Why do I get bad values when using the PCI-QUAD04 incremental encoder board with xPC Target 2.5 (R14)?

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I am using the PCI-QUAD04 Incremental Encoder Driver block. If the index inputs to the PCI-QUAD04 (or CIO-QUAD04) Incremental Encoder board are left unconnected, occasional bad values are read from the block.

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 27 Jun 2009
This bug has been fixed in Release 14 Service Pack 2 (R14SP2). For previous releases, please read below for any possible workarounds:
We have verified that there is a hardware limitation with the Measurement Computing CIO- and PCI-Quad04 boards, which are supported by xPC Target 2.5 (R14), in the way that they handle unconnected inputs. Even with the the "Index input resets counter" checkbox disabled, noise on the index input line might cause the counter chip on the board to latch an incorrect value. This value differs from the correct value by a multiple of 256. The frequency of occurrence of the bad reads depends on the count frequency coming from the encoder. High count rates result in more bad reads.
To work around this issue, try the following.
1. If the index signal from an encoder is not being used, connect the index inputs.
2. Connect the index+ input to +5 volts.
3. Connect the index- input to ground.
These connections should be made as close to the board as possible to minimize any crosstalk at high count frequencies.

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