Honeywell Experion PKS User Manual page 125

Configurable channel types
The PROFIdrive technology defines the following two configurable channel types.
• PKW Inputs
• PKW Outputs
These channels use the acyclic PKW data area to read parameter information from the drive. These channels are
slow. Therefore, they can be used to read information such as statistics, configuration information, and other
miscellaneous data from the drive.
Attention
The PKW area is supported in the PPO types 1, 2, and 5.
Guidelines for using PKW channels
While using the PKW channels, you must remember the following guidelines.
• Do not use PKW channels for accessing or writing time-sensitive data - Typically, the PKW channels
send and receive data at a rate proportional to the device subscription rate. If there is no PDA connection, the
rate is approximately 1 sec. If there is a PDA connection, the rate depends on the PDA subscription rate.
Also, this rate is dependent on the number of PKW channels. For example, if the subscription rate is 1 sec
and you have 10 channels configured, the rate at which each channel will be updated is 10 sec (subscription
rate * n = 1 sec * 10).
However, due to the acyclic nature of the parameter requests, there is no guarantee that the response comes
in the data cycle after the request. Therefore, you can use the above information only for approximation
purposes and not consider as the actual rates. Because the PKW channels update the date at a non-
deterministic rate, you should not use these channels for accessing or writing data that is time sensitive.
• Configuring the channel high range and low range as "NaN" to enable or prevent scaling of the
parameter values - Because the PKW channels are analog channels, you must configure the channel high
and low range. If you know the parameter range and if it is possible to scale the parameter value within 0%
to 100%, you can enter the high and low ranges.
However, in some cases having a scaled value does not make sense. For example, counters and for
configuration enumeration values (which may have a range of 0, 1, 2, 3, and 4, each with a specific
meaning). In such cases, you can configure the channel high and low range as "NaN" (IEEE 754 "Not a
Number"). Configuring the channel high and low range as "NaN" prevents scaling of the parameter value.
• Do not use PKW channels with Regulatory Control blocks - You must be careful while using the PKW
channels in control strategies because of the following reasons:
– They are slow and non-deterministic in the timing of their updates.
– You can read and write the same parameter which can lead to race conditions. This may result in error
reading old data or error writing new data.
– The status of the output channels does not follow the typical channel behavior.
For example, consider a scenario where you provide a value to an output channel that writes to a PKW
parameter, and that value is outside of the range of that parameter. In this scenario, the channel status is
set to "Bad" because of the PKW error response. Also, if this channel is wired to a block that is used in a
control (for example, PID, REGCALC, RATIOCTL, and so on) you must force all upstream blocks into
the "Initialization Manual" state. Also, you will not be able to recover without deleting the Control
Module containing the strategy. This is one of the reasons you should not use PKW channels with
regulatory control blocks.
• Difference in PKW channel displays - You can use the displays for the PKW channels for displaying
statistics, counters, and other information about a drive. However, the displays of the PKW channels differ
depending on whether or not that drive is under control (PIOMB is loaded or not).
For example, consider a display displaying all 32 channels of a PKW input PDC.
7 DEVICE SUPPORT BLOCK (DSB)
125