Rockwell Automation 1771-DA,D17716.5.13 U MNL 1771-DA ASCII I/O MODULE User Manual

Page 143

Function of Control/Status Bits

Chapter 6

6Ć8

Bit: IW2(17)
Function: Single or Multiple String Transfer (Rate)
Description: Reset it to zero (default) when you want the module to send
a single string to the PC processor in a block transfer, or when more than
one block transfer is required to transfer the string.

Set it when you want the module to send more than one string in each
block transfer. This feature is limited to strings that fill 31 words or less.
Select this feature when the transmission rate of data from the ASCII
device to the ASCII module’s input buffer exceeds the transmission rate
of single string block transfers to the PC processor. The block transfer
time in a remote system is approximately the same regardless of the
number of strings contained in a transfer. So send as many full strings as
you can fit in 62 words. (Two words of each block transfer are reserved
for status or command words). The module will not divide a string
between two or more block transfers when set for multiple string transfer.

If the rate of transfer between ASCII module and processor cannot keep
up with the communication rate from the ASCII device, data will be lost
when the ASCII module’s input buffer becomes full. You can program
the examination of the input buffer 75% full bit, SW1(02), to alert the
operator to turn off the ASCII device before the loss of data occurs.
When you use RS-232-C with control lines, the module turns off the
clear-to-send (CTS) signal when the input buffer is full.

Initialization Word Three, IW3

Bit: IW3(00-02)
Function: I/O Buffer Split
Description: The I/O buffer capacity is 1024 words. You can subdivide
the buffer between percentage of input and output according to the
transmitting and receiving capacity of your ASCII device. Select the
percentage of input to output.

Input/Output (%)

50/50

100/0 (invalid for simplex write)

75/25

25/75

0/100 (invalid for simplex read)

all

other

50/50