I don't have a full answer but I can say that not all USB-to-RS232
converters or cables are equal. A similar problem was reported in one
of my astronomy groups (re: telescope control) and one enterprising
person did some sleuthing.

Success or failure depends on the USB-RS232 converter chipset. Here's
what the guy discovered:

Available Chip(set) Names:

FTDI
Prolific PL230
MCT U232/F5U109
USA-19HS

Of all the chips used, only the MCTs are known to be problematic on
the EQDIR interface. The MCT chips are used in the Belkin/Targus
models.