Re: 6522 VIA inputs

From: Gerrit Heitsch <>
Date: Fri, 21 Apr 2017 18:10:11 +0200
Message-ID: <>
On 04/21/2017 11:21 AM, wrote:
> In 1541 bits five and six of the VIA port at $1800 are used to initialise the device id. In the older drives, both respective pins are grounded via bridged trace fields. In the newer there is a DIP switch block. I saw this schematic uncountable number of times, yet only recently it struck me that when the trace bridges are cut the pins are left floating (!). There are no pullups whatsoever. Now the question - is this a documented feature of the VIA that can be safely relied upon? Something like "unconnected pins will always read 1 when configured as inputs". I had a look at the 6522 datasheet and found only "Port B lines represent one standard TTL load in the input mode [...]"

Take a look at the MOS datasheet for the 6522 (If you can't find it, I 
can mail it to you), it has schematics for the port drivers for port A 
and port B, they differ, but both ports have a permanent pullup in form 
of a MOS transistor for each bit.


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Received on 2017-04-21 17:00:02

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