Re: 6522 VIA inputs

Date: Fri, 21 Apr 2017 11:34:07 +0200
Message-Id: <>
> On 2017-04-21, at 11:21, 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 [...]"

Even only deciding to post the question seems to help. I _think_ I found the answer immediately after posting the question... In the datasheet there is a Figure 3. Although it's caption says: "Port B Output Circuit", which originally made me believe that it's only the part used when the pin is configured as output, it also contains a line described as "INPUT DATA". This line seems to be in fact pulled up. I assume this is what makes it reliable - a kind of internal pullup. Would still appreciate corrections if I am missing something here.

SD! -

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Received on 2017-04-21 10:02:38

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