Article 2019-07-08 18:39:16

_GM High Speed GMLAN

Circuit Description

A High-Speed GMLAN serial data bus is used where data needs to be exchanged at a high enough rate to minimize the delay between the occurrence of a change in sensor value and the reception of this information by a control device using the information to adjust vehicle system performance.

Properties

The High-Speed GMLAN serial data network consists of two twisted wires. One signal circuit is identified as GMLAN-High and the other signal circuit is identified as GMLAN-Low. At each end of the data bus there is a 120 Ω termination resistor between the GMLAN-High and GMLAN-Low circuits. The resistors are used to reduce noise on the High Speed GMLAN bus during normal vehicle operation.

The High Speed GMLAN is a differential bus. The GMLAN-High and GMLAN-Low signals, are driven to opposite extremes from a rest or idle level. Data, (1s and 0s), is transmitted sequentially at a rate of 500 Kbit/s. The data transmitted over the bus is represented by the voltage difference between the GMLAN-High signal voltage and the GMLAN-Low signal voltage.

GMLAN-High

Serial Data (+). Logic "1". Idle. When the two wire bus is at rest the GMLAN-High and GMLAN-Low signal circuits are not being driven, which is approximately 2.5 volts, the transmitted data is considered recessive and represents a logic "1". In this state both signal circuits are at the same voltage of 2.5 V. The differential voltage is approximately 0 V.

GMLAN-Low

Serial Data (-). Logic "0". In driving the lines to their extreme difference, one volt gets added to the GMLAN-High wire and one volt gets subtracted from the GMLAN-Low wire. In doing so the GMLAN-High signal circuit is driven higher to about 3.5 V and the GMLAN-Low circuit is driven lower to about 1.5 V. The differential voltage becomes approximately 2.0 (+/- 0.5) V.

Network Management

High Speed GMLAN network management supports selective start up and is based on virtual networks. A virtual network is a collection of signals started in response to a vehicle event. The starting of a virtual network signifies that a particular aspect of the vehicles functionality has been requested. A virtual network is supported by virtual devices, which represents a collection of signals owned by a single physical device. So, any physical device can have one or more virtual devices. The signal supervision is the process of determining whether an expected signal is being received or not. Fail-softing is the ability to substitute a signal with a default value or a default algorithm, in the absence of a valid signal. Some messages are also interpreted as a heartbeat of a virtual device. If such a signal is lost, the application will set a no communication code against the respective virtual device. This code is displayed on the Tech 2 screen as a code against the physical device.