Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 beta 3/9/83; site sdcrdcf.UUCP Path: utzoo!linus!philabs!sdcsvax!sdcrdcf!darrelj From: darrelj@sdcrdcf.UUCP Newsgroups: net.micro Subject: Re: Apple //e braindamaged keyboard Message-ID: <350@sdcrdcf.UUCP> Date: Sun, 26-Jun-83 13:57:43 EDT Article-I.D.: sdcrdcf.350 Posted: Sun Jun 26 13:57:43 1983 Date-Received: Thu, 30-Jun-83 11:30:47 EDT References: <108@iwu1c.UUCP> <915@uwvax.ARPA> <203@packet.UUCP> Reply-To: darrelj@sdcrdcf.UUCP (Darrel VanBuer) Organization: System Development Corporation--a Burroughs Company Lines: 14 Part of a full N-key rollover keyboard implementation requires that the connection made in the scanning matrix (typical scanning chips have the potential for somewhere between 70 and 110 keys in a not too far from square array, e.g. 8x11) have a nonzero connection impedence under some characteristics. The possibilities include plain (0 ohm) switches with diodes (so switch is unidirectional) and switches which create a resistor or capacitor with measurable impedance, thus sneak paths have at least 3X the impedance, so a carefully tuned threshold detector won't see false keys. Another approach in some Hall-effect keyboards (in which each key contains a little detector chip) is that the key sends simultaneous brief pulses onto it's two lines in the scanning matrix on key closure. Ghosts then can occur only in the event two keys are pressed in one pulse time (less than a millisecond, usually).