Issues with WAIT line on clones

A project log for Tek V2

Version 2 of PS/2 keyboard adapter for Zeddies

danjovicdanjovic 08/09/2019 at 23:120 Comments

On the ZX80/81 the /WAIT line is used to synchronize the Z80 with NMI timing twice during each video frame. The inverted /HALT line is ORed with /NMI using a transistor and some resistors.

Some clone machines (even modern clones) have been designed using logic gates instead of a transistor.

The problem with the latter designs is that a peripheral device (like my keyboard adapter) would fail to bring down the /WAIT line to hold the Z80, as the signal is generated by an ordinary TTL gate (instead of an Open Collector).

It can be fixed though with one diode and one resistor.