Delta RMC101 - Manual (Page 615)

SSI 6.6
Transducer Interface Modules 6-71
2. Set all options to your desired settings.
3. Click the Axis 1 tab.
4. Set all options to your desired settings.
5. Click Update RMC.
6. The Update Module Configuration dialog box will be displayed to indicate the progress. If the
module could not be reset automatically, you may be prompted to reset the module yourself.
7. In the RMC Configuration dialog box, click Close.
Data Length
SSI devices may be purchased which provide a specific number of bits of data. Some common
values include 13, 24, and 25 bits of data. Set the data length field to match your device.
Certain SSI devices have a number of bits but pad it with a zero, which means the total Data
Length must be one greater than the manufacturer's stated data length.
If the most significant bit from the SSI feedback is set, the RMC100 will see the value as a
negative number and will report a transducer overflow. The counts will go to zero.
Output Format
SSI devices may be purchased which use either Gray code or binary data output. Although the
RMC supports both, binary is preferred. Gray code is generally used internally by most absolute
rotary encoders, but it offers no advantage when the data is transmitted to the RMC over the SSI
interface. More importantly, there are some cases where using Gray code can interfere with
transducer error detection. Under Output Format, click the appropriate option to match your
device.
Error Method
Choose the method of indicating an overflow error on the transducer.
• Bit 20
If bit 20 in the SSI data turns on, or if the SSI data bits are all zeros, the Overflow bit in the
Status word will turn on. While the SSI standard does not specify the use of an overflow bit,
Balluff does provide an overflow bit (bit 20) on some of its BTL-5 series of transducers. When no
magnet is detected, the Balluff transducer sets this bit to indicate an error.
• All Zeros
If the SSI data bits are all zeros, the Overflow bit in the Status word will turn on. Most linear SSI
transducer manufacturers set the SSI data bits to all zeros if a magnet is not detected.
Count Offset
The RMC imposes a restriction that all positions must be specified as a 16-bit value in position
units. A position unit is a user scaled value and therefore is quite flexible. However, there are
limitations to this situation. Consider the following example:
A user is using a 2m-long, 5µm-resolution transducer, and wishes a position unit to equal 10 µm.
Therefore, the scale is set up so that 1 (one) position unit equals 2 (two) counts.
The user wishes to control the positions beginning at 1500 mm and ending at 2000 mm on the
transducer. This would result in position units of 150,000 and 200,000. Because the maximum value
for a 16-bit number is 65,535, this would not work. This is where the Count Offset is used.