REXGEN: log OBD2 Mode 1 PIDs

Make any REXGEN logger ask a vehicle for standard OBD-II Mode 1 parameters, such as engine speed and coolant temperature. Then record the answers and decode them with a DBC file. You need REXDESK and a logger connected to the vehicle's OBD socket.

How Mode 1 logging works

OBD-II is designed for emission control. It defines a broad set of standard parameter IDs (PIDs) that most vehicles answer. A logger does not see these values on the bus unless it asks for them. The REXGEN logger sends a request for each PID at a fixed interval. The engine ECU answers, and the logger records both.

Mode 1 (service 01) and its PIDs are defined in SAE J1979. For the background, see on-board diagnostics basics.

Set up the requests

1. Create a new project. Select New Project in REXDESK.
2. Add a periodic transmission. Drag the Periodic tool from Transmitting into the Output area of the configuration window.

   

3. Set up the bus. If the bus is not configured yet, drag Log All onto it, select its Bus icon and set the bit rate. Use Bit Timing for advanced settings.
4. Add one request per PID. In the Periodic messages window, select Add for each request and fill in its fields.

   

5. Send the configuration. Select Run to send it to the logger. Keep Log All on the bus so the logger records the answers.

Ident

The CAN identifier of the request. 7DF is the standard 11-bit OBD-II broadcast (functional) request. The main engine ECU answers on 7E8. Some vehicles need physical addressing, a request sent to one ECU, for example 7E0.

Linked

Links a message to the one above, so they go out as a sequence. Optional.

Period

How often the request is sent. Optional.

Delay

The delay before a linked message is sent. Optional.

CAN 0 to CAN 3

The bus that carries the request. Use only the channels your logger has.

Type

CAN Standard (11-bit) or Extended (29-bit) identifier.

DLC

The data length, in bytes. OBD-II requests use 8.

Data

The request bytes, for example 02 01 0C 00 00 00 00 00 for engine speed.

To reuse a list, save it with Save CSV and load it later with Load. This sample list asks one ECU, by physical address, for six PIDs.

Download the sample transmit list (CSV, under 1 KB)

Read a request and an answer

Each request and answer is a short byte string:

• Length. The first byte counts the bytes that follow. In the request 02 01 0C, 02 says two bytes follow: the mode and the PID.
• Mode. 01 asks for current data. The answer replaces it with 41.
• PID. The parameter, as defined in SAE J1979.
• Data. The answer's value bytes, called A, B, C and D in order.

Engine speed (PID 0C) uses two data bytes, and the value is (256 × A + B) / 4 rpm. An answer of 04 41 0C 1A F8 gives (256 × 26 + 248) / 4 = 1726 rpm.

Every PID has its own formula. For the full list, see Wikipedia's OBD-II PIDs page.

Decode the answers with a DBC file

REXDESK can decode the recorded answers with a DBC file, so you do not work out each value by hand. The export includes only the messages and signals you choose. The sample DBC covers the standard 11-bit Mode 1 answers.

Download the sample OBD-II DBC file (DBC, 16 KB)

1. Open the logs. Select Internal Storage, select the logs and open the Download menu.
2. Open the export window. Select Advanced to open Export data.
3. Set the output. On General, choose the output format (.mf4, .mat or .csv), the time format (CSV only) and the output location. Tick the merge option to combine several logs into one file.
4. Load the DBC file. On Database, select the load DBC icon and open the file.
5. Assign it to the bus. Drag the messages or signals you need onto the bus they were recorded on, for example CAN 0.

   

6. Process. Select Process. REXDESK decodes the raw data and saves the result where you chose.

For more export options, see REXDESK: download and export your logs.

Watch: Log OBD2 data with REXGEN (2:02)

Export the configuration as an RXC file

To load the same OBD-II configuration onto another logger without REXDESK connected, for example over the air, export it as an RXC file. Select Export on the Config menu, then Export ReXgen Configuration (rxc).

To send a configuration over the air, see REXDESK: send a new configuration over the air.