Rockwell Automation 57C570 AutoMax PC3000 User Manual - Manual (Page 161)

14Ć3
14.1.2 About Continuous and NonĆContinuous
BlockĆTransfer Requests
BlockĆtransfer requests operate in one of two ways:
D continuous
D nonĆcontinuous
Use a continuous blockĆtransfer when you want the blockĆtransfer
request to execute at regular intervals. This allows the blockĆtransfer
modules like analog I/O to act like discrete I/O, meaning you get
predictable data transfer. When you set up the blockĆtransfer, you
can specify how often you want the blockĆtransfer to execute in
milliseconds. Once initiated, the continuous blockĆtransfer request
performs its operation by executing every n ms, which you defined
as the update time, until an error occurs. When an error occurs, the
blockĆtransfer request is disabled.
When choosing an update time for a continuous blockĆtransfer, keep
in mind that only one blockĆtransfer request can be executed per
rack per remote I/O scan. When defining an update time, consider
these factors:
D remote I/O scan time
For information about estimating this time and designing your
system to optimize blockĆtransfers, see chapter 4, Designing
Control Systems That Use the AllenĆBradley Remote I/O Link."
D blockĆtransfer processing restrictions for target blockĆtransfer
modules
The time you define is the minimum time in which the
blockĆtransfer will execute.
Define an update time that is:
D longer than the minimum time required to complete all the
blockĆtransfers in the program
If you choose a time that is less than the minimum time required
to complete all the blockĆtransfers, the blockĆtransfer data is read
or written as fast as the remote I/O scan allows.
D not too fast for the processing capabilities of the target
blockĆtransfer module
Use nonĆcontinuous blockĆtransfers when:
D you only need intermittent block data exchange, such as writing
a message to an operator interface or gathering nonĆcritical
status information
D your application requires orderly handshaking with I/O modules
For every nonĆcontinuous blockĆtransfer you want, you must write a
separate command. However, with continuous blockĆtransfers, you
write one command for each target module.