Max HN50 Service Manual download pdf (Page 2)

2 HN.50.Y1.02

CAN communication

CAN communication works on the prioritising

of messages, thus CAN uses familiar and

established methods such as CSMA/CA

(Carrier Sense, Multiple Access with Collision

Avoidance) with improved capability to avoid

collision (non-destructive bit arbitration). This

means that the message with the lowest iden-

tification code will have access to the bus

before other messages, ensuring that the

capacity of the bus can be utilised to the maxi-

mum.

The speed of the bus is limited by its length,

see below.

CANopen CAN components communicate using a

protocol. A protocol can be compared to a

language. The different protocols on the

market are adapted to the applications in

which they are used.

The CANopen protocol is particularly suitable

for mobile applications. There are many

suppliers on the market whose products work

together with CANopen, therefore it is easy to

put together a comprehensive CANopen

system.

CANopen uses objects for communication.

The most common are:

Service Data Object(SDO)

SDOs transfer large amounts of information

that is not time-critical eg setting-up parame-

ters.

Process Data Object (PDO)

PDOs are used to transfer data that are time-

critical. For example, joysticks transfer signals

via PDOs.

NMT is a special part that handles emergency

situations and other network administration.

Via an emergency object, the individual nodes

(components) are able to send a warning of

emergency situations. In this way, other CAN-

open components are able to identify the node

point from which the emergency call was sent.

CANopen specifies an Object Dictionary (OD)

that describes all parameters in the product.

This OD does not function solely as a specifi-

cation file, but also as an interface with other

CANopen devices. In other words, a descrip-

tion is given detailing which parameters are

necessary to activate the different functions

the product can perform.

CAN communication is best understood in the

following way:

Instead of sending a message from compo-

nent A to unit B, it is broadcast. Each compo-

nent, a PVG CIP for example, is then able to

listen in and col-lect information relevant to it

selv. The message format is designated COB

(Communication Object), which applies to all

messages.

A COB has an identification code (COB-ID)

that makes it possible for a component, a PVG

CIP for example, to sort and prioritise trans-

mitted communication objects (COBs). The

COB-ID clearly identifies the COB in a network.

Baud rate

Bus length

1 2 3 4 5 6 7 ... 31 32

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