I wrote:
There was also a command or sequence of commands that
could be sent to
a 1540 that changed its timing to meet the C64 requirements, until the
next time the 1540 was power-cycled or otherwise reset. When I got my
first C64 in 1982, it came with that information, but I no longer have
it.
I am of the impression that on a VIC one would get slightly higher
performance out of a 1540 than a 1541, but I don't know for sure. And
I don't know whether there is a corresponding command or sequence that
can be sent to a 1541 to switch it to 1540 timing.
Found it. The "UI+"
command will slow down a 1540 for use on a C64, and
"UI-" will speed it up again. Apparently when they wrote the 1540
firmware they anticipated that the bus might have to be slowed down for
use with some future computer.
Although the 1541 defaults to slow mode, the same commands work, so when
using a 1541 on a VIC-20 you might want to give the "UI-" command.
Example: OPEN 15,8,15,"UI-":CLOSE 15
Eric