RS-485 INTERFACE
STANDARD FOR ELECTRICAL CHARACTERISTICS OF GENERATORS AND RECEIVERS FOR USE IN
BALANCED DIGITAL MULTIPOINT SYSTEMS
RS-485 INTERFACE
STANDARD FOR ELECTRICAL CHARACTERISTICS OF GENERATORS AND RECEIVERS FOR USE IN
BALANCED DIGITAL MULTIPOINT SYSTEMS
Introduction: The RS-485 is the recommend standard by the Electronic Industries Association (EIA) that specifies the
electrical characteristics of generators and receivers that may be employed for the interchange of binary signals in multipoint
interconnection of digital equipments. When implemented within the guidelines, multiple generators and receivers may
be attached to a common interconnecting cable. An interchange system includes one or more generators connected by a
balanced interconnecting cable to one or more receivers and terminating resistors.
Electrical Characteristics: The
electrical characteristics that are
Rt Rt
specified are measured at an
interconnect point supplied by the
devices manufacturer. Figure 1
A/A1
A1
A
shows an interconnection
R G/R Balanced
G
application of generators and Interconnecting
B B1 B/B1
Cable
receivers having the electrical C1
C L C/C1
parameters specified. The Interface
elements in the application are: Described in
LEGEND: This Standard
generators, receivers, transmission G = Generator
cables, and termination resistances R = Receiver
(Rt). The loads on the system G/R = Combination Generator / Receiver
caused by each receiver and L = Length of stub: the guideline assumes length of stub to be effectively zero
R t = Termination resistance; location and value are not specified in this
passive generator shall be defined standard, but a generator can drive 32 unit loads plus two termination
in terms of unit loads. Each resistances of 120 ohms each.
generator can drive up to 32 unit
Figure 1. Multipoint Interconnect Application
loads consisting of both receivers
and generators in the passive state. The loading caused by receivers and passive generators on the interconnect must be
considered in defining the device electrical characteristics. Two areas are of concern: the DC load and the AC load
characteristics. The DC load is defined as a number or fractions of "unit loads". The AC loading is not standardized but
must be considered in the design of a system using the devices meeting this standard.
General System Configuration: The generators and receivers conforming to the RS-485 standard can operate with a
common mode voltage between -7 volts and +7 volts ( instantaneous ). The common mode voltage is defined to be any
uncompensated combination of generator-receiver ground potential difference and longitudinally coupled peak noise voltage
measured between the receiver circuit ground and cable with the generator ends of the cable short circuited to ground, plus
the generator offset voltage (Vos).
Grounding Arrangements: Proper operation of the generator and receiver circuits requires the presence of a signal return
path between the circuit grounds of the equipment at each end of the interconnection. The grounding arrangements are
shown in Figure 2. Where the circuit reference is provided by a third conductor, the connection between circuit common
and the third conductor must contain some resistance ( e.g., 100 ohms ) to limit circulating currents when other ground
connections are provided for safety. Some applications may require the use of shielded interconnecting cable for EMI or
other purposes. The shield shall be connected to frame ground at either or both ends, depending on the application.
9-4.1
Balanced Interconnecting
Rt Rt
Cable Pair
Third Conductor
No Connection No Connection
A/A1
A1
A
R G/R
G
B B1 B/B1
C/C1
C C1
100 S 100 S 100 S
Logic reference
100 S * 100 S *
100 S * Optional
1/2 W 1/2 W
1/2 W Strap
Chassis reference
Earth (safety) reference
NOTE: Third conductor not required if earth
* Resistor must become an open
reference provided in each using equipment.
circuit when overloaded
Figure 2. Grounding Arrangements
Similarity with RS-422-A:
In certain instances, it may be possible to produce generators and receivers that meet the requirements of both
RS-422-A and of RS-485. Table 1 depicts the differences in parameter specifications which exist between the two
documents.
Table 1. Comparison of RS-422-A and RS-485 Characteristics
Characteristic RS-422-A RS-485
Min. output voltage 2V into 100 ohm 1.5 V into 54 ohms
> 1/2 open circuit V
Ishort to ground 150 mA maximum
Ishort to -7, +12 volts 250 mA peak
trise time < 0.1 tb , 100 ohm load < 0.3 tb , 54 ohm, 50 pF load
Where tb = time duration of the unit interval at the applicable data signalling rate (pulse width).
9-4.2