Design Guide
Revision 1.12
1998-08-12
HMS F
IELDBUS
S
YSTEMS
AB
Page 20 (35)
4.3.2 Data Exchange Mode
In the data exchange mode is the actual update of input, output, fieldbus and control area performed. In
order to ensure consistent data on both sides a simple handshaking procedure is introduced. This
handshaking procedure comes in two outlines:
•
POLL mode
•
INTERRUPT mode
In poll mode the data exchange is handled by polling the AB-DT control register (3FFh) during the DPRAM
access procedure.
In the interrupt driven mode the polling is substituted by waiting for the interrupt.
Both handshaking forms have in common, that all action will be started by the application. To reduce
update overhead, they can check the new data flag (3FCh) before the updating of the output field is
performed. When the last feature is used, the new data flag should be cleared during the access cycle. In
order to prevent deadlocks, a time-out function should be implemented by the application, which will
either leave the procedure or re-enter it.
