Central gateway control unit, component description - GF54.21-P-4170RT

MODEL 251.1/0 up to Model Year 8

Fig 1: Identifying Central Gateway Control Unit Component

Courtesy of MERCEDES-BENZ USA

Location

The central gateway control unit is located in the driver footwell, on the left, toward the front of the vehicle

Task

Gateway between:
- CAN B
- CAN C
- CAN D
Version coding; on networked control units, the central gateway control unit transmits information (e.g. model series or national version) to the corresponding control unit via CAN B, CAN C and CAN D bus.
Router between CAN B and CAN D
Router between CAN C and CAN D
Diagnosis between CAN C and CAN D
Diagnosis between CAN B and CAN D
Data container

Three CAN buses connected via the central gateway control unit are installed in the vehicle:

Controller area network bus, class B (interior compartment) (CAN B) 83.3 kbit/s
Controller area network bus, class C (engine compartment) (CAN C) 500 kbit/s
Controller area network bus, class D (diagnosis) (CAN D) 500 kbit/s

Gateway functions

Explanation of terms:

DLC (Data Link Control), length information for following data
Identifier, information for receiver and priority information

Functional gateway

During normal vehicle operation, information present on the CAN bus and required on another bus for control unit functions, are routed by the central gateway control unit.

Diagnosis gateway

In the diagnosis mode, i.e. when an external tester is connected, the diagnosis request messages are routed to the appropriate control units by the tester. In the other direction, it is necessary for the control unit to route the diagnosis response messages to the tester.

Types of routing on function gateway

Two types of routing are differentiated:

Routing entire CAN message
Routing individual signals (a CAN message)

Routing entire CAN message

When routing an entire CAN message, the message received on the source bus is transferred 1:1 to the destination bus. Routing therefore has the following characteristics in terms of the source and destination bus:

The data bytes are the same.
The cycle time is the same.
As a rule, the DLC is the same. However, it can be smaller in the destination message than in the source message.
As a rule, the identifier is the same. However, it can be different on the source and destination bus. As a rule the identifier is not changed. However, it may be necessary to change the identifier in order to avoid overlaps on the destination bus.

If the DLS for the message on the destination bus is smaller than the DLC for the message on the source bus, the data bytes are copied 1:1 beginning with byte 1 to byte DLC on the destination bus.

All diagnosis messages have a length of eight data bytes (DLC = 8).

Routing individual signals of a CAN message

When routing individual signals in a CAN message, certain signals are extracted from the CAN message received from the source bus and transferred to the destination bus in a different CAN message.

A signal is a constituent of a CAN message and can be structured as follows:

Bit
Bit sequence
Byte
Bit sequence

Since the signals on the source and destination CAN can be contained in different messages, the cycle times with which the signals are transferred can also change.

Routing therefore has the following characteristics in terms of the source and destination bus:

Signals in different CAN messages
Therefore different DLC and identifier
Different cycle times

Type of routing (routing of complete CAN message)

The message received on the CAN bus in question (CAN B/CAN C/CAN D) is transferred directly to the CAN in question after receipt.

Behavior in case of time-out for received messages

Transfer of gateway messages from CAN D to CAN B is accomplished only when a message has been received on the CAN D

If a gateway message has not been received on the CAN D, such a message is not sent to the CAN B

Influence of circuit 15

The functional gateway messages are transferred to the CAN B regardless of the circuit status. The control units on the CAN C transmit, by contrast, only when circuit 15 is "ON".

The functional gateway messages are therefore transmitted to the CAN C only when circuit 15 is "ON".

Influence on network management

Routing from CAN B to CAN D is accomplished only when CAN B is active.

Influence of global version coding

(with special equipment when communications platform is present)

On the basis of the global version coding stored in the central gateway control unit it is recognized whether the special equipment communication platform is present on the CAN D. If the communication platform is not present, no functional gateway messages are transferred to the CAN D.

Special case, reverse gear signal

The "Reverse gear" engaged signal is transferred by the central gateway control unit to the CAN B for all transmissions. The reverse gear signal is configured by evaluation of the "Selector lever position" signal by the CAN C.

If the signal coincide, the "Reverse gear" signal is set. The reverse gear signal is a special case for a functional gateway signal.

Data container

The data container in the central gateway control unit represents a mobile data memory.

Within the throughput cycle data specific for the vehicle, test location and control unit is stored in the data container.

This data can be used for test sequence control in the following tests. The total size of the data memory is 128 bytes