Data transfer from the DSP memory to the host computer can be performed in two different ways: via the VME bus and via the Host Port of the DSP.
The first method is the fastest one and it is used in the experimental data taking. There is a master VME CPU performing the readout of different sections of the GDH experimental apparatus [2]. This CPU directly accesses the DSP memory. The VME addresses of the DSP memory are obtained by multiplying by 4 the DSP internal addresses.
The second method requires a handshake protocol using the Host Port registers [9]. It has been adopted to test the MIDAS device independently from the rest of the experimental apparatus.
The data transfer software has been realized using the Labview code [10] installed on a QUADRA 900 computer. The data transfer loop is only controlled by two flags, HF2 and RXDF (Receive Data Register Full). When the DSP is ready for data transfer (HF2), the host tests the RXDF flag which is set to 1 when the DSP has filled the 3 8-bits Receive Data Registers.
A control of the number of word to be read is not implemented: the event transfer ends when both HF2 and RXDF are set to 0. A final test on RXDF is performed to ensure that the last word of the event has been transferred.
The program disables the IRQA interrupt during all the acquisition cycle. In such a way the program does not accept new triggers arriving while the data transfer of the previous event is in progress.