Xycom 74212-001B, Руководство пользователя

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XVME-212 Manual
August, 1989
3.11 PROGRAMMING CONSIDERATIONS
Use the following procedure to ensure that no change bits in a Change Register are zeroed
before they are read.
The Change Register must be read first, which will stop the
scanner. Then the Data Register is read, which will zero the Change Register and restart
the scanner. The reason for this sequence is explained in Section 3.9.2. Reading any
Change Register stops the scanner and reading any Data Register restarts it. The user-
should therefore read the Data registers as logical pairs (i.e., read Change Register 3, then
Data Register 3, not Change Register 3 then Data Register 2). These logical pairs may be
words or bytes. For example, to read all channels on the board as words, the correct
sequence is:
1 . read Change Registers 0 and I (as one word)
2 .
read Data Registers 0 and 1 (as one word)
3 . read Change Registers 2 and 3 (as one word)
4 .
read Data Registers 2 and 3 (as one word)
During the time that the scanner is stopped, no changes in state will be detected and the
Data Registers are not updated. In systems which must keep up with the fast changing
inputs, the time that the scanner is stopped must be minimized. It is therefore necessary
to read the Extended Status, Status/Control, or Data Registers (restarting the scanner)
quickly after the Change Register is read (stopping the scanner). It is suggested that the
user disable all CPU interrupts between the two reads. If an interrupt is taken after the
Change Register read but before the data/status registers are read, the scanner would be
off for the duration of the interrupt service routine. The following chart shows the
relationship of input channel signals with respect to stopping the scanner.
3-13
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