No need external power supply. - Serial port feature: compatible with RS-232C，RS-485 standard of EIA/TIA. - Interface: DB9 hole type interface of RS-232 DB9 pin type interface of RS-485. - Working mode: asynchronous half-duplex. - Transmission media: common wire double wire or shielded wire. - Transmission speed; 300~115-21BDS. - Transmission distance: 5 meters (RS-232 port) 1.200 meters (RS-485 port). - Tel-communication agreement: transparent. - Work temperature: -10'C to 85'C relative humidity: 5% to 95%. - Signals: RS-232: TXD RXD RTS CTS DTR DSR DCD CND; RS-485: Date+ Date Date. GND.