IV25101A_e.DOC / Feb-08
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7.3.
SSI Specific Settings:
SSI Low Bit:
Defines the lowest bit (LSB) for evaluation, when the bit blanking function is used.
Must be set to “01” for evaluation of the full encoder range.
SSI High Bit:
Defines the highest bit (MSB) for evaluation, when the bit blanking function is used.
Must be set to the total number of encoder bits for evaluation of the full encoder range.
The following example uses a 13 bit encoder where High Bit is set to 12 and Low Bit is set to
03, resulting in evaluation of bits 03 to 12 only and blanking out positions 01, 02 and 13.
13 12 11 10 09 08 07 06 05 04 03 02 01
(Hi_bit = 12, Lo_bit = 03)
Evaluated encoder bits
(LSB)
(MSB)
High order bit
Low order bit
Hint for the use of the bit blanking function:
Bit blanking results in a different evaluation of the encoder information, and you should be fully
aware of what happens with the resolution and the number of registered turns when you use
this function.
The subsequent example uses a 13-bit single-turn encoder to explain two different results
when blanking out one bit:
•
Without blanking, a 13 bit encoder would provide a 0 – 8191 information with a 0 - 360º
turn of the encoder shaft.
This would assume a setting of “High Bit = 13” and “Low Bit = 01”.
It is easy to understand that there are two different ways how to use only 12 of the 13 bits
available:
•
When we set High Bit to 12 while Low Bit remains 01, we have blanked the high order bit.
The result corresponds to an encoder providing information 0 – 4095 while we turn from
0 - 180º, and again the same 0 – 4095 information while we continue from 180º to 360º.
The resolution remains unchanged with respect of the number of steps per revolution.
•
We can also leave High Bit to 13 and set Low Bit to 02 instead. This means we blank the
low order bit now. As a result, within one turn of 0 - 360º, we receive the encoder
information 0 – 4095 one time only, but the total number of steps per revolution has been
halved.
