Re: Questions about system bus

Ed Casas (edc@ece.ubc.ca) Thu, 13 Apr 2000 06:32:48 -0700 

Date: Thu, 13 Apr 2000 06:32:48 -0700
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: Questions about system bus



On Thu, Apr 13, 2000 at 01:15:56AM -0700, Pauline Siu wrote:

> 1. What is a VME bus ( How does it differ from ISA and PCI buses) ? 

VME is a system bus that was widely used in the 1980's. It was
designed around the Motorola 68000 processor bus in the same way
as the ISA bus was designed around the 8088 processor bus.  Due
to the cost of the DIN-style bus connectors, the relatively large
card size (``form factor'') and the relatively low bus bandwidth
it is now only used in high-end military and industrial
microcomputer systems.

> 2. What does it mean by "sink" and "source" current?

Sink means that current flows in (to a pin), source means that
current flows out.

> 3. How to calculate bus bandwidth? 

Compute the following:

(bytes read or written per bus cycle) / (bus cycle duration)

For example, if an 8-bit bus with a 5 MHz (200ns) clock requires
9 clock cycles per bus cycle then the bandwidth is:

1/(9*0.200) = 555,000 Bytes/s

-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592