P38 EAS Communication Interface and Cable Details



Introduction
Many Range Rover owners are tired of being at the mercy of the dealer or of shops with expensive diagnostic software whenever they experience an air suspension fault (which is usually all too often). Alert Range Rover owner Storey Wilson has been researching the EAS ECU microprocessor, interface and codes that appear at the vehicle's diagnostic port (under the glove box) and are accessed by these sophisticated diagnosis systems for such functions as monitoring and clearing EAS faults. Storey has kindly provided the following summary of what he has found out, including details of how to build your own cable for interfacing your notebook computer to the vehicle's diagnostic connector.

Interface Protocols
The Range Rover Electronic Air Suspension (EAS) communications interface is a very simple three wire transmit (Tx), receive (Rx) and ground (Gnd) arrangement. These three wires can be connected to any RS-232 PC serial port interface and establish a communications session very easily without any level shifting. To further simplify matters, the Range Rover EAS is the only Electronic Control Unit (ECU) on these communication wires.

Interface Cable Details
The hardware serial interface between the PC and the EAS, can be constructed using off the shelf parts for under 20 USD. The parts required for this interface cable can be obtained from www.mouser.com and www.OBD2cables.com,

Once the parts are obtained, a few simple solder connections need to be made. For example, if you opt for the DB9 Serial connector, connect pins 2(Rx), 3(Tx) and 5(Ground) on that connector to the OBDII pins 11(White)(Tx), 12(Pink)(Rx) and 5(Yellow)(Ground) respectively. These OBDII pins go straight to the EAS ECU communication lines. Make sure and double check the wire color to pin map from the OBDII pass through connector. The manufacturer could change the wire colors without notice. This is what your completed cable will look like:

EAS ECU Microprocessor and Programming Details
The microprocessor in the EAS ECU is a very commonly used Motorola 68HC705B series single chip microprocessor. This is great news for us, as it limits the possible serial protocol communication options. The EAS ECU operates the serial interface at a very low speed of 600 bps, using 8 data bits, no parity, 1 stop bit and no hardware control (8,N,1, None). The data transmitted and received across the serial interface is in a Hexadecimal format, 0xF7, 0xF8, 0xF9, 0xFA, etc…

Any terminal program or serial protocol analyzer program will work for this serial interface, providing it meets the above configuration settings. The program I use is called “Realterm” and is considered freeware. This program can be downloaded from http://realterm.sourceforge.net/. Display settings should be “Hex + Space” and “Half Duplex”. Port settings should be “Baud 600”, “Parity None”, “Data Bits 8”, “Stop bits 1”, “Hardware None”. Under the send tab, enter a HEX address (0xF7) in the blank field and press “Send Numbers”. The EAS should respond with a HEX value.

The ultimate goal of this cable PC interface is to reset the EAS microprocessor. Currently, we do not have the ability to actually reset the EAS but have made some significant advances. With this physical and software interface, more people will be able to test this and move us towards a more complete solution. So far we know a few memory address locations inside the Motorola Processor and are trying to understand the message framing necessary to change those locations. For example, the following memory locations hold the Diagnostic Troubleshooting Codes (DTC), (0xF7, 0xF8, 0xF9, 0xFA, 0xFB, 0xFC). This information is one step closer to resetting the EAS but we still need to know what to do with those memory locations. Eventually a simple string of HEX commands will be used to reset the EAS system. A logical assumption would be that the memory address locations of the DTC codes would need to be surrounded or framed by other function HEX codes.

More Information
RSWSolutions.com is the website of Range Rover enthusiast Storey Wilson who cracked the EAS interface and developed software for calibrating and resetting it.
Sourceforge.net is the host for the freeware -- you can download it from this site. The software can read and reset faults, as well as calibrating the air suspension. The Range Rover specific page for free downloads is at  this link
Range Rover Diagnostics Forum: A discussion forum (including manufacturers) all about Range Rover computer diagnostic problems and solutions.
See also the page on Clearing EAS Faults using a Notebook Computer and Free Software for a typical user experience.