Shares of Xilinx ( XLNX) plunged in early Monday trading after the company's announcement that September-quarter sales will come in below its previous estimate.The semiconductor maker said it now expects sales to be down 1% to 2% sequentially, below its prior forecast of sales being flat to up 4% sequentially. Analysts surveyed by Thomson First Call were expecting the company to post sales of $415.3 million in the quarter, up about 2% sequentially. Xilinx said that its turns business was less than forecast, and that sales from companies with manufacturing operations in the Asia Pacific region fell by double digits sequentially. Sales of mainstream products also were below expectations, Xilinx said. Shares of Xilinx were recently off $3.92, or 14.5%, to $23.20. The news also affected other chipmakers: Altera ( ALTR) slipped 40 cents, or 2.2% to $17.94, while Broadcom ( BRCM) was off 47 cents, or 1%, to $46.