Bus communication

Bus communication

•	Control module, ACC (216)

•	Radio switch C (267c)

•	Control module, airbag/SRS (331)

•	Radio (353)

•	CD changer (355)

•	Control module, PSM (357Dk)

•	Control module, TRIONIC (430)

•	Control module, TCM (502)

•	Control module M2.10.3 (505)

•

MIU (540)

•

SID (541)

•	Control module (ABS) 547

•	Control module, STC (565)

•	Control module, Trionic OBDII (589)

•	Control module, EDC 15 (595)

•	Control module, auxiliary heater (597)

•	Diesel pump (598)

•	Control module, DICE (628)

•	Control module, TWICE (632)

P-bus and I-bus

A bus is understood to be the leads over which information is sent digitally and serially. Digital means that the voltage difference between the leads has only two values, roughly 0 V and 5 V. The information is coded so that different combinations of 0 V and 5 V pulses have different meanings.

Serial means that the information is sent in ”packets” which are transmitted one after the other in rapid succession.

On the Saab 9-3, half of the car's control modules are connected to a bus.

The buses consist of a P-bus (Powertrain Bus) and an I-bus (Instrument Bus). Both buses are connected to the MIU (main Instrument Unit). The buses are electrically isolated from each other.

The diagnostic tool is not connected direct to the bus but communicates via the DICE, one of the control modules connected to the I-bus, and so has access to all control modules connected to the bus.

The data transfer rate on the P-bus is ten times faster than on the I-bus. The reason for this is that the powertrain systems need information as quickly as possible with the least possible delay.



All the information sent from one control module is accessible for all other control modules on the bus. The MIU is responsible for ensuring that information available on one bus is also available on the other bus.

The control modules send out information on the bus at regular intervals. The time between two transmissions depends on the information being sent and varies between 10 milliseconds (0.010 seconds) and 1 second. Information is also sent out by the control modules whenever the information changes.

The transfer of information between control modules takes place on two leads, bus+ (green lead) and bus- (white lead). The leads are twisted to increase their resistance to electrical interference.

The MIU uses the following information:

Information	Unit	Sensor system	Description
LH direction indicators	ON/OFF	DICE	The MIU lights the indicator lamp for LH direction indicators.
RH direction indicators	ON/OFF	DICE	The MIU lights the indicator lamp for RH direction indicators.
Rear fog light	ON/OFF	DICE	The MIU lights the indicator lamp for the rear fog light.
A/C	ON/OFF	DICE	Request from DICE for activation of A/C. The MIU passes on the request by grounding the output to the engine management system.
Current consumption	ON/OFF	DICE	The Current consumption ON message is sent out on the bus 0.1 seconds before DICE activates the radiator fans. The MIU grounds a lead from Motronic which then compensates for the increased load in its idle speed control circuit.
Driver's door	OPEN/CLOSED	TWICE	The MIU lights the driver's door symbol.
Passenger's door	OPEN/CLOSED	TWICE	The MIU lights the passenger's door symbol.
LH rear door	OPEN/CLOSED	TWICE	The MIU lights the LH rear door symbol
RH rear door	OPEN/CLOSED	TWICE	The MIU lights the RH rear door symbol.
Boot lid	OPEN/CLOSED	TWICE	The MIU lights the boot lid symbol.
Steering wheel location	LHD/RHD	TWICE	Control module programming. The MIU uses the information to indicate open door in the pictogram, driver door/passenger door.
Immobilization	ON/OFF	TWICE	Immobilization function
Info Disp Lamp	ON/OFF	SID	The MIU lights the Info Display indicator lamp in the event of a text message.
Rheostat level	0-100 %	SID	The MIU uses information on the rheostat setting to control the brightness of the instrument lighting.
Night Panel	ON/OFF	SID	The MIU turns off the lighting of certain instruments and displays.
Display lighting	0-100 %	SID	The MIU uses the information to control the display lighting.
Light intensity in cabin	Lux	SID	The MIU uses the information to control the brightness of indicator lamps.

The MIU sends the following information:

Information	Unit	Used by	Description
Vehicle speed	km/h (mph)	DICE, SID, ACC, Audio, TWICE, STC	Wheel speed, RL (lead from ABS). The MIU controls the speedometer
Engine speed	0-7000 rpm	ACC	The main instrument unit sends engine speed information which is received by the MIU via a lead from Trionic or Motronic.
A/C relay	ON/OFF	DICE	The MIU receives information via a lead to Trionic/Motronic. When the A/C relay is active, the MIU sends this information out on the bus.
Acoustic warning	Sound information	SID	Request for acoustic signal when central warning lamp or Info Displ goes on.
Tighten fuel filler cap	ON/OFF	SID	If a fault develops in the tank venting system, the MIU receives information about it via a lead from Trionic. The MIU sends Tighten fuel filler cap information out on the bus and the message is displayed on the SID unit. OBDII only.
Coolant temperature	-40-215°C	DICE, ACC, SID	The MIU has a coolant temperature sensor function on petrol-engined cars. The current temperature is sent in °C.
Fuel Consumed Since Start	0-65535 ml	SID	Lead from Trionic, Motronic 2.10. Value adjusted by a factor in the MIU. The factor can be adjusted using the diagnostic tool.
Fuel level	litres	SID	The MIU sends information on the current amount of fuel in the tank.
Odometer reading	km, miles	SID	Information on distance driven.
Identification number	---	TWICE	The number is generated in the MIU when the immobilization code is programmed in TWICE.
Diagnostics communication	---	DICE/ Diagnostic tool	Control module should be live and the bus leads intact.
Engine has started	ON/OFF	SID	When engine speed is higher than 310 rpm the MIU sends the Engine has started ON message out on the bus.