Rockwell Automation 1734-SSI POINT I/O Synchronous Serial Interface Absolute Encoder Module User Manual - Manual (Page 36)

Publication 1734-UM007D-EN-P - December 2005
4-8 Set and Operate Your Module
At point B of the CLK signal is the rising clock edge. The sensor
begins to send its serial data to the 1734-SSI module. The module
actually starts reading the sensor position data on the next rising clock
edge, denoted by MSB in the figure above. MSB is the most significant
bit of the data word.
The 1734-SSI module supports only MSB aligned data. This means that
the SSI sensor sends the MSB of its data word first, and the least
significant bit (LSB) is sent last. Notice that it takes 25 rising clock
edges to read in a 24-bit data word from a 24-bit SSI sensor. A 25-bit
sensor needs 26 rising edges and a 13-bit sensor needs 14 edges.
Configure the module for the SSI Word Length as stated in the sensor
specifications and the module adds the extra rising edge
automatically.
The value t
p
shown in the CLK signal in the figure is the period of the
SSI clock signal and is equal to the inverse of the SSI Data Rate; thus
SSI Data Rate = 1/t
p
bits/second (or communication rate). You can
verify that your module is operating at the Data Rate that set in the
module configuration by measuring t
p
on an oscilloscope and
calculating the Data Rate.
The actual bit values of the current SSI sensor position are shown
beneath the Data signal in the figure. Note that a bit value is always
the binary (0 or 1) value just before a given rising clock edge. The
next binary value is sent by the SSI sensor just after the rising clock
edge and is read by the module on the next rising clock edge.
When you group the binary values in sets of 4, starting with the LSB at
the right of the DATA signal, and working up towards the MSB, you
can calculate the hexadecimal value of the SSI sensor position.
The hexadecimal values are shown in the figure, below the brackets
that denote each group of 4 bits. The Present SSI data word for this
sensor position would be transmitted by the module as follows:
Produce0 = 0x1C, Produce1 = 0x92, Produce2 = 0xAE, Produce3 =
0x00.
Produce3 is 0x00 because the 1734-SSI module always sends a 32-bit
hexadecimal value and the sensor in this example is 24-bit. The
Latched SSI data word, Produce 4 to 7, is similarly transmitted (low
byte first) by the module when the data latch is activated.