Keyboard PS/2 to Parallel converter

Because for a parallel keyboard you need a key-down strobe. If you just set the strobe on keydown and clear it on key up, you will not correctly perform rollover - If you are unfamilier with the term, rollover describes what often happens when a typist is trucking along at speed - one key is pressed, then another before the first is released, then the first released, and finally the second (and you usually have a third key happning after the second etc.) If you simply assert key-down on a make, and release it on a break, you get: Press A - Write A to port, assert key-down Press B - Write B to port, key-down is still asserted Release A - Release key-down Release B - key-down is already released In this scenario, you only got one keystrobe at the emulated parallel interface, and one of the keys will be lost. A true typist would throw your keyboard out the window. One way to resolve this is to simply "pulse" key-down, and not hold it down while keys are down - this works for most applications, but not for some games and things which actually use the duration of key-down. If you take this approach, you can ignore key-up codes, and simply write the ASCII value to the parallel port on key-down codes, and strobe the key-down signal on the parallel interface - but you still have to queue data from the keyboard because you cannot process the next key until after you have released key-down on the parallel interface. If you prefer to implement a true key-down signal on the parallel interface, you assert key-down on the key-down scancode, and queue anything occuring until the matching key-up scancode - then you process the codes in the queue, which arranges all the keys "in order". Btw, most parallel keyboards have "two key rollover" which means that you can press one more key while one is down, but not two - this ability is inherent in the key scanning matrix - the keyboard scans until it finds a key and stops the scanner, watching only those lines until the key is released - if another key is pressed in the meantime, it will be detected as soon as the scanner resumes - if more than one extra key is pressed, the one detected may not be the next one that was actually pressed (hence "two key" rollover). This type of keyboard also usually output a key-down strobe for as long as the key is held (bascially the same control signal that holds the scanner). I've implemented a number of keyboards this way, and it's actually quite sufficient for most typists. Fancier keyboards feature 'n' key rollover, which means the scanner runs all the time, and keystrokes are queued so that any number of pending keys can be serviced. These typically have only a key strobe output, so that the queue can be emptied to "keep up" with the guy mashing down keys. Yes they do, but they only repeat the key-down code - you need to maintain your own repeat logic if you want to be able to toggle the key-down strobe while keys are held down (just asserting it again won't make it toggle). My usual approach is once I've received key-down, and am waiting for key-up to release the key-down strobe, I queue anything else received, EXCEPT for the matching key-down code - these I ignore and implement my own repeat logic. But there are many possibilities - you could provide a "positive pulse" in the key-down strobe when you receive secondary key-down codes - this would provide parallel repeat at the same rate as the PC keyboard is providing (and you can program that to a certain extent if you like - I personally like the repeat logic in my code where I am not reliant on the variances between "compatible" keyboards. Regards, Dave -- dave06a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.classiccmp.org/dunfield/index.html