I had two different Serial-to-USB adapters that caused me problems.
One was a Dynex brand that came from Best Buy and the other was a Cables-To-Go brand.
These both had problems and they also both had Prolific chip sets in them.
There is a checkchipversion tool from here
http://prolificusa.com/pl-2303hx-drivers that will confirm these chip sets.
I eventually found three solutions to my problems that really worked.
1) Purchased a Dell E-Legacy adapter that has a real COM1 port that is mapped to the same Port and Interrupt that most software understands.
2) Purchased a Tripp-Lite Keyspan model USA-19HS from Tiger Direct
3) Purchased a FTDI CHIPI-X USB-to-Serial adapter from Digi-Key
All of these items worked just fine with a SLC-5/05 using the DF1 serial protocol.