TMS9995 sbc

PCB design

Following is a PCB design of Stuart Conner's TMS9995 sbc (breadboard) microcomputer. The pcb design was made with DipTrace software purely from pcb components (not from original schematic). Major difference with the pcb board and the original schematics is:
  • the addition of 40 pin expansion socket,
  • led with a resistor,
  • a bank of links for pullup resistors (so they can be easily disconnected if not needed-say with possible expansion options)
  • 10 pin socket that connects to 9 pin panel RS232 socket
  • terminal block for power and reset switch connection
Layout of components is better shown below:

pcb top layout

In addition to original Stuarts schematic here is connection for expansion connector, terminal connector and the 10pin IDC socket to 9 pin D connector. Note that the expansion connector is pin for pin compatible to CPU chip (except pins 1 and 2 - N/C). J2 pins are numbered 1-20 on LHS and 40 to 21 on RHS from top to bottom (see image above). Links must be placed on J3 for pullup resistors.

The following is info about address and data bus for TMS9995 chip (note the reversed significance of numbers). That hopefully explains the wiring above.

Power Consumption

If all Ok ... the power consumption should be less then 170mA @5V.

Putting all in a box

With the pcb reasonably small the whole lot was placed in a small ABS type box.