Can Bus - Leica M500 N Service Manual

Description
08/2007 Gahe

CAN bus

CAN-bus principle
The CAN-bus (Controller Area Network) is a bi-directional, two-wire bus. It is used to handle the communication
between the control unit, the optics-carrier, the stand and other CAN devices. The CAN-bus uses a protocol which
largely suppresses transmission errors.
The CAN bus in the M500 N/M520/M525 system is operated in the configuration "standard – CAN".
Data transfer
The bus is looped through each of the controllers. Data is transferred serially, bit by bit, with differential signals. The
data transfer is rated at with 50kBit/s.
The data can be exchanged between each of the controllers. A data transfer consists of:
•
Start bit,
•
Identification, enabling the priority to be determined,
•
The data itself, and
•
CRC (Cyclic Redundancy Check).
Each controller recognizes the data related to its task. If a controller wants to send data, it has to wait until the bus is
free. The feature of setting the priority of commands is not applied in the M500 N/M520/M525 system; all commands
have the same priority.
Command interpretation by control unit
The control unit reads CAN commands from CAN handles and from other CAN members. It interprets the received
commands and releases appropriate commands to the CAN bus. The concerning CAN device will then release a
required action.
Safety concept - brake watch dog
The control unit must periodically (100ms) send a brake state message. If no periodic brake state message is received
by the connected stand/brake controller, all brakes will lock.
Terminating impedance
In control unit: the end of the CAN-bus is terminated with an impedance of 120Ω. The controller detects if it is at the
end of the bus or if another device is connected (CAN cable is coded: Pin 4 is bridged to pin 8 in the connector
housing).
In optics carrier: the controller of the optics carrier is always terminated by 120Ω; other devices, connected to the bus
at the optics carrier are not terminated.
Service manual M500 N, M520, M525
1-8
Doc Code 10 665 751