Preset counts

Data may be acquired in a preset count mode in which counting is initiated manually and ends when the preset count number (up to 1 million) has been reached. 

Start the patient s emission scan collecting data for a preset count or time. 

During periods of maximum use, minimize counting time by selecting the proper terminators (e.g., use a low count reject where appropriate or use a preset count terminator if this is appropriate, rather than a preset time or percent error setting). 

The signal to be counted can, in general, be easily followed through its path in the counter, and defective ICs can be located without too much difficulty. Circuitry associated with preset count and gating is generally more difficult to troubleshoot because of interactions among several control signals. 

Preset count is frequently limited to the combination pqxlO r where p.q and r represent decimal numbers. Generally it involves either a comparison of the BCD-coded numbers p and q with the contents of the sealer, or the zero detection in a count-down process of two decades previously loaded with contents p and q. Troubles may easily arise from glitches or spikes in the power lines. 

In timers or counter/tlmers, time marks are derived from the mains (typically at l/lOO or 1/120 second period) or, most frequently, from a crystal oscillator. These generally oscillate at a frequency of several MHz, which Is suitably divided to give time marks either In a seconds or in a minutes scale. In preset type counter/tlmers, the preset count can be associated with either the counting events or the time marks. 

The first step in troubleshooting is to establish the block diagram of the Instrument and to identify the faulty block or blocks. This Is generally easy for an absolute (no signal) failure, but may be rather tricky for the faults related to degraded performance (spurious counts, erroneous preset counting, etc. ). ... 

Preset count circuitry may be checked starting from the storage of preset information, followed by observation of the count-down, or by examination of the comparator circuits that test when preset count is reached, according to the approach used in the instrument to implement the preset function. 

The preset count value takes the form pqxlO r where r is set to a value in the range 0 to 6, and pq is a two-digit number taking values from 0 to 99. This information is loaded in the down counters A42 (units) and A43 (tens). The power r is used to control the output of the six decade counter A51 if r = 0, the output has the same number of pulses as the input in general, the output corresponds to a division by 10 r. 

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