VSIC Code Switch Modes
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JP2 "TEST/MODE" pin #    7531  -- 1==connect to GND, LED lights up
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alt_VLBA_sampler: 	"0000"; -- all 8 USBs in low, LSBs in high bits
VLBA_sampler: 		"0001"; -- BBC order
alt_MK4_sampler: 	"0010"; -- not implemented yet
MK4_sampler: 		"0011"; -- not implemented yet
(vacant)                "0100";
(vacant)                "0101";
decrement_testmode:	"0110"; -- decrementing 32-bit word
testmode:		"0111"; -- VSI-H test pattern
MKIV_IOB: 		"1000"; -- Mk4 formatter headstack #1 (50-pin)
MKIV_IOB_no_parity: 	"1001";
MKIV_WDB:		"1010"; -- Mk4 formatter headstack #2 (40-pin)
MKIV_WDB_no_parity:	"1011";
VLBA_recorder: 	        "1100";
VLBA_recorder_no_parity:"1101";
S2_sampler              "1110"; -- 16 low bitstreams
milkingmachine:	        "1111"; 



VSIB Control Register Mode Bits
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alias cmode: STD_LOGIC_VECTOR(3 downto 0) is capture_mode(19 downto 16);
all32bits:	"0000";	-- MK5 32-bit mode, all the VSI data
even16bits:	"0001"; -- MK5 16-bit mode or all sign bits from VLBA sampler
low16bits:	"0010"; -- alt: 2-bit USB from 8 BBCs
                        -- norm: 2-bit USB+LSB from 4 BBCs
                        -- also S2 16-bitstream mode
low8bits:	"0011"; -- alt: 2-bit USB from 4 BBC:s
                        -- norm: 2-bit USB+LSB from 2 BBCs
loweven8bits:	"0100"; -- MK5 8-bit mode or 8 sign bits from VLBA sampler
                        -- alt: 1-bit USB from 8 BBCs
                        -- norm: 1-bit USB+LSB from 4 BBCs
autoc4x2bits:	"0101"; -- as 'low8bits'; Autocorrelator 2-bit mode
autoc4x1bit:	"0110"; -- 4-bit low even; Autocorrelator 1-bit mode
test2bits:	"0111"; -- 2-bit mode for test
test1bit:	"1000"; -- 1-bit mode for test
VLBA_142:	"1001"; -- VLBA 1:4 fanout mode with 2-bit sampling
VLBA_141:	"1010"; -- VLBA 1:4 fanout mode with 1-bit sampling
VLBA_122:	"1011"; -- VLBA 1:2 fanout mode with 2-bit sampling
VLBA_121:	"1100"; -- VLBA 1:2 fanout mode with 1-bit sampling
VLBA_112:	"1101"; -- VLBA 1:1 fanout mode with 2-bit sampling