Andrew It's something my Boss taught me he Designed some OPL 3 stuff back in the mid to late 90's.
He worked for A trend, and did some mother boards and video cards for them.(He passed away last month.)
With ISA Bus you should assume signals up to 16MHz, with the XT bus you should keep in mind some third party boards exist that run up to 12MHz.
Besides that Near the back plane there is this one trace making sharp corners where those can be eliminated.
On U16 between the Pins are copper pour where there shouldn't because they will cause signal interference especially there!
P18 the first FD connector should be rotated 180 degrees so that the ground plane can be continues without disturbing the signal wires.
Design technical we would need to bring down the number of components, since then we regain some board space for some proper routing.
And besides that Andrew in general I design for low speed applications, at least i Used to.
PS: Problem area's.
-GND/Copper pour, should be have a be forced to a higher width so they don't create fingers between signal lines.(The FD connectors and U2 and U16)
-Sharp corners in the design these should be evaded whenever possible.
-Not all Via's are connected correctly some even have sharp corners before their connection.(Pin 17 on U4)
Besides the copper pour the 74 logic part is done in a excellent manner you said this was not done by Hand?
