Introduction

Introduction

The I and K buses are a serial communications bus in which all connected control units can send as well as receive information over one wire. The I and K bus are technically identical, the only difference is their use by model. From this point forward they will be referred to as the I/K bus and differences will be pointed out separately.

The I bus was originally introduced in the E31 to provide a standardized, reliable and cost-effective communications bus for a cars electronics to combat the increasing size of wiring harnesses.

The E38 expanded the use of bussing in BMW vehicles by adding three more busses (K, P and M) and adding more control units to the network.

The data transfer rate is approximately 9.6Kbps (bits per second).

The I/K-bus is always active when terminal R is switched on. If the bus line is quiet more than 60 seconds, all of the control modules will go into Sleep Mode.

When receiving messages over the bus line, the control unit first determines if the message is error free before accepting it.

The information sent over the bus is configured serially. Each message consists of:
1. Transmitter address (8 bit address)
- The senders name.
2. Length of data (number of following message bytes)
- How long the sender will speak.
3. Receiver address (8 bit address)
- Whom the sender wishes to speak to.
4. Command or Information
- What the sender wants done.
5. Detailed description of message (maximum 32 bytes of data)
- How the sender wants it done.
6. Summary of transmitted information (check sum)
- The sender summarizes everything said.

The sender of the message then waits (100ms) for an acknowledgement that the message was received.

All of the connected control units will receive the information, but only the module addressed will accept and react to the data.

The rules for communication on the bus line are:
- Only one module speaks at a time.
- Everybody speaks at the same speed.
- Messages are acknowledged by the recipient.
- The message is repeated if the addressed module fails to respond.
- The Master Controller has priority.
- Quit sending message after 5 failed attempts.

Communication between busses: On vehicles equipped with an I-bus (E38, E39, E53 High) messages to be sent back and forth between the K-bus and I-bus have to be transferred via a Gateway. This Gateway is the IKE. The IKE determines by the address of the message recipient whether the message needs to be passed along to the other bus. The D-Bus and CAN-Bus also utilize the IKE or KOMBI as a gateway.

Polling: Each module on the I/K bus is informed by a message from the Master Controller as to the ready status of all of the other connected modules. The modules polled are according to the coding of the Master Controller. Every 30 seconds after KLR is switched on, each module on the bus line is polled.

A message concerning bus subscriber status is updated continuously based on the results of these polls. If a subscriber fails to respond with "device status ready" the Master will try again after 1 second.

If the module fails to reply again, the Master will assume that the subscriber is defective and send the message "subscriber inactive" to all connected modules. The inactive module will continue to be polled until the key is switched off in case the module resets itself.