Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rutgers!mit-eddie!genrad!decvax!ima!johnl From: johnl@ima.UUCP (John R. Levine) Newsgroups: comp.sys.ibm.pc Subject: Re: LPT3 support Message-ID: <380@ima.UUCP> Date: Wed, 17-Dec-86 10:24:27 EST Article-I.D.: ima.380 Posted: Wed Dec 17 10:24:27 1986 Date-Received: Thu, 18-Dec-86 02:39:58 EST References: <133@parcvax.Xerox.COM> <295@ima.UUCP> <1348@cit-vax.Caltech.Edu> Reply-To: johnl@ima.UUCP (John R. Levine) Distribution: na Organization: Javelin Software Corporation Lines: 13 Summary: lpt interrupt request isn't latched The problem with the PC's printer interrupt is quite simple. After you print a character, the printer sends back a 5us ACK pulse to say it's ready for another character. The interrupt request is generated from that pulse. If your PC has its interrupts masked during those 5us, or if it takes longer than 5us for the instruction that's executing to finish, you lose the interrupt. Had the guys at IBM who designed the interface been allowed to speak to the people who bought the printers, they would probably have decided to generate the interrupt from the not-busy line which stays up until you send the next character, and which other machines with working printer interrupts use. Pfui. -- John R. Levine, Javelin Software Corp., Cambridge MA +1 617 494 1400 { ihnp4 | decvax | cbosgd | harvard | yale }!ima!johnl, Levine@YALE.EDU Where is Richard Nixon now that we need him?