Article 2019-07-08 18:31:21

_GM Low Speed GMLAN

Circuit Description

Low Speed GMLAN serial data bus is used in applications where a high data rate is not required which allows for the use of less complex components. It is typically used for operator controlled functions where the response time requirements are slower than those required for dynamic vehicle control.

Properties

The Low Speed GMLAN serial data network consists of a single wire, ground referenced bus with high side voltage drive. Unlike the high speed dual wire networks, the single wire low speed network does not use terminating resistors at either end of the network.

During normal vehicle operation data symbols (1s and 0s) are transmitted sequentially at the rate of 33.3 Kbit/s. This protocol produces a simple pulse train sent out over the Low Speed GMLAN. For programming components, a special high speed data mode of 83.3 Kbit/s may be used.

The data to be transmitted over the bus is represented by different voltage signals on the bus.

Logic "1"

When the Low Speed GMLAN is at rest and is not being driven, there is a low signal voltage of approximately 0.2 V. This is translated as a recessive logic state representing a logic "1".

Logic "0"

When a module pulls the Low Speed GMLAN bus high, around 4.0 volts or higher, this creates a dominant logic state or logic "0" on the bus.

Network Management

To wake up the control modules connected to the Low Speed GMLAN, a high voltage wake-up pulse is sent over the bus. The voltage level of the pulse is 10 volts or higher. Modules connected to the Low Speed GMLAN can be part of a virtual network. Most modules on the Low Speed GMLAN are connected to the bus in a parallel configuration. Refer to the schematics to determine modules that are not in parallel.