BALANCE SHEET COMPONENTS (Components of significant balance sheet accounts) (Details) - USD ($) $ in Thousands		Mar. 28, 2020	Sep. 28, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Short-term investments	[1]	$ 248,000	$ 229,000
Inventories, net:
Raw materials and supplies		60,181	52,853
Work in process		33,798	32,026
Finished goods		42,764	33,742
Inventory, gross		136,743	118,621
Inventory reserves		(30,565)	(29,313)
Inventories, net		106,178	89,308
Property, plant and equipment, net:
Land		2,182	2,182
Buildings and building improvements		22,284	41,961
Leasehold improvements		22,712	24,441
Data processing equipment and software		37,082	36,302
Machinery, equipment, furniture and fixtures		73,957	71,465
Construction in progress		7,082	6,512
Property, plant and equipment, gross		165,299	182,863
Accumulated depreciation		(109,652)	(110,493)
Property, plant and equipment, net		55,647	72,370
Accrued expenses and other current liabilities:
Accrued customer obligations (2)	[2]	33,766	26,292
Wages and benefits		23,419	18,188
Dividend payable		7,557	7,582
Commissions and professional fees		2,606	2,024
Deferred rent		0	1,721
Severance		880	1,500
Other		10,068	7,226
Accrued expenses and other current liabilities		$ 78,296	$ 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 and six months ended March 28, 2020 and March 30, 2019.

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