ALTO DISK TOOL
I spent another day iterating with the online free design verification tool associated with the 4PCB foundry and with my design on DesignSpark PCB. I really cleaned up the design substantially, ensured a good silkscreen layer, but had to accept some spurious errors that reflect limitations of the checking software.
Specifically, a metal plated hole through a board is called a 'via' and links a copper trace on one layer to another trace on a different layer. Mostly, these are linking the top and bottom layers of the board, where traces run left to right on the top layer and top to bottom for the bottom layer. Thus, a signal can pass over another using the 'other' layer.
I designed this for a four-layer board, which has two internal layers in addition to the top and bottom. One of the internal layers is ground and the other is the +5V power. If a component is connected to ground or +5V, it has a larger via which is electrically connected to one of the inner layers.
The design review software spots vias when they have signals connected to them on both top and bottom, the typical usage, but if the via is connected internally, the software confuses the plated hole with a hole that a component lead will fill. If a component is attached to the hole, it has to be soldered on.
Any spot where soldering occurs must have an opening on the solder mask layer, all other parts of the circuit board to and bottom are insulated with a thick green coating. Since the software falsely thinks the power or ground via is a solder pad for a through-hole component, it is flagging to me that the solder mask is covering this pad. That would render the pad or component hole un-solderable but in this case, no solder is needed.
I placed the order for four boards, to arrive in a bit more than a week, at a net cost of just under $300 or $75 per board. I should have all the components to attach by the time the boards arrive, thus I can assemble them at that time.
It is time to begin designing the complementary board, for the disk emulator role, which will use the tried and true components and circuit elements, only connecting them in a different order to suit the disk signals going in and out of the Alto computer.