There is a not-so-obvious problem of using a datassette -and- a 
1581 at the same time on a C64. Someone wanted to transfer files from 
their datassette to their 1581, but it wouldn't work and they thought
there was something wrong with their drive because accessing the drive 
locked up the serial bus with that configuration of hardware. Since 
the C64 doesn't use the SRQ line anyway, the "fix" was to disconnect 
that line either at the computer serial port or in the cable to the 
1581 so it can't pull the line LOW. Note that the C128 doesn't show 
that problem because its hardware configuration is different!

Ray Carlsen