AN1990 Rev.0.00 Page 1 of 11
Feb 7, 2018
AN1990
Rev.0.00
Feb 7, 2018
RS-485 Transceivers
RS-485 Transceivers in J1708 Physical Layer Applications of
Heavy Duty Vehicles
APPLICATION NOTE
Abstract
In 1986 the Society of Automotive Engineers (SAE) issued the recommended practice J1708, which defines a
bidirectional, multi-master network that enables the communication between Electronic Control Units (ECUs) in heavy
duty vehicles. The standard specifies a data rate of 9.6kbps and was intended to be replaced by the much faster (250kbps)
Controller Area Network (CAN) a decade later.
Still, J1708 remains alive and well, not only in refurbished networks of older vehicles, but also as a diagnostic bus in
modern heavy duty vehicles. Here it coexists with CAN, which is predominantly used for the exchange of high-speed
data (Figure 1
).
Figure 1. Electronic Control Unit Network in Heavy Duty Vehicles
This application note gives an overview of the hardware design aspects of a J1708 network. It explains the configuration
and operating mode of RS-485 transceivers in J1708 applications, and introduces a highly robust bus node design that is
immune to overvoltages of ±60V
DC
and ±80V transients, while operating over a wide common-mode voltage range of
±20V.
Contents
1. Network Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Bus Node Interface Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Data Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Message Priority and Buss Access Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Collision Detection and Bus Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Fault-Protected Bus Node Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Signal Waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Revision History. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
ICI
(Infotain)
PSCU
(Steer)
TCU
(Transm)
EMS
(Engine)
LCM
(Lights)
ABS
(Brakes)
DCU
(Doors)
BCM
(HVAC)
BMS
(Batt)
HMI
(Diag)
ECAS
(Susp)
SCU
(Speed)
CTI
(Tires)
J1708/J1572
CAN/J1939
AN1990 Rev.0.00 Page 2 of 11
Feb 7, 2018
RS-485 Transceivers
RS-485 Transceivers in J1708 Physical Layer
Applications of Heavy Duty Vehicles
1. Network Parameters
SAE J1708 defines a serial communication link for the exchange of data between standalone microcomputer-based
modules, known as Electronic Control Units (ECUs), in heavy duty vehicle applications.
The topology of a J1708 network is that of a common or global bus, supporting a minimum of 20 bus nodes. The
transmission medium is an 18-gauge twisted-pair cable. Although there is no restriction in cable length between nodes,
the total length of the data link (A+B+C+D+E+F) shall not exceed 40m. The applied data rate is 9.6kbps, which
translates into a bit time of (t
B
= 104.17μs).
2. Bus Node Interface Circuit
Each bus node interface consists of an RS-485 transceiver and a resistor-capacitor network, known as the load
(Figure 3
). Thus, J1708 networks do not have bus termination resistors as referenced in EIA-485.
The receiver stage within the RS-485 transceiver is always enabled through the fixed connection of the receiver enable
pin (RE
) to local ground. This allows for monitoring the data traffic on the bus, as well as the detection of bus
collisions.
The driver is configured to operate in the dominant/recessive mode. Its data input (D) is fixed to ground while its
enable pin (DE) is used as the new data input. A high at DE enables the driver, driving the low potential at D onto the
bus. A low at DE disabled the driver, making its outputs high-impedance. In this case, bus line A is pulled up towards
Vs through R
1
, and bus line B is pulled down to ground using R
2
. This condition constitutes a bus-high. To ensure the
UART output and the differential output of the driver have the same signal polarity, a logic inverter is inserted between
TxD and DE.
Because the actively driven low-state always overwrites a passively generated high-state on the bus, the low becomes
the dominant, and the high becomes the recessive bus state. This operating mode enables bus nodes to detect idle lines
and bus contentions, both of which are important conditions, used for synchronizing the network, prioritizing bus
access, and aborting transmissions.
The remaining components of the load circuits serve as low-pass filters. R
3
and R
4
in combination with C
1
and C
2
form a 1.6MHz receive low-pass filter for EMI suppression. The driver output impedance together with C
1
and C
2
form a 6MHz transmit low-pass filter for EMI and transient suppression.
Figure 2. J1708 Network Topology
Figure 3. Bus Node Interface Circuit
ECU 1
ECU 2
ECU 3 ECU 20
ECU 4
A
B
DF
E
C
TxD
RxD
1001
1001
V
CC
GND
High
(Rec.)
Low
(Dom.)
High
(Rec.)
Low
(Dom.)
UART
RxD
MCU
TxD
Vs
B
A
R
D
DE
RE
Vs
R
D
4.7k
47
2.2nF
2.2nF
Vs
V
CC
GND
R
1
R
3
C
1
C
2
47
R
4
4.7k
R
2
J1708
Bus
A
B