Hi all, just to set the scene, this is my first post, (so sorry if I ramble) and, as of the beginning of this year, by electronics knowledge peaked when I did my A-level physics in 1989, so sorry if I ask stupid questions or annoy people!
At the beginning of this year I dug my old Commodore 8-bit computer collection out of an attic and tried to get them working. Had fun repairing broken C64's, Plus4's and Vic's and it's now officially a hobby.
To start with it was just a case of swapping chips but to take it to the next stage, I'm slowly beginning to understand the basics of digital circuits (boolean logic is not a problem, being from an IT background) and trying to understand phrases like "open collector" and "tri state" outputs.
I am in the process of building a test harness for the +4/C16 range of 8-bits, and to do this I want to have dongles on all the ports to "spoof" the signals that would come from the outside world. My original (and sadly fatal to some IC's) approach was to short a load of pins together and check the appropriate registers in my ROM code (on an eprom.)
I now want to be a bit more sophisticated and generate the appropriate 'active low' signals in a more graceful manner. For each of the two joystick ports, there is a total of 5 active-low inputs, and an active-low 'select' line output (as well as +5v and 0v). The computer pulls the 'select' line low and reads the 5 inputs on a latched register to determine the current state of the joystick.
If possible, I want a single chip solution which will pull the outputs low in turn, every time the select line goes low, all outputs being high when the select line is high. At the moment, I think I could do it with a 74HC4017 Johnson counter and 10 inverters, which is at least 3 IC's. I am not yet familiar enough either 7400 or 4000 series logic chips to know whether there is a better way...
Any tips or guidance appreciated. More info on request.
Cheers,
Rob
At the beginning of this year I dug my old Commodore 8-bit computer collection out of an attic and tried to get them working. Had fun repairing broken C64's, Plus4's and Vic's and it's now officially a hobby.
To start with it was just a case of swapping chips but to take it to the next stage, I'm slowly beginning to understand the basics of digital circuits (boolean logic is not a problem, being from an IT background) and trying to understand phrases like "open collector" and "tri state" outputs.
I am in the process of building a test harness for the +4/C16 range of 8-bits, and to do this I want to have dongles on all the ports to "spoof" the signals that would come from the outside world. My original (and sadly fatal to some IC's) approach was to short a load of pins together and check the appropriate registers in my ROM code (on an eprom.)
I now want to be a bit more sophisticated and generate the appropriate 'active low' signals in a more graceful manner. For each of the two joystick ports, there is a total of 5 active-low inputs, and an active-low 'select' line output (as well as +5v and 0v). The computer pulls the 'select' line low and reads the 5 inputs on a latched register to determine the current state of the joystick.
If possible, I want a single chip solution which will pull the outputs low in turn, every time the select line goes low, all outputs being high when the select line is high. At the moment, I think I could do it with a 74HC4017 Johnson counter and 10 inverters, which is at least 3 IC's. I am not yet familiar enough either 7400 or 4000 series logic chips to know whether there is a better way...
Any tips or guidance appreciated. More info on request.
Cheers,
Rob