Two parallel modes of data acquisition are required to receive the result from all the DSP nodes of each sub-band. In one mode, the results flow at a reasonably slow rate ( upto 64KBytes per second ), and are acquired locally by the control PC, for typical observations and diagnostic runs. In the second mode, the results may be generated at a high-speed ( upto 8 MBytes per second ) and are needed to be acquired continuously at this rate. To handle these two modes, a data acquisition interface has been developed to read out data from the DSP nodes at a programmable speed. It delivers the results to the PC for lower speed acquisition, and in parallel, to a separate high-speed DAS ( such as the S2 - Canadian recorder ).
The DAS controller has a node-address-sequencer, which is programmed with apriori information regarding the number of available nodes and the sequence they have to access the results read-out. The controller gets flags from each DSP node which indicate that their respective result link FIFO are empty, half-full ( HF ) or full. The controller polls these flags in the sequence mentioned above. For each node, the controller keeps waiting till that HF-flag gets set, after which it supplies read-strobes to the FIFOs of the corresponding node, and reads out half-FIFO worth of data in a continuous block. After each block, the node sequencer changes to the next node-address in the prescribed sequence and starts polling its HF flag. Within the DAS interface, a 16 bit X 32 K FIFO bank is hosted to buffer the data for PC recording. The incoming 32-bit result is split into two halves of 16-bits each and is loaded into this bank. The FIFOs are memory, mapped onto the PC via ISA bus. They generate flags to the PC to show whether they are empty, half-full or full. The PC monitors these flags once in a while and initiates a transfer to the hard disk from the FIFOs whenever the half-full flags are set. The data acquisition is but one of the tasks of the main control routine that runs on the control PC. The data entering local FIFO bank is also buffered in parallel to a connector for interfacing to a high speed recorder.