CAN Newsletter magazine
This year the new EU exhaust regulation for industrial diesel engines stage-V becomes active. The demands on engine management that result from these regulations can no longer be satisfied by mechanically governed engines.
This article originally appeared in the June issue of the CAN Newsletter magazine 2019. This is just an excerpt.
Modern electronically governed diesel engines are the upcoming technology in industrial machinery from 2019 on. Such engines come with whole new set of options that offer a new level of comfort for the end user. They no longer rely on mechanical or electrical signals only - they can be controlled by digital commands and also send information about their current status. Commands and status information are no longer transmitted via analog connections but via a digital bus – the CAN network.
The fact that most manufacturers are forced to switch from mechanical to electrical engines means that many of them come into contact with the CAN network for the first time. The de facto standard is CAN with the SAE J1939 protocol which is well proven from the automotive industry. The standard defines a set of messages and the data that is transmitted with each message. At maximum 254 nodes can be connected to a CAN network. These are either master or slave and generally have a pre-defined, fixed, and unique address. They communicate at a fixed global bus speed of 125 Kbit/s, 250 Kbit/s, or 500 Kbit/s and information is packed into messages.
The majority of J1939 messages are expected to be broadcasted. This means that any device connected to the bus can read this data. An advantage of this technique is that less request messages are needed which saves bandwidth for other data. Nevertheless messages can be sent to specific addresses. CAN messages have a 29-bit identifier and an 8-byte data frame. The identifier has the 3-bit priority field, the 18-bit PGN (parameter group number), and the 8-bit source address. The priority can be 0 to 7 where 0 is the highest priority. If a node tries to send a message and reads a message from a different node with higher priority it stops its own transmission. That means that in theory nodes with high priority can block the bus communication if they send lots of messages. PGNs are furthermore broken into four fields: 1 bit is reserved, the data page has also 1 bit, 8 bit PDU format, and also 8 bit PDU specific.
CAN data types are identified by unique so called SPNs (suspect parameter number). These SPNs have a different length depending on the data they contain. Engine speed (SPN 190) e.g. has a length of 2 bytes where engine starter mode (SPN 1675) only needs 4 bit. These are not long enough to fill the whole 8-byte data frame. For an efficient bus communication multiple SPNs are thus combined to PGNs.
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