Datasheet

~
~
~
~
μP
SN65 HVD257
An
RXD_A
TXD
FAULT_A
RXD
RXD_B
SN65HVD 257
Bn
FAULT_B
S_B S_A
μP
SN65HVD 257
A3
RX D_A
TXD
FAULT_A
RX D
RXD_B
SN65 HVD 257
B3
FAULT_B S_B S_A
μP
SN65HVD 257
A2
RX D_A
TXD
FAULT_A
RX D
RXD_B
SN65HVD 257
B2
FAULT_B S_B S_A
μP
SN65HVD 257
A1
RX D_A
TXD
FAULT_A
RX D
RXD_B
SN65HVD 257
B1
FAULT_B S_B S_A
SN65HVD255
SN65HVD256, SN65HVD257
SLLSEA2C –DECEMBER 2011–REVISED SEPTEMBER 2013 www.ti.com
Example: Functional Safety Using the SN65HVD257 in a Redundant Physical Layer CAN
Network Topology
CAN is a standard linear bus topology using 120 Ω twisted pair cabling. The SN65HVD257 CAN device includes
several features to use the CAN physical layer in nonstandard topologies with only one CAN link layer controller
(μP) interface. This allows much greater flexibility in the physical topology of the bus while reducing the digital
controller and software costs. The combination of RXD DTO and the FAULT output allows great flexibility, control
and monitoring of these applications.
A simple example of this flexibility is to use two SN65HVD257 devices in parallel with an AND gate to achieve
redundancy (parallel) of the physical layer (cabling & PHYs) in a CAN network.
For the CAN bit-wise arbitration to work, the RXD outputs of the transceivers must connect via AND gate logic so
that a dominant bit (low) from any of the branches is received by the link layer logic (μP), and appears to the link
layer and above as a single physical network. The RXD DTO feature prevents a bus stuck dominant fault in a
single branch from taking down the entire network by forcing the RXD pin for the transceivers on the branch with
the fault back to the recessive after the t
RXD_DTO
time. The remaining branch of the network continues to function.
The FAULT pin of the transceivers on the branch with the fault indicates this via the FAULT output to their host
processors, which diagnose the failure condition. The S pin (silent mode pin) may be used to put a branch in
silent mode to check each branch for other faults. Thus it is possible to implement a robust and redundant CAN
network topology in a very simple and low cost manner.
These concepts can be expanded into more complicated & flexible CAN network topologies to solve various
system level challenges with a networked infrastructure.
A. CAN nodes with termination are PHY A, PHY B, PHY An and PHY Bn.
B. RXD DTO prevents a single branch-stuck-dominant condition from blocking the redundant branch via the AND logic
on RXD. The transceivers signal a received bus stuck dominant fault via the FAULT pin. The system detects which
branch is stuck dominant, and issues a system warning. Other network faults on a single branch that appear as
recessive (not blocking the redundant network) may be detected through diagnostic routines, and using the Silent
Mode of the PHYs to use only one branch at a time for transmission during diagnostic mode. This combination allows
robust fault detection and recovery within single branches so that they may be repaired and again provide
redundancy of the physical layer.
Figure 20. Typical Redundant Physical Layer Topology Using the SN65HVD257
20 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: SN65HVD255 SN65HVD256 SN65HVD257