Rockwell Automation Allen-Bradley 871FM-M10BA30-x, Руководство пользователя, Страница: 83 / 90

Rockwell Automation Publication 871FM-UM002A-EN-P - April 2017 83
Explicit Messaging Examples Appendix D
Read Subindex
Figure 6 - Read Subindex
If you want to read/write at an index level that contains multiple subindexes,
you must send all subindex information when writing to read all subindex
information. When you are at the index level, you must know which subindex
the setpoint is located if the sensor has multiple setpoints.
Byte Swapping
Allen-Bradley little-endian format for its ControlLogix® systems via
Studio 5000®.
• Endian: Endian refers to where the most significant byte is in an integer
value. The big-endian stores the high-order byte at the starting address
while little-endian stores the low-order byte at the starting address. The
low-order byte contains the bits for the lowest possible values (for
example, 0…255) while the high-order byte contains the bits that specify
the large values (for example, 256…65535).
•
Little-endian:
The low-order byte of the number is stored in memory at
the lowest address, and the high-order byte at the highest address.
• Big-endian: The high-order byte of the number is stored in memory at
the lowest address, and the low-order byte at the highest address.
•
Byte Swapping:
Messages that come from a ControlLogix system to a
PLC-5® controller with a PLC-5 type message know to swap the bytes
before sending the data. Messages that come from a device that worked
with a PLC-5 controller previously can have an issue with bytes that
swap when the information is sent to the ControlLogix system. To help
arrange the byte in the proper order, the ControlLogix processor has an
instruction SWPB.