ncoplin_at_orbeng.com
Date: 2003-10-10 07:34:19
Hello All,
-----Original Message-----
From: Davison, Lee
>> There must be a way to protect the parallel port using simple components.
>There is, simply connect a resistor in series >with each bi-directional
line
>you wish to protect. A value of 180 to 220
>ohms should work.
I would agree with this method as the simple approach, however my experience
so far is with software handshaking only and make sure your routines are
robust. The problem with a tri-stating buffer etc is it will either be under
software control (on the PC side) or reset control on the "gadget" side.
I've been through the issue recently for 64HDD/Pro/GEOS/DriveGhost code
(Commodore specific details below):
Whilst developing /debugging the new Bi-Directional PwrLoad support I'm sure
I had bus contentions at various stages (due to development bugs). However,
I never fried the LPT or the userport CIA - lucky perhaps... The
recommendation for Commodore users trying this is always reset the C64 first
as it will put the CIA into input mode. If your software doesn't hang (and
any "good" product should not) the bus contention will never arise if the
handshaking confirms (not just tells) what the other side is doing. To be on
the safer side, only momentarily switch the CIA into output mode, then
switch it back... simple dec$dd03/inc$dd03
And.... check that the LPT supports input mode... if it doesn't the PC must
tell the C64 to abort!!!
In the beginning I thought about connecting the ATN line (also available on
the userport) to a 74xxx buffer chip... but didn't take it further.
- Nick
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