GE 90-30 PLC - Manual (Page 68)

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3-35 GFK-1084B Chapter 3 Programming Communications Requests
Each channel has a dedicated pair of bits as follows:
(Status Bits 17, 19, 21 ... 47) Data Transfer Bit: This bit is normally set to 0. It is pulsed
to 1 and back to 0 on successive PLC scans each time a transfer completes successfully.
Do not assume that when the Data Transfer bit goes to 1 that a transfer has just completed during
the last scan. The Data Transfer bit is not closely synchronized in time with the transfer.
The bit only indicates that a transfer has occurred during the preceding read (or write)
period. A rising edge on the Data Transfer bit indicating that a transfer has completed
successfully does not guarantee that the next transfer has not begun or completed. In
the case of an Establish Channel command, the CRS word is always updated before the
Data Transfer bit is set to 1.
(Status Bits 18, 20, 22 ... 48) Channel Error Bit: This bit is set to 1 when an error is de-
tected on this channel. It is set to 0 when the channel is initially established and if the
channel resumes normal operation after a transient error condition subsides. The Chan-
nel Error bit is also set to 0 when the channel is aborted by an Abort Channel command
or when the PLC CPU transitions from RUN to STOP. In the case of an Establish Chan-
nel command, the CRS word is always updated before the Channel Error bit is set to 1.