Vansco VMM3120, User Manual, Page: 55 / 92

User Guide 45
Communication
Shield should be connected to an available ground terminal attached to the
negative battery. The CAN cable must have an impedance of 120 Ω.

The CAN cable is very susceptible to system noise; therefore, CAN shield
must be connected as follows:
a. Connect CAN Shield to the point of least electrical noise on the CAN bus.
b. Connect CAN Shield as close to the center of the CAN bus as possible.
c. Use the lowest impedance connection possible.
Note: Ground loops can damage electronic modules. The CAN Shield can
only be grounded to one point on the network. If grounded to multiple points,
a ground loop may occur.

CAN Connectors: Industry-approved CAN connectors are manufactured by
ITT Cannon and Deutsch, and come in either T or Y configurations.

CAN Harness: The CAN harness is the main backbone cable that is used to
connect the CAN network. This cable cannot be longer than 40 meters and
must have a 120 Ω terminating resistor at each end. The 120 Ω terminating
resistors eliminate bus reflections and ensure proper idle-state voltage levels.

CAN Stubs: The CAN stubs cannot be longer than 1 meter, and each stub
should vary in length to eliminate bus reflections and ensure proper idle state
voltage levels.

Max Number of Modules in a System: The CAN bus can handle a maximum
of 30 modules in a system at one time.
The following shows a typical CAN connection using the SAE J1939 standard:
120 ohm
Terminator
120 ohm
Terminator
Node
Node
Node
Node
Node
T connectors
Variable length
CAN stub (<1m)
CAN Network Backbone
(less than 40 meters long)
Figure 19: J1939 CAN connection