SATELLINE-3AS and SATELLINE-3ASd
12
5.1.1.3
RS-422 Interface
RS-422
defines a serial interface much like RS-232. However, RS-422 uses balanced (or
differential) transmission lines. Balanced transmission lines use two transmission lines for each
signal. The state of each signal is represented by the relative voltage of the two lines to each
other. For example, the TX signal is carried on two wires, wire A and wire B. A logical 1 is
represented by the voltage on line A being greater than the voltage on line B. A logical 0 is
represented by the voltage on line A being less than the voltage on line B. Differential voltage
transmission creates a signal which is more immune to noise as well as voltage loss due to
transmission line effects. Thus, you can use RS-422 for greater cable distances (up to 1 km) than
RS-232. (See also paragraph 7.2.2 for wiring)
5.1.1.4
RS-485 Interface
RS-485
expands on the RS-422 standard by increasing the number of devices you can use from
10 to 32 and by working with Half-duplex bus architectures. Only one pair of cables is needed
compared to the two pairs needed in RS-422. Unlike the RS-422 standard, RS-485 addresses
the issue of using multiple transmitters on the same line. RS-485 defines the electrical
characteristics necessary to ensure adequate signal voltages under maximum load, short-circuit
protection, and the ability to withstand multiple drivers driving conflicting signals at the same
time. (See also paragraph 7.2.3 for wiring)
5.1.1.5
Termination
Each differential pair of wires is a transmission line. You must properly terminate the line to
prevent reflections. A common method of terminating a two-wire multidrop RS-485 network is to
install terminating resistors at each end of the multidrop network. If you daisy chain multiple
instruments together, you need a terminating resistor only at the first and last instruments. The
terminating resistor should match the characteristic impedance of the transmission line (typically
100~120 ohm).
