Specifications

Fieldbus Communications Support
31004624 8/2009 65
Control and Status Words
Introduction
Understanding the manner in which the INTERBUS master’s control word
corresponds to the NIM’s status word is crucial to gathering diagnostic information
from the STB Advantys island. Output image and input image are defined relative to
the master.
When the INTERBUS master requests diagnostic data from the island, the
requested information will be delivered in the low byte of the NIM’s status word.
NOTE: In this discussion, input and output describe the process image from the
master device’s perspective. For example, the master receives the input process
image and transmits the output process image.
Control Word
The control word is the first word in the output image. By setting the word’s output,
the INTERBUS master can select up to 256 bytes of status information from the
NIM. Because the NIM control word is treated like a data word, you can choose
different status information at the application program level.
Status Word
The status word is the first word in the input image. The NIM automatically responds
to the master’s control word by providing the requested data in the low byte of its
status word.