Rockwell Automation Publication 1734-UM007E-EN-P - October 2015
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Set and Operate Your Module
Chapter 4
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.