During the early years of the New Mexico Tech Interferometer, the data acquisition was performed partly with tape recorders which were notorious for losing synchronization and corrupting data. Also, the lack of an absolute timing reference made it difficult to compare interferometer data with other data sets. In 1995, that all changed when a versatile new VME-based digital system replaced the tape recorders. New modes of data acquisition were introduced such as sferics recording. More importantly, precise GPS timing was integrated into the data.

A picture of the VME system is shown below with numbers denoting the various VME boards and letters denoting other parts and peripherals. The boards and parts are described in the tables below. This picture was taken during 1999 when the system was configured for sferic acquisition only.

Table 1. VME Board Descriptions

Board	Manufacturer & Product Number	Description
1	Performance Technologies PT SBS-915	This is an interface board which allows a host computer equipped with an S-Bus adapter to communicate with the VME boards (especially the DSP board).
2	Performance Technologies PT VME-240	This is a 32 MB memory board which is currently only used for the triggered interferometer mode. In this mode, interferometer data is block-transferred almost continuously between the DSP and the memory board. Unfortunately, the VME bus must be locked for these transfers, which forces a data acquisition dead time after a trigger is completed since the transfers must be disabled in order for the host computer to gain access to the VME bus and the data.
3	Sonitech Technologies SP-40 Dual DSP	This is essentially the "brains" of the entire system. The dual DSPs process the data real time and make the crucial decisions on which data should be stored and which should be discarded. The DSPs are Texas Instruments C40 processors which run at 40 MHz (slow by comparison to todays CPUs, but they are computationally efficient). Each of the 2 DSPs has it's own dedicated memory area. The DSPs are linked to each other through an external communication port (see "B" description below) which is configured for bidirectional communication between the DSPs.
4	KSI Odetics GPS VME	This board interfaces to a GPS Lowrance receiver. The board phase locks to the GPS and outputs time codes which are accurate to within 1 microsecond of actual universal time (UT). The time codes are output onto the VME P2 connector and are intercepted by each A/D board, which mixes the time codes in with the data stream (see "5-8" below).
5-8	New Mexico Tech Analog/Digital (A/D) boards	The inputs of these analog/digital conversion boards interface directly to the voltage output of electric field, log-RF, sine/cosine phase, etc.. and convert these voltages to digital values. Most of the inputs are configured for 8-bit conversion, but some of them can be (and have been) reconfigured for higher precision 12-bit sampling (see "D" and "E" below). The boards sample data continuously and can be configured (jumpered) for either 500 kHz or 1 MHz sample rates. The boards arrange the digital data into 100 microsecond data blocks, the beginning of which is always time-stamped with GPS time codes. The most common use for each individual board is listed below: Interferometer Z-baseline? (8-bit sine/cosine for short/long baselines) Interferometer X-baseline? (8-bit sine/cosine for short/long baselines) Interferometer Y-baseline (8-bit sine/cosine for short/long baselines) 8-bit Log-RF, 12-bit slow-antenna electric field, and 12-bit fast-antenna electric field (for both sferic and interferometer modes). Other data sources have been substituted in the past when needed. Most notably, photometry and magnetic field data were sampled in conjunction with electric field data during late 1997.

Table 2. VME and Supplementary Part Descriptions

Item	Name	Description
A	S-BUS cable	A 25' long cable which connects the VME crate to a host (Sun) computer. The host computer controls the DSPs and retrieves data through this cable.
B	ASM-C cables	High speed cables which connect each A/D board to a COM port on either the upper or lower DSP. The A/D boards output digitized data and time codes through these cables. The DSPs can also reset and synchronize the A/D boards through these cables.
C	A/D board cable	This cable interconnects the A/D boards. One A/D board is configured as the master and synchronously controls the other slave boards through this cable. This arrangement insures that data is sampled synchronously.
D	Fast Antenna input	This is an input for a 12-bit A/D converter. Normally, fast antenna electric field data is the input source. Whatever data is input here becomes the trigger source for the sferic data acquisition mode.
E	Slow Antenna input	This is an input for another 12-bit A/D converter. Normally, slow antenna electric field data is the input source.
F	log-RF input	This is an input to an 8-bit A/D converter. Normally, the interferometer log-RF (274 MHz) is the input source. In RF-thresholded interferometer mode, this data is the trigger source.
G	Horita GPT-50 unit	The Horita unit(s) are actually not integrated into the VME system, but usually must be in close proximity in order to use the GPS output (see H below). The Horitas, one of which is shown above (viewed from the side), inserts GPS timing information onto video.
H	GPS serial cable	GPS information obtained by a Lowrance receiver passes through the Horita unit(s) down into the KSI Odetics GPS VME board. The numerous connectors on the back of the Horita unit are combinations of gender changers, null modem adaptors, and a custom 1 PPS video pulse insertion adaptor.