The diodes in the original version have been removed, because they served no meaningful purpose and an R2R ladder design was used for the DAC DA conversion. This not only improves the linearity of the DAC greatly, bit it also ensures that the output impedance remains stable and of course means not having to buy 8 or 9 different non-standard value resistors. The completed devices look like this:
Track layouts have been made for both Centronics and Edge-connector. As seen in the picture above, the layouts use isolation displacement connectors (the type usually intended for flat cable) which have been modified (ie: the plastic band and clips removed) so that it can be mounted directly to the PCB. 1.5mm FR4 PCB should be used preferably to give the circuit stability.
== Resources ==