dataTaker CANgate, User Manual

UM-0086-A2

CANgate User’s Manual 

Page 28 

Notes and Examples 

Extracting and Formatting Bit Fields 

In the following example two 16-bit fields plus four 4-bit fields are packed into the CAN message with identifier 0x118. The 
most recent value for each of these fields will be returned when polled by the host. 

  CONNECT 1 500   'Enable CAN port 1 at 500kbps 

 BEGIN

12 RECV 1 0x118 1 2 FORMAT "P1:%d\n" 

  13 RECV 1 0x118 3 4 
  14 RECV 1 0x118 5.8 5.5 

  15 RECV 1 0x118 5.4 5.1 FORMAT 10 -40 
  16 RECV 1 0x118 6.8 6.5 FORMAT .25 "Gibble Freq. %6.3f Hz\n" 

  17 RECV 1 0x118 6.4 6.1 
 END 

So if the last message that CANgate received (with CAN ID 0x118) happened to have the following 64-bit data field: 
 

019266401A9F0000 

and the above six memory slots were polled, for example using the command: 
 

RP 12 17 

then the following data would be returned: 
 

P1:402                  

(bytes 1-2 = 0x0192 = 402)

  6640                    

(bytes 3-4 = 0x6640)

  01                      

(byte 5 = 0x1A, bits 8-5 = 0x1)

  60.00                   

(byte 5 = 0x1A, bits 4-1 = 0xA, scaled by 10 with offset -40 = 60)

  Gibble Freq.  2.250 Hz  

(byte 6 = 0x9F, bits 8-5 = 0x9, scaled by 0.25 = 2.25)

  0F                      

(byte 6 = 0x9F, bits 4-1 = 0xF) 

In this case three of the memory slots use the 

FORMAT

 specifier to return their values in scaled engineering units, rather than 

raw hexadecimal. Slot #15 uses the default format string of 

"%f\n"

, ie. floating point, 2 decimal places, followed by CRLF. 

J1939 PGNs & SPNs 

Broadcast PGNs 

J1939 based systems typically have a multitude of SPNs (Suspect Parameter Numbers), each representing an individual 
measured quantity or status. A group of related SPNs will be broadcast over the CAN network under a particular Parameter 
Group Number (PGN), which is specified as part of the 29 bit identifier.  
Each 29 bit identifier carries with it one 64 bit packet of CAN data. Some PGNs with a data length larger the 64 bits require 
several packets to send them. CANgate automatically uses the J1939 transport protocol to send or receive large PGNs. 
Refer to 

RQSTJ – Request J1939 Data

(P20)

 or the SAE J1939/21 (Data Link Layer) standard for a more detailed description 

on PGN numbers and how they are encoded into CAN identifiers. 
An example of a J1939 SPN would be “Engine Speed”. In order to capture engine speed using CANgate, it is first necessary 
to determine the PGN which contains this parameter. This can be found in SAE J1939/71 (Vehicle Application Layer), which 
specifies: 

Electronic Engine Controller #1 

Update rate: 

Engine speed dependent 

Data length: 

8 bytes 

 Data 

page: 

0 

 PDU 

format:  240 

 PDU 

specific:  4 

 Default 

priority 

3

Parameter Group Number:  

61,444

    (0x00F004) 

 Byte: 

1 

Status_EEC1 

Bit: 8-5 

Not defined 

Bit: 4-1 

Engine/retarder torque mode 

Byte: 2 

Driver’s demand engine – percent torque 

Byte: 3 

Actual engine – percent torque 

Byte: 4-5 

Engine Speed 

Byte: 6-8 

Not defined 

 
The precise format of each SPN is also specified in SAE J1939/71: 

Engine Speed 

Data Length: 

2 bytes 

 Resolution: 

0.125 rpm/bit gain, 0 rpm offset