Thanks for the Cassette CAS information. I have found that my System 80 will not load computer audio CAS files. The problem is caused by there not being a long enough delay after the Sync Byte A5 and the start of the three D3 bytes. Found this out by using an Arduino program to simulate CAS audio input to the System 80. Guess System 80's are not as fast as Tandy TRS-80's.