You should choose the 4-to-1 Mode. 

Please follow those steps : 

1. Turn off all devices

2. Connect the AS8144B to the computer with DB-25 straight-though male-to-male cables 

3. Connect the AS8144B to the printer with a DB-25 to C36 male-to-male cable 

4. Set DIP switch segment 3 - located on the side panel - to 4>1 mode 

5. Set DIP switch segment 4 for a time out of either 15 or 45 seconds 

6. Turn on your computers and the printer. The four LEDs should flash alternately as the switch scans for print jobs. If AS8144B doesn't get enough power from the connected devices, its operation becomes erratic. In this case you must use a power Adapter (DC9V,0.5mA)