If you have the proper Zip 100 plus blue Iomega cable connect the end marked AutoDetect to the Autodetect male DB25 bulkhead connector, and the other end with the Iomega logo to your Mac. If you are using improvised cabling possibly with gender converters make sure all 25 pins and also shield ground are passed thru direct without handshake signal swaps. A barebones RS232 patch cable with a subset of wires will not cut it.