CAN Transmission Types

Introduction

The XMP CANOpen interface uses four messages (serial packets of data on the CAN bus) to pass I/O data between the XMP and an I/O node. Each message contains either the digital input, digital output, analog input, or analog output data. The XMP supports two standard communication methods to transmit I/O data between the XMP and each of the I/O nodes-cyclic transmission and event transmission. For most applications, cyclic messaging (the default) will be sufficient, but the transmission type fields within the MEICanNodeConfig structure allow the user to select an alternative transmission type for each of the I/O messages going to and from a node.

Cyclic Transmission

The Cyclic Transmission type, transfers I/O data messages between the XMP and the nodes using a cyclic protocol. The trigger for each cycle is a synchronization message that is transmitted at a regular rate by the XMP. When a node receives the synchronization message, it latches and transmits the current state of its inputs. Immediately after receiving the synchronization message, the master also transmits command messages to all the nodes with their new output states, which will get applied on the next synchronization message. An idle period is also needed to allow time for any non-cyclic messages to be transmitted.

The advantage of this scheme is that it generates a predicable loading of data on the bus. The latency on transmitted data is predictable, but the latency is not the absolute minimum that can be achieved.

Cyclic Period

The cyclicPeriod field within the MEICanConfig structure allows the user to specify the period (in milliseconds) that the XMP will use between the successive transmission of synchronization messages. The minimum cyclic period that can be used is dependent upon the chosen bit rate and the number of nodes. Assuming that all the nodes have inputs and outputs that are analog and digital, the minimum cyclic period that can be used is given in the following table.

CANOpen Cyclic Period

Bit Rate	< 5 Nodes	< 10 Nodes	< 50 Nodes	< 128 Nodes
1M	3	5	30	60
800k	3	6	30	80
500k	5	10	50	200
250k	10	18	89	300
125k	19	36	200	500
50k	46	90	500	2000
20k	200	300	2000	3000
10k	300	500	3000	6000

Event Transmission

The Event Transmission type, only transmits I/O data messages when an "event" occurs on the source node (either the XMP or the I/O node) to change the I/O data. The event that forces the transmission is either a new state of an input that is detected on an I/O node or a new output state that is commanded on the XMP.

The advantage of this type of messaging is that short reaction times are attainable, but this is accomplished at the expense of variable network traffic, and the possibility of saturating the network. In many cases, the reaction time is not significant in relation to other time delays in the system (ex: the user's application or delays in task switching).

Inhibit Time

If the source node's events occur at a very fast rate, the number of messages generated can swamp the network and consequently block out other messages. To prevent an excess of messages, nodes can optionally support inhibit times for their transmit PDOs. This value defines the minimum time between two successive PDO messages.

The inhibitTime field within the MEICanConfig structure allows the user to specify the period (in milliseconds) that all nodes on the network will use. A reasonable inhibit time is half a cyclic period.