Hardware setup

To set the VSIC card to S2 input mode,  it must be set to mode E. This is done either using the toggle switch or setting the 4 rocker switches to 1110 (binary E). The VSIC also has a useful test mode where it produces a sequence of monotonically decreasing integers. This is useful for thorough testing of the capture software. To use the test mode set the VSIC to mode 6/0110.

The disk recorders can be used in a number of ways. Either directly connected to the LBA DAS, recording in parallel with the S2 and there are a number of special modes for recording higher data rates

Direct disk recording

The VSIC should be connected to the S2 output port on the DAS using a normal 50-way twist and flat cable. In this mode up to 256 Mbps data rates can be recorded (4x16 MHz) depending on the DAS mode.

Parallel S2 recording

For realtime fringe testing, and some special experiments, it is desirable to record data in parallel with the S2. The S2 C2a port allows this to be done, but the pin layout is different than for the S2 input (ie the data and clock signals are on different pins). To record this data, the BG2 cable (actually a small mounted PCB) is needed. Plug the BG2 into the S2 C2a  output port then plug a normal 50way twist and flat from the BG2 to the VSIC. Note that the C2a output on the S2 only makes available the signals that the S2 is recording, so that in this mode only 128 Mbps rates can be recorded. The VSIB recording hardware cannot record data rates less than 256 Mbps, so that half of the received data is rubbish. The vsib_record "compression" mode (-c) allow this data to be discarded before recording to disk.

Dual DAS recording (Huygens mode)

Two DAS can be recorded in parallel using the "Huygens" cable. This is a 3 headed cable where two connectors plug into the S2 output of two DAS and the other end of the cable plugs into the VSIC. In this mode 512 Mbps data rates can be recorded (8×16 MHz). This means vsib_record needs to be run in mode "2" (16 bits per sample).

64 MHz bandwidth

The correlator output ports on the DAS can be configured to output 2 bit 64 MHz data directly (with no digital filtering). This allows data rates of 512 Mbps from a single DAS (twice that from the S2 output port). As the signal format from the correlator ports are different from the S2 port a patch cable is needed to produce a pin assignment in the S2 specs. The BG3 cable does this. This PCB has 3 inputs and one output. Two of the inputs are 20 way twist and flat cables which connect into the two correlator output ports on the back of the DAS (this supplies the sampled 64 MHz data from the two IFPs and the 32 MHz reference clock from the first correlator output). The third input plugs into the S2 output of the DAS to supply the 1 PPS tick. The 50 way twist-and-flat output from the BG3 then plugs into the VSIC. For this recording mode, vsib_record needs to be run in mode "2" (16 bits per sample).

Please not that currently the BG3 cable does not work reliably. These problems are being investigated.