Toshiba V Series Manual page 31

(2) Asynchronous mode
Output : The S controller writes output data to slave devices into the DN311.
Input :
S controller
DN311
Slave
Transfer of output and input data between the S controller and the DN311 and their
transfer between the DN311 and slave devices are asynchronous. Transfer between the
S controller and the DN311 is synchronized with the output/input cycle of the S controller
program, while transfer between the DN311 and slave devices is synchronized with the
scan cycle of the DN311.
Data is guaranteed in units of word (16 bits), but the sequence program is simplified for
data transfer processing, compared with the synchronous mode.
When the scan cycle of the DN311 is shorter than the that of the S controller, there will be
less delay in data refresh between the S controller and slave devices.
The scan time of the DN311 varies depending on the number of slave devices
connected, size of transmission data by slave device, and the performance of the slave
devices connected.
6F8C0903
Regardless of the timing of output data write by the S controller, the DN311
sends the written output data to slave devices on the scan cycle of the local
station.
Unless the output data is refreshed by the S controller, the DN311 sends the
same data to slave devices.
The DN311 receives data from slave devices by a polling response/bit
strobe response. After having received data from all the slave devices, the
DN311 refreshes the input data. The DN311 doesn't sends a receive
completion notice to the S controller.
Regardless of the timing of input data refresh by the DN311, the S controller
reads input data.
S controller data refresh cycle
Output
data
Output Input
processing processing processing
Response
Figure 1-11 Data Transfer in Asynchronous Mode
Input
data
DN311 scan
Output Input Output
processing processing
Response Response
1.4 Basic Functions
1
Output
data
Input
processing
11