should not be too much effort to lay out a board. I've never used kicad for PCB layout or even laid out other than very simple single-sided boards, so this would be a good learning experience. Now that the design is proven, the risk of a PCB layout should be quite low.

Replace Baud Rate Generator With Fixed Clock Oscillator/Counter

The Motorola MC14411 baud rate generator chip used in the current design is a little difficult to obtain and overkill if only one baud rate is ever used. Newer UART chips have on-chip baud rate generators, but assuming the design stayed with the 6850 to remain software compatible, a baud rate generator could be created with simpler and more easily obtained circuitry. This link, for example, shows how to do it with a 74HC4060 and the existing 1.8432 MHz crystal.

Support Higher Baud Rates (Through Jumpers or Software)

It would be desirable to be able to run the serial port at baud rates higher than 9600, when downloading programs for example. A new baud rate generator circuit would allow this, since the 6850 can run at higher rates given a suitable clock.

Add One Or Two PIA, VIA, or PIT Chips

A parallel port would allow controlling hardware. This could be the Motorola 6820 or 6821, the more powerful MOS Technology 6520 or 6522 VIA, or the Motorola 68210 Parallel Interface/Timer (PIT) which was present in the Motorola ECB.

Optionally Support Line Drivers/Receivers For True RS-232 Serial Ports

True serial ports might be desirable for some users, so the line driver/receiver circuitry from the original TS2 could be included as an option. This could be simplified by using a chip like the MAX232 which does not require +/12V supplies.It might also be useful to support hardware handshaking with RTS/CTS, which currently do not connect through to the FTDI connectors.

Add Connectors For External Parallel I/O, Interrupts, Etc.

A connector for some external signals would be useful if there was a parallel port. The interrupt lines could also be available here.

Replace 25LS2548 Chip

The 25LS2548 decoder chip is quite hard to obtain and may be an impediment to people who want to reproduce the circuit. This could be replaced by more commonly available chips. Probably a 74LS138 decoder plus a 74LS06 open collector driver and maybe and another gate or two.

On-board 5 Volt Power Regulator

To allow running off of unregulated power, an on-board 5 volt regulator could be added, either a 78L05 series regulator or a more modern buck converter.

Add an LED Display

A seven segment LED display would be useful for diagnostics and other purposes. This could be driven from a PIA (if present) or with dedicated circuitry (see the Clements book p.649 for a possible approach to drive a 7 segment LED).

Prototype Area

If there is room on the PCB, an area with a grid of 0.1" holes could be provided for prototyping.

Support Either RAM or EPROM/EEPROM In All Sockets

With a few changes, the design could support either RAM or EPROM/EEPROM chips on any of the memory sockets (the boot code would need to be in ROM). This would allow, for example, 48K or RAM for users who only needed 16K of ROM). It might also be handy to optionally (via a jumper) allow the EEPROM devices to be written to.

Support Larger RAM and ROM Chips

Larger RAM and ROM chips (e.g. 27128, 27256, etc.) are available and could be supported to allow more memory. This would impact the memory map and address decoding circuitry and make it more complex. Given more RAM, the board could potentially run a stripped down version of an operating system such as Linux or Minix.

Mass Storage for File System

To run a "real" operating like Linux would require some form of mass storage for the file system. This could be done with some flash memory and some suitable interface, maybe using an SD card interface intended for Arduinos. This is probably more of a version 3.x feature (at which point maybe I could add video, a real-time clock, Ethernet, etc.)