Date: Thu, 2 Jan 1997 13:21:34 +1030 (CST) From: Michael Smith <msmith@atrad.adelaide.edu.au> To: luigi@labinfo.iet.unipi.it (Luigi Rizzo) Cc: msmith@atrad.adelaide.edu.au, hmmm@alaska.net, hackers@FreeBSD.ORG Subject: Re: Ints (fwd) Message-ID: <199701020251.NAA14993@genesis.atrad.adelaide.edu.au> In-Reply-To: <199701020146.CAA00972@labinfo.iet.unipi.it> from Luigi Rizzo at "Jan 2, 97 02:46:28 am"
next in thread | previous in thread | raw e-mail | index | archive | help
Luigi Rizzo stands accused of saying: > > > > Work it out; 5V / 24mA (LSTTL sink limit) = ~200R. Assuming 50pf of > > parasitic capacitance (not unreasonable), you get RC = 10usec. Not so > > good. (220R is actually the accepted value for LSTTL and compatible > > families, witness the 220/330R resistor pairing in passive SCSI > > terminators.) > > of course all this reasoning breaks since 50pf = 50e-12 F so RC is > 10 nanoseconds... :) Ah whoops, forgot some zeroes there. Still, 50pf is a bit on the light side, and consult Solari for some amusing stories about the negative-time requirements on ISA-bus signals 8) Just checking with Solari, actually; the spec is 15pf/slot, and 24mA sink. OC lines on the bus are tied with 300R, not 220. So on a modern 4-slot mixed ISA/PCI bus backplane that's 300*60p = 18ns. IEEE P996 specifies 11ns bus settling time for a fully-loaded 8-slot backplane, so that loses again. But Solari observes : "[EISA uses open-collector interrupt inputs] The open collector approach for a level triggered INTERRUPT signal line relies on a pull-up resistor to deactivate the interrupt request. The EISA bus specification has the largest resistor value; consequently, 500 nanonseconds are required for deactivation on the E-ISA platform." (1ed, p5-76) > > scheme. But not ISA. YOU CANNOT SHARE INTERRUPTS SAFELY ON ISA. > > true, but that's because of edge triggered lines as you say later. > Were they level sensitive, there would be no problem in sharing > them. Err yes. But like we said, they ain't. 8) > Luigi -- ]] Mike Smith, Software Engineer msmith@gsoft.com.au [[ ]] Genesis Software genesis@gsoft.com.au [[ ]] High-speed data acquisition and (GSM mobile) 0411-222-496 [[ ]] realtime instrument control. (ph) +61-8-8267-3493 [[ ]] Unix hardware collector. "Where are your PEZ?" The Tick [[
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199701020251.NAA14993>