BALANCE SHEET COMPONENTS (Components of significant balance sheet accounts) (Details) - USD ($) $ in Thousands		Dec. 28, 2019	Sep. 28, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Short-term investments	[1]	$ 119,000	$ 229,000
Inventories, net:
Raw materials and supplies		51,651	52,853
Work in process		37,481	32,026
Finished goods		36,122	33,742
Inventory, gross		125,254	118,621
Inventory reserves		(30,045)	(29,313)
Inventories, net		95,209	89,308
Property, plant and equipment, net:
Land		2,182	2,182
Buildings and building improvements		22,282	41,961
Leasehold improvements		24,542	24,441
Data processing equipment and software		36,626	36,302
Machinery, equipment, furniture and fixtures		73,018	71,465
Construction in progress		6,769	6,512
Property, plant and equipment, gross		165,419	182,863
Accumulated depreciation		(108,593)	(110,493)
Property, plant and equipment, net		56,826	72,370
Accrued expenses and other current liabilities:
Accrued customer obligations (2)	[2]	26,484	26,292
Wages and benefits		18,473	18,188
Dividend payable		7,651	7,582
Commissions and professional fees		2,432	2,024
Deferred rent		0	1,721
Severance		1,205	1,500
Other		6,277	7,226
Accrued expenses and other current liabilities		$ 62,522	$ 64,533

[1] All short-term investments were classified as available-for-sale and were measured at fair value based on level one measurement, or quoted market prices, as defined by ASC 820. The Company did not recognize any realized gains or losses on the sale of investments during the three months ended December 28, 2019 and December 29, 2018.

[2] Represents customer advance payments, customer credit program, accrued warranty expense and accrued retrofit obligations.