New Holland CX720 Repair Manual page 1452

DISTRIBUTION SYSTEMS - ELECTRONIC SYSTEM
Data bus - Overview (A.50.A.92 - C.10.A.10)
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CAN NETWORK
The CAN network is a multiplex system which follows the guidelines established in SAE J1939. Multiplexing simply
stated is linking two or more digital devices through a network. In the past, if an RPM sensor's information was needed
by a tachometer, an engine controller and a transmission, all three devices would need to be hard wired to the RPM
sensor. Through the CAN Data Bus only one wire is needed. The information is then accessed through the network by
other systems that need it. Any other system on the network that does not care about RPM data ignores the message
on the network. On today's high tech machinery, the complexity of wiring can be greatly reduced through the use of
the CAN data bus network.
The network is made up of a twisted pair of wires, identified as CAN HI (yellow) and CAN LO (green). These two
wires are used to form a "linear bus" network, in that the wires run in parallel from one end of the vehicle to the other,
and each module is connected to both wires as a "node". These two wires are connected together at each end of the
network using a 120 ohm resistor, which is known as a "termination" resistor. Because there is a 120 ohm resistor at
each end of the network, the resistance should always be 60 ohms between the CAN HI and CAN LO wires.
On the CX combine, one of these termination resistors is located in the Infoview monitor itself, while the termination
resistance on the other end of the network is located in the ECU (engine control unit) on combines equipped with an
electronic engines. On mechanical engine units, there is a connector next to the starter relay in the engine compart-
ment that contains the necessary 120 ohm termination resistor.
Each control module "tees" into the network, and acts as a separate "node" on the network. As a control module
receives data from sensors, switches and potentiometers, it broadcasts this information on the network for other mod-
ules to pick up and use as needed. Each module knows which information it needs through the software programmed
into it.
When the network is initially powered up (operator turns key on), each module on the network sends out a message
announcing its existence, and "looks" for messages from other modules on the network. After this initial message,
each control module sends out data messages as necessary, and, if necessary, it will send out another broadcast
message announcing its existence if it has not sent any messages within the last 5 seconds. This is done in order
to monitor each module's status on the network; if a certain module has not transmitted any messages for more than
5 seconds, then the other modules on the network will generate an alarm message indicating that the module is
"offline".
NOTE: If a control module never powers up and broadcasts its initial message, the other modules will not be aware
that it exists, and no "offline" alarm will be generated.
Optional control modules, such as the ECU (engine control unit), CCM3 (installed for Precision Farming and other
options), DLU (data logger unit, used for Precision Farming) and DGPS (differential GPS receiver), must be set to
"ON" in the Configuration screens when installed in order for their alarm messages to become active. In addition, like
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