Title: Hardware Installation Guide.pdf
uses a balanced differential transmitter/receiver to reject common mode noise. The following table is a comparison of interfaces commonly used in access/alarm systems.RS-485RS-232CModem20mA LoopMode of Operation:Differential DC coupledSingle-ended DC coupledDifferential AC coupledSingle-ended currentDC Isolation:NoNoYesUsually IsolatedDistance:4000 feet50 feetPhone Line1000 feetNo. of Devices on 1 Line:3222Limited by Loop VoltagesData Rate:10M bps20K bps19.2K bps2400 bpsUnlike the RS-232C or current loop interfaces, the RS-485 interface allows multiple devices to communicate at high data rates on a single cable, over long distance. Obviously, the RS-485 interface provides advantages in cost savings for installation and improved system performance, but it also brings about problems which would not commonly be seen on systems using RS-232C or current loop interfaces.