Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site ucbvax.UUCP Path: utzoo!linus!decvax!tektronix!ucbcad!ucbvax!daemon From: daemon@ucbvax.UUCP Newsgroups: fa.info-vax Subject: Re: 730 Unibus speed revisited. Message-ID: <823@ucbvax.UUCP> Date: Thu, 15-Sep-83 03:10:34 EDT Article-I.D.: ucbvax.823 Posted: Thu Sep 15 03:10:34 1983 Date-Received: Thu, 15-Sep-83 13:54:45 EDT Sender: daemon@ucbvax.UUCP Organization: U. C. Berkeley Computer Science Lines: 51 From GEOFF5@SRI-CSL Thu Sep 15 00:09:41 1983 Date: Wed 31 Aug 83 23:57:33-EDT From: Richard Garland Subject: 730 Unibus speed revisited. I asked several days ago if anyone had ideas why my Versatec printer/ plotter (heavy character IO) runs faster on a 730 than on the 780. One response ... mentioned the 730 Unibus goes 20% faster than the 780's. Is this true? I thought the Unibus was clocked at some fixed speed? One of the nice things about the UNIBUS from the interface designer's perspective, is that it is asynchronous, i.e. each transaction consists of discrete steps that go through a handshake sequence before they proceed. In general, synchronous buses such as the VAX/780 SBI are considered faster than asynchronous buses such as the UNIBUS or the IBM 360/370/4300 data channel. There are however some design guidelines about what expectations you can make about UNIBUS timing. These used to be published in something called the UNIBUS HANDBOOK and the PDP-11 INTERFACING HANDBOOK, but I hear that such information has not been easily accessible in recent years, and recent DEC CPUs have not adhered to the OLD specs. Recent shortenings of timing windows took place with 11/44 and 11/750, so it is not surprising if the 11/730 is shaving off yet another little bit, since it is so dependent on its UNIBUS. However, the important part is not the UNIBUS speed, but the number of instructions that must be executed for each character interrupt (see below). Another idea I had is that the Unibus is connected more directly to the 730 and so maybe there is one less layer of software to go through. (I think interrupts are directly vectored as in the 750 for example.) The VAX HARDWARE HANDBOOK 1982-83 describes the UNIBUS adapter of the 11/730 in just about the same terms as the 11/780. But you really ought to write a device driver that will drive your VERSATEC in DMA mode.... That should make your paper roll flow rather than crawl!! med venlig hilsen Lars Poulsen