ELF/LEM data format

ELF data are stored on unformatted, 512 byte blocked, disk drives in the following format:

Blocks 0 to 255 are reserved for a disk directory, containing up to 4096 entries corresponding to each 1/2 Mbyte disk write (a record). Each 32-byte directory entry is as follows:

Byte number(s)	Function
1,2	2-byte unsigned integer: record number starting at 1
3,4,5,6,7,8	Year, Month, Day, Hour, Minute, Second as binary coded decimal
9	1-byte unsigned integer: total number of channels (1,2,3,4,or 6)
10	1-byte unsigned integer: frequency number (see below)
11,12	2-byte unsigned integer: number of blocks per record (always 960)
13,14,15,16	4-byte unsigned integer: block number where data start for this record
17,18,19,20	4-byte unsigned integer: time of first sample in record (see below)
21,22,23,24	4-byte unsigned integer: time of last sample in record (see below)
25 - 32	Unused

All integers are stored using PC byte ordering (low-byte, high-byte). Times are stored as high-word, low-word.

Frequency number is as follows:

1 = 256 Hz	2 = 128 Hz	3 = 64 Hz	4 = 32 Hz
5 = 16 Hz	6 = 8 Hz	7 = 4 Hz	8 = 2 Hz
9 = 1 Hz	10 = 2 s	11 = 4 s	12 = 8 s
13 = 16 s	14 = 32 s	15 not valid	16 not valid

Times are in ticks (1/256 second) since the reset of the sample clock. The sample clock is set on power-up and so time is arbitrary unless the sample clock is reset by a hardware synch pulse. The software real time clock can only be set via a keyboard stroke, and so a synch pulse after instrument power up allows the actual time of the first sample (recorded in the first directory entry) to be calculated. A second synch pulse on recovery, followed by enough time for the instrument to write another disk record before reset, allows the time of the last recorded sample to be computed and thus a clock drift to be estimated.

Data starts at block 256 and is a continuous stream of 2-byte, sample multiplexed, data. All data are unsigned integer with PC byte ordering (low-byte, high-byte).

Center frequencies of the VCOs are switched depending on sample frequency to give the following counts during a sample interval for a 0V input:

256 Hz = 3906.25	128 Hz = 7812.5	64 Hz = 15625.	32 Hz = 31250.
16 Hz = 7812.5,	8 Hz = 15625.	4 Hz = 31250.	2 Hz = 7812.5
1 Hz = 15625.	2 s = 31250.	4 s = 1953.125	8 s = 3906.25
16 s = 7812.5	32 s = 15625.

-5V gives 0 counts, and +5 V gives 2 times the number in the above table. Thus, the instrument is true 16-bit only at sample frequencies of 32 Hz, 4 Hz, and 2 s.