TB506 Rev.1.00 Page 1 of 5
Jun.25.19
RS-485 Drivers and Receivers
Functional Principles of RS-485 Drivers and Receivers
Common Information
Abstract
The RS-485 standard specifies the electrical characteristics of differential drivers and receivers in multi-point
networks but does not explain their functional principles. This document explains how the differential line signals
are generated by the driver and processed by the receiver.
Contents
1. Driver Functional Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Receiver Functional Principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Revision History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
List of Figures
Figure 1. Driver with Drive Logic and H-Bridge Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Figure 2. Driver Differential and Common-Mode Output Voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Figure 3. Mixed, Differential, and Common-Mode Representations of an RS-485 Driver . . . . . . . . . . . . . 3
Figure 4. Input Voltage Divider with Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Figure 5. Input Voltage Attenuation and Biasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
RS-485 Drivers and Receivers Functional Principles of RS-485 Drivers and Receivers
TB506 Rev.1.00 Page 2 of 5
Jun.25.19
1. Driver Functional Principle
An RS-485 driver consists of a drive logic and four output transistors (Q1 to Q4) in an H-bridge configuration. The
drive logic becomes active when the driver enable pin (DE) is asserted high.
A high applied to the data input (DI) turns on Q2 and Q4 and disables Q1 and Q3. This configuration causes
current to flow through R
L
from Output Y to Output Z. A low applied to DI turns Q2 and Q4 off and enables Q1 and
Q3, which causes the current to flow in the opposite direction, from Z to Y.
Each transistor has a diode in series to prevent reverse leakage current from flowing into the transistor if the bus
voltage either rises above V
CC
or drops below ground. When a driver drives a loaded bus, the forward voltages of
the diodes and the voltage drops across the r
DS(ON)
resistance of the transistors causing the output voltages, V
Y
and V
Z
, to never reach the supply rails. Instead, both outputs switch alternately between the high and low voltage
levels, V
H
and V
L
(Figure 2).
In general, the differential output voltage, V
OD
, is the difference between the high and low-level output voltages:
However, in practice V
OD
is defined as V
Y
– V
Z
, so V
Y
is referenced to V
Z
. For V
Y
> V
Z
, V
OD
is positive and
represents a binary 1 or logic high at DI. For V
Y
< V
Z
, V
OD
is negative and indicates a binary 0 or logic low at DI.
As both outputs switch within the positive voltage range, the DC-component is common to both outputs. This
voltage is the driver output common-mode voltage, V
OC
, and is defined in Equation 2:
Inserting Equation 1
into Equation 2 presents the output voltages in their common-mode and differential
components:
The driver can therefore be shown as a common-mode voltage source superimposed by two complementary,
differential voltages:
Figure 1. Driver with Drive Logic and H-Bridge Output Figure 2. Driver Differential and Common-Mode
Output Voltages
Q4
Q1
Q2
Q3
Q3
Q2
Drive
Logic
DE
DI
V
CC
Y
Z
R
L
V (V)
t
V
H
V
L
V
CC
V
Y
V
Z
+V
OD
-V
OD
V
OC
V
OC
+ V
OD
/2
V
OC
- V
OD
/2
Logic 1 Logic 0
V
OD
/2
V
OD
/2
V
OD
V
H
V
L
–=
(EQ. 1)
V
OC
V
H
V
L
+
2
---------------------=
(EQ. 2)
V
H
V
OC
V
OD
2
------------
and V
L
V
OC
V
OD
2
------------–=+=
(EQ. 3)
V
Y
V
OC
V
OD
2
------------
and V
Z
V
OC
V
OD
2
------------
==
(EQ. 4)