Re: Drive disconnect switch for SX 64

From: Nate Lawson <>
Date: Tue, 30 Nov 2010 17:39:38 -0800
Message-ID: <>
On 11/30/2010 2:56 PM, Ingo Korb wrote:
> Nate Lawson <> writes:
>> Jim's analysis is good. I've found that with one drive that is off but
>> connected to the IEC bus, it doesn't drag the idle level too low (3.3v
>> high versus 5v).
> Unfortunately that depends a lot on the drive. I've seen a high level of
> just 1.5V on the bus with a C64, a 1541-II (off) and an sd2iec-device
> (with 4k7 pullups). I assume that the manufacturer(s) of the bus-driving
> 74xx chips in the disabled drive is just one of the factors that
> determine how high you can get the voltage in this case.

Very interesting. This was an original 1541 that I measured. When a
drive is off, the 7406 is not powered. So it should be dragging down the
lines via the ESD diodes in its package and you get a voltage divider:

drive-on (+5v --- 4.7K)
drive-off (7406 --- ESD diode --- GND)

In this case, the voltage at the bus should be one diode drop above GND
(0.7V). I don't know details about the internals of the 7406 though, so
it's likely there's an additional effect here.

Also, this assumes no pullups in C64 or host. If the host has them, it
will increase the bus idle level.


       Message was sent through the cbm-hackers mailing list
Received on 2010-12-01 02:00:22

Archive generated by hypermail 2.2.0.