29-11-2020, 07:43 PM
The DGZ80 design relies upon some unconnected TTL inputs to float to a logic High. This poor practice because not only does it make it susceptible to be unreliable from EMI but the behaviour of floating inputs can vary between semiconductor manufacturers. There are even written articles on how this practice can also damage parts from overheating.
The easiest way to address this is to connect suitable pull up resistors for user selectable TTL inputs. A common value is 4k7 but this can vary according to the circumstance.
IC13 - 81LS97, (Sense Port)
IC33 and IC36 - 74LS257, (POJ Switch)
IC21 and IC25 - DM8131, (Memory and I/O Address decoders)
The easiest way to address this is to connect suitable pull up resistors for user selectable TTL inputs. A common value is 4k7 but this can vary according to the circumstance.
IC13 - 81LS97, (Sense Port)
IC33 and IC36 - 74LS257, (POJ Switch)
IC21 and IC25 - DM8131, (Memory and I/O Address decoders)