Use of the SAE J1708 standard to automatically switch the flow of RS-485 serial data transmission

The SAE J1708 standard was used for serial data transmission between the electronic control unit (ECU) in the vehicle and also between the computer and the vehicle. With respect to the Open Systems Interconnection (OSI) model, J1708 defines the physical layer. The protocol, which operates on a higher layer of the J1708 standard, is defined by SAE J1587 and SAE J1922. The protocol is maintained by SAE International.

The SAE J1708 (Society of Automotive Engineers) standard is based on the electrical characteristics of the RS-485 standard, which is a serial transmission interface standard that only specifies electrical characteristics for balanced multidrop transmission lines. The complete RS-485 interface standard specifies electrical, mechanical and functional parameters.

In contrast, SAE J1708 only specifies functional characteristics for balanced circuits. In terms of technical parameters, SAE J1708 is compatible with RS-485 but with several circuit modifications and defines hardware solutions.

The implementation of the SAE J1708 standard recommends the use of an RS-485 transceiver, such as the DS75176B.

Implementation of the DS75176B chip according to the SAE J1708 standard

If we activate the receiver in these systems (/RE=Lo), the integrated circuit connected to the bus on which there is no active transmitter will set the Hi state at the R output (the receiver behaves in the same way when disconnected from the bus or with the A, B lines shorted). . However, if we activate a transmitter (DE=Hi) and its D input is in the Hi state, the D output will still be in the Hi state. Since the receiver interprets the „passive state” of the bus and „transmitting number one” in the same way, the conclusion is simple – we do not need to transmit number one (just „unplug” from the bus). To give „zero” in a traditional application, we do it by controlling DE=Hi and D=Lo. So we permanently connect the D input to Lo. We supply the negated data signal to the DE input. The effect is that the transmitter is activated only for the duration of sending a data bit with the value „zero”. The transmitter becomes inactive immediately after sending the last zero bit of data. In the extreme case, when we transmit byte 0x00, the transmitter is turned on only during the transmission of the start bit. Despite this, all receivers connected to the bus will correctly decode the received byte.

SAE J1708 implementation diagram with opto-isolation

The solution shown in the diagram above (RS-485 with opto-isolation) was used in the SU 1.2 controller and the SU 1.5 controller.

Useful documentation:

  • SAE J1708 standard documentation,
  • documentation of the DS75176B system.
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