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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-485
Mode of
Differential DC
Operation:
coupled
DC
No
Isolation:
Distance:
4000 feet
No. of
32
Devices on
1 Line:
Data Rate:
10M bps
Unlike 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.
Using long communication cable with multiple devices often necessitates powering devices from different
power sources. This can result in ground faults and ground loops, which can cause communication problems
and possible equipment damage. Because the RS-485 interface communicates in the base band and provides
no DC isolation, ground fault places devices at different electrical ground levels and causes large ground
currents to flow. Possibilities of ground fault call for careful system planning and installation verification.
Communication cables exceeding 4000 feet can also create noise and signal reflection problems if proper
cable is not used or if the cable is not correctly terminated.
Belden Wire Specifications
Trade Number
Number
UL NEC Type
of Pairs
CSA
Certification
9841
1
NEC CM CSA
9842
2
NEC CM CSA
88102
2
NEC CMP CSA
RS-232C
Single-ended DC
coupled
No
50 feet
2
20K bps
Nominal
Shield
D.C. R.
Conductor
24.0 ohms/M
3.35 ohms/M
78.7 ohms/
11.0 ohms/K
km
24.0 ohms/M
2.2 ohms/M
78.7 ohms/
7.2 ohms/K
km
24.0 ohms/M
15.5 ohms/M
78.7 ohms/
50.9 ohms/km
km
Hardware Installation Guide
Modem
20mA Loop
Differential AC
Single-ended
coupled
current
Yes
Usually Isolated
Phone Line
1000 feet
2
Limited by Loop
Voltages
19.2K bps
2400 bps
Nominal
Nominal Capacitance
Impedance
pF/feet
(Ohms)
120
12.8
120
12.8
100
12.95
pF/meter
42
42
42
21
revision 7 —
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