Re: petSD project page

From: Nate Lawson <>
Date: Mon, 20 Jun 2011 07:57:50 -0700
Message-Id: <>
On Jun 20, 2011, at 2:56 AM, Ingo Korb wrote:

> Jim Brain <> writes:
>> Scanning both busses might be an issue, but maybe not insurmountable.
> The easiest way would be to wait for an ATN signal from either bus at
> startup, then servicing only that bus until the AVR is reset. This
> scheme might switch to the wrong port if there are other devices
> connected on both ports and not all of them are powered on.

The xum1541 firmware does this for IEC and IEEE support. The first time it is accessed, it detects a bus and then sticks to it.

> A bit less easy to implement is to allow switching whenever the
> currently-serviced bus is idle and there are no open files. The last
> condition avoids situations where someone might shoot themselves in the
> foot, e.g. by accidentally accessing files on the same secondary number
> from both computers at the same time.

I chose not to go this route for several reasons:

1. Shared IO pins. I didn't want to confuse IEC devices when accessing IEEE using the same pins.

2. Electrical concerns. Users shouldn't be plugging and unplugging devices while we are alive (powered over USB). It could lead to a short if a plug was inserted wrong or a surge on device power-up.

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Received on 2011-06-20 15:00:35

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