SMA1
5 - SSI interface
Order code: SMA1-GA2-5-…
5.1 SSI (Synchronous Serial Interface)
SSI (the acronym for
Synchronous Serial
Interface
) is a synchronous point-to-point serial
interface engineered for unidirectional data
transmission between one Master and one Slave.
Developed in the first eighties, it is based on the RS-
422 serial standard. Its most peculiar feature is that data transmission is
achieved by synchronizing both the Master and the Slave devices to a common
clock signal generated by the controller; in this way the output information is
clocked out at each controller's request. Furthermore only two pairs of twisted
wires are used for data and clock signals, thus a six-wire cable is required.
The main advantages in comparison with parallel or asynchronous data
transmissions are:
•
less conductors are required for transmission;
•
less electronic components;
•
possibility of insulting the circuits galvanically by means of
optocouplers;
•
high data transmission frequency;
•
hardware interface independent from the resolution of the absolute
encoder.
Furthermore the differential transmission increases the noise immunity and
decreases the noise emissions. It allows multiplexing from several encoders, thus
process controls are more reliable with simplified line design and easier data
management.
Data transmission is carried out as follows.
At the first falling edge of the clock signal (
1
, the logic level changes from high
to low) the absolute position value is stored while at the following rising edge
(
2
) the transmission of data information begins starting from the MSB.
At each change of the clock signal and at each subsequent rising edge (
2
) one
bit is clocked out at a time, up to LSB, so completing the data word
transmission. The cycle ends at the last rising edge of the clock signal (
3
). This
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5 - SSI interface
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