You want to send some command to your arduino or monitor serial output at somewhere away from your workbench, but you do not want to carry your heavy laptop. An old PDA is my answer. OK, you can do this with an android smartphone. But many of us would not feel comfortable when our expensive smartphone or tablet got involved with a home brewed circuit mess. Or your smartphone doesn’t support serial port. I picked up CASIO BE-500 (a Japanese version of BE-300) from my junk box and installed Pocket Teraterm, a terminal emulator. If you have a genuine serial cable, it goes quite simple. You need MAX232C or a level shifter with two NPN transistors. Unfortunately, I didn’t have one. I cut a cellphone cable. The pin assign of CASIO 20-pin PDA cable is described in this page. I connected TxD, RxD and GND respectively, but I got no response at all. This page (in Japanese) shows why it was. Some modification is needed in 20-pin connector to make the COM port available. A 4-bit combination of pin 2 (SEL3), pin 4 (SEL0), pin 5 (SEL1) and pin 6 (SEL2) tells BE-300 what kind of cable is connected. To turn a cell ohone cable into a serial cable, pin 2 must be open and pin 4, 5 and 6 must be connected to GND. Note that COM port is TTL UART not RS-232C.