Yes, it's true: The PLG boards are not supported by the Motif XS. Here is what Yamaha have to say about this (originally posted on the Motifator forum):
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Okay, now for some news that may stir up some posts. The Motif XS does not support PLG boards. We looked at this long and hard. We supported the forum when there was a call for a new Piano PLG card and got Yamaha Japan to develop it. However, the PLG cards were not particularly successful in the market. The basic architecture was developed almost 10 years ago and had certain inherent limitations. The ones that some people are most passionate about (the VH and VL boards) were not that successful partly because they need more expertise to use (knowledge of vocal harmony settings in the former and use of a breath controller in the latter). The VL which focused on solo wind and brass is partially addressed by the new 8 element voices with note offs and legato.
The VH is replaced by an on board, built in 10 band Vocoder. The Vocoder allows up to 128 notes of harmonies to be created from the keyboard and also is of course useful for cool synth effects as well. It can be used in Voice and Performance mode and recorded directly into the Integrated Sampling Sequencer.
We realize that the decision to stop support for PLG cards in the XS will not sit well with some people, but we always have to balance legacy support with the need to move forward.
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Yamaha PSR-S770, Korg Krome 61