Rockwell Automation 20-COMM-H RS485 HVAC Adapter FRN 2.xxx - Manual (Page 37)

Using Modbus RTU 4-5
20-COMM-H RS-485 HVAC Adapter User Manual
Publication 20COMM-UM009D-EN-P
Setting the Logic Command and Reference
On Modbus RTU, there are two ways to set the logic command: discrete
outputs (
Table 4.E on page 4-6 ) and register outputs ( Table 4.F on
page 4-7 ).
•
When you need to set only one bit in the logic command word, you can
use a discrete output. For example, to stop a PowerFlex 70/700 drive (bit
0), you can use a discrete output (Modbus Address 0x0001).
•
When you need to set multi-bit fields in the logic status word or to set the
entire logic status word, you must use the register output to maintain data
integrity. For example, to set the direction of a PowerFlex 70/700 drive
(bits 4 and 5), you must use a register output (Modbus Address 4x0001).
A 16-bit product logic word is buffered in the adapter, holding the last logic
command sent to the drive regardless of whether it was sent through the
discrete outputs or through the product logic command register output.
When a bit is updated through the discrete outputs or the register output, a
new logic command is generated and sent to the drive.
To set the Reference, you must use a register output (Modbus Address 4x0002
and/or 4x0003 in
Table 4.F on page 4-7 ). Remember that the Reference value
is a scaled value; it is not an engineering value. For example, in PowerFlex 70/
700 drives, the reference is scaled based on the value of Parameter 55 -
[Maximum Freq], but the commanded maximum speed can never exceed the
value of Parameter 82 - [Maximum Speed].
Table 4.D shows example
References and their results on a PowerFlex 70/700 drive that has its
Parameters 55 - [Maximum Freq] set to 130 Hz and 82 - [Maximum
Speed] set to 60 Hz.
!
ATTENTION: Select and use either the “Product Logic
Command Discrete Outputs (0x000x)” or the “Product Logic
Command Register Output (4x0001)” as a control method, but
not both. Conflicts caused from using both methods can result in
dangerous operation. Failure to observe this caution could cause
bodily injury and/or damage to equipment.
Table 4.D Example Speed Reference and Feedback for a PowerFlex 70/700
Reference
Value Scale
Output Speed
Feedback
Value
Percent Value
32767
(1)
(1) A value of 32767 is equivalent to 100%. The effects of values greater than 32767 depend on
whether the DPI product uses a bipolar or unipolar direction mode. Refer to the
documentation for your DPI product.
100% 130 Hz 60 Hz
(2)
(2)
The drive runs at 60 Hz instead of 130 Hz or 65 Hz because Parameter 82 - [Maximum
Speed] sets 60 Hz as the maximum speed.
15123
(3)
(3)
The Feedback value is also scaled based on the value of Parameter 55 - [Maximum Freq],
For example, 60/130 = 0.46 so 32767 x 0.46 = 15123.
16384 50% 65 Hz 60 Hz (2) 15123 (3)
8192 25% 32.5 Hz 32.5 Hz 8192
0 0% 0 Hz 0 Hz 0