User guide

26 MDX-ETHERNET User Guide
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7 Cyclic data
Modbus does not feature a dedicated cyclic data channel in the same sense as many
other networks. In the MDX-ETHERNET implementation, cyclic data can however still
be accessed from the network via dedicated entries in the Modbus register map. Just as
with regular Parameter's, the cyclic data is converted to a format suitable for Modbus.
7.1 What is cyclic data?
Cyclic data transfer is a method of transferring data on a regular time period, often
known as 'polled data'. High-speed data transfer is achieved by transmitting only data
bytes over the Modbus TCP network and using local mapping information within the
MDX-ETHERNET and Modbus TCP master controller to ensure that the correct data is
sent to the correct locations. The flexibility of the MDX-ETHERNET means that each
cyclic data OUT channel can be directed to any read/write drive parameter. Similarly
each cyclic data IN channel can use any drive parameter as a source of data.
See section 13.2 Compression of Cyclic data for information on using data compression
with 8 or 16-bit parameters.
7.2 Data formats
The MDX-ETHERNET can be configured with up to ten 32-bit or ten 16-bit cyclic OUT
and IN data. OUT and IN cyclic data are mapped using ten mapping (pointer)
parameters, one for each mapping.
Cyclic data is more easy and fast acess than acyclic, when various parameters is
needed (no contiguous). it is strongly recommended to use cyclic.
WARNING
• The term OUT data refers to data that is transmitted out of the master to the
slave.
• The term IN data refers to data that is returned from a slave into the master.
• Cyclic data mapping cannot be changed dynamically, as changes to the
configuration (mapping parameters, etc.) will only take effect during
initialisation of the MDX-ETHERNET.
• The maximum number of 8-bit mappings parameters that is possible is: 10 (10
bytes if cyclic data compression is on and 20 words if cyclic data compression
is off ).
• The maximum number of 16-bit mappings parameters that is possible is : 10
(10 words if cyclic data compression is on and 20 words if cyclic data
compression is off).
• The maximum number of 32-bit mapping parameters that is possible is : 10
(20 words).
NOTE
By default all drive parameters are cast as 32-bit (two 16-bit words) therefore twenty
cyclic words give ten possible drive parameters. Data compression reduces the number
of cyclic words required for drive parameters of 16-bit to 16-bits and 8-bit (or less) to 16-
bits.
Any 32-bit parameters mapped will still require two 16-bit words even with compression
turned on.
NOTE