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7 SSI Input Units
NX-series Position Interface Units User’s Manual (W524)
7-6 I/O Refreshin
g Method
Setting
7
7-6
-2 F
ree-
Run Refr
esh
ing
Precautions for Correct Use
• If you use a Position Interface Unit and EtherCAT Coupler Unit together and you use
Free-Run refreshing, set the task period to a value that is greater than or equal to the refresh
cycle of the Position Interface Unit.
• If you use synchronous I/O refreshing or task period prioritized refreshing, set the task period
to a value within the specified refresh cycle range of the Position Interface Unit.
For the communications cycle specifications of the built-in EtherCAT port on an NJ/NX-series CPU
Unit, refer to the
NJ/NX-series CPU Unit Built-in EtherCAT Port User's Manual (Cat. No. W505). For the
communications cycle specifications of the EtherCAT Coupler Unit, refer to the NX-series EtherCAT
Coupler Unit User’s Manual (Cat. No. W519-E1-05 or later).
Use Free-Run refreshing to ignore the data refresh time of the SSI Input Unit and simply exchange data
with the Controller.
The SSI Input Unit will refresh data through SSI communications asynchronously with the Controller
processing cycle.
Data is exchanged with the Controller based on the I/O refreshing timing of the Controller.
The data that is exchanged is based on the SSI data that was obtained in the last I/O refresh.
For this method, the SSI Input Unit sends a clock signal to the encoder that is timed to the internal cycle
of the Unit and receives data from the encoder.
After the data is received, the updated data is written to memory for I/O refreshing.
For Free-Run refreshing, the Unit’s internal cycle is always 125 μ s. The cycle for receiving and refresh-
ing data through SSI communications depends on the SSI baud rate and the data length.
The timing of refreshing the data that is exchanged with the Controller depends on the data refresh
cycle of the SSI communications.
Use the following equations to calculate the data refresh cycle.
You can use data traces on the Sysmac Studio to check the data update timing. Assign the Encoder
Present Position Refresh Count to an output and check the timing when the value changes in the data
trace.
7-6-2 Free-Run Refreshing
I/O refreshing
Data received and refreshed Data received and refreshed
Data
processing
I/O refreshing
Data
processing
Controller
SSI Input Unit
Controller cycle
time
Position data
Clock output
Data input
Data refresh cycle
Wait Time for
Receive Enabled
Unit’s internal cycle: 125
μ
s
Unit’s internal cycle Unit’s internal cycle
Data refresh cycle = Conversion wait time + (Number of leading bits + Valid data length) × Clock
Monoflop time + Unit processing time