User manual
Functional Description
ComTec GmbH 4-1
4. Functional Description
4.1. General
Basicly there exist two modes of operation: SINGLE and COINCIDENCE and of course any
mixture of both. The mode of operation of each port is individually selectable.
Generally all transfer of ADC data to the PC is done blockwise and in listmode, i.e. one after the
other with some overhead. The overhead consists of words for synchronization purposes and
some header information. The synchronization words provide a means to reconstruct data
structure if a transmission error accidentally occured. The header words contain the information
which data will follow in the data block. Deadtime or actually Livetime data of the ADCs is also
transfered in listmode.
The BASE module fetches the ADC data from the ADC input ports and transmits it in listmode to
the FIFO buffer on the PCI card. In COINCIDENCE mode it first checks if the coincidence
conditions are met. Every millisecond LIVETIME information of the ADC ports is inserted into the
list data stream.
4.2. SINGLE Mode
In SINGLE mode of operation the ADCs operate independently of each other. When a valid
converted event of an ADC is received from an ADC it is directly transfered to the PC. When
several ADCs send data simultaneously all the data can be transmitted to the PC in one block
thus saving system deadtime.
When the ADC asserts DRDY the data is registered in the input register of the corresponding port
and DACC is answered to the ADC. Then the ADC will remove DRDY and DACC is also
deasserted. After registering the data a BUSY is signaled to the BASE module to indicate that
new data is present and can be transfered to the PC.
When an ADC port detects a DRDY signal it checks if previous (not yet transmitted) data is still
present in its input register. If the port is still busy with old data it asserts DACC but does not
register the new data. Thus - in case of old data still being left to transfer - it discards the new
ADC data and makes sure the old data is not overwritten before it is transfered.
4.3. COINCIDENCE Mode
The basic purpose of introducing COINCIDENCE mode is to accumulate only data that meets
some predefined timing relationship. Events that will be accumulated must fall within the
coincidence resolving time window. Thus, events that do not fall within this time window will be
discarded and do not show up in the histogrammed spectra.
The versatility of the coincidence definitions in the MPA-3 system allows to select various
combinations of ADC events that either have to occur within the coincidence time window or
where just some have to occur to accumulate them.
In COINCIDENCE mode of operation the active going edge (programmable rising or falling edge)
of the ADC's DEADTIME signal is used as time-of-arrival of an ADC event. This is done because
the DEADTIME signal usually is the most accurate time information obtainable from an ADC.
On detecting an active going edge of a DEADTIME signal a coincidence resolving time window is
opened. While a coincidence window is open all arriving DEADTIME edges are stored. After the
coincidence time has elapsed the system waits for all BUSY (DRDY) signals of the corresponding
ADC ports (meaning that the port contains valid data) to become true. The maximum time to wait
for this is software selectable (ref. DRDY timeout - Figure 5.7: ADC Settings and Presets dialog
box on page 5-5). When all expected BUSYs have arrived - or at the latest when the DRDY