Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE - Manual (Page 151)

Function of Control/Status Bits
Chapter 6
6Ć16
Bit: SW1(15)
Function: Read Data Available
Description: When the module detects a change in its status or receives
new data from the ASCII device, it toggles this bit. Then the module
sends new status and/or data to the processor with the toggled status of
this bit. The module must receive acknowledgment in a subsequent write
block transfer (toggled status of CW1(15)) before it can send new status
and/or data. When sending new data to the processor, the module also
enters the number of transferred data words into status word two (SW2)
which accompanies the transfer.
Bit: SW1(16)
Function: Write Data Acknowledge
Description: The module acknowledges receipt of a valid write block
transfer by copying the status of CW1(16) into this bit, which is returned
to the processor in the next read block transfer.
Bit: SW1(17)
Function: Channel Active
Description: The module sets this bit to tell the processor that the ASCII
device is enabled. It is reset when the ASCII device is turned off or
disconnected.
Status Word Two, SW2
Bit: SW2(00-07)
Function: Number of Words Per String (Read, Only)
Description: This 2-digit BCD number shows the number of words used
by the module to transfer the data string that you selected in IW2(00-13).
The number of words will be approximately equal to the number of
characters per string divided by the type of data conversion such as 2
ASCII characters or 3 BCD characters per word.
When a data string requires more words than the maximum for a read
block transfer, these digits display the maximum number of data words
(62) in the read block. A data string could require several read block
transfers.
Bit: SW2(10-17)
Function: Number of Strings Per Block Transfer (Read, Only)
Description: This 2-digit BCD number shows the number of data strings
that are transferred to the PC processor in each read block transfer when
you select the transfer of multiple data strings, IW2(17)=1.