Re: EMC compatable computers

[Matt Shaver <mshaver-at-erols.com>]

On Saturday 11 January 2003 12:36 am, john wrote:
> The observation that ethernet does not always need a protocol
> stack is useful, and also applies to USB I suspect.

This is true, in an electrical sense, but USB protocol (what little there is, 
compared to Ethernet) is enforced by the USB Host Controller in the PC. It 
operates more or less autonomously by processing a linked list of commands it 
fetches from a shared area of system memory. The one millisecond frame time 
is essentially hard coded in the host controller's logic. The highest 
priority isochronous transfers can only monopolize 90% of the time in any one 
frame and interrupt transfers (from a peripheral back to the PC) can be 
delayed till the next frame.

In comparison, an ethernet controller is like a complex, multibyte UART.

Matt