Campbell Scientific CR10 CR10 Measurement and Control - Page 126

SECTION 9. INPUT/OUTPUT INSTRUCTIONS tN4 4.7 FIGURE 9-1. Conditioning for Long Duration Voltage Pulses When this happens, the excess voltage is shunted to the CR10 5 VDC supply, with the current limited by an internal 10 Kohm resistor. When this extra current source exceeds the quiescent current needs of the CR10 (about 0.6 mA), the 5 VDC supply willstad to rise, upsetting all analog measurements. Thus, pulses whose positive voltage portion exceeds 5.6 VDC with a duration longer than 100 milliseconds need external conditioning. One method would be to use a 4 to 5.6 V zener diode from the signalto ground. The simplest method, however, is to add an external20 Kohm resistor in series with the signal (Figure 9-1). This will limit the current for pulses to 20 VDC to the point that it will not upset the CR10 5 VDC supply. LOW LEVEL AC This configuration is used to count the frequency of AC signals from magnetic pulse flow transducers or other low voltage, sine wave inputs. The minimum input voltage is 6 millivolts RMS. Input hysteresis is 11 millivolts. The maximum AC input voltage is 20 volts RMS. The maximum input lrequency ranges from 100 Hz at 20 mV RMS to 1000 Hz at 150 mV or greater. Consult the factory if higher f requencies are desired. SWITCH CLOSURE ln this configuration, the minimum switch closed time is 5 milliseconds. The minimum switch open time is 6 milliseconds. The maximum bounce time is 1 millisecond open without being counted. The 2 pulse count input channels each have eight bit counters. lnput frequencies greater than 2000 Hz (the limit of the eight bit counter, 255 counts at the reset interval of 0.125 second) can be counted by combining two counters on one input channel. When this option is selected, channel 1 is used for the pulse input. Channel2 is not used. Every 0.125 seconds, the CR10 processor transfers the values from the 8 bit pulse counters into 16 bit accumulators (max count is 65,535) and the 8 bit counters are hardware reset,to zero. The pulses accumulate in these 16 bit accumulators untilthe program table containing the Pulse Count Instruction is executed. At the beginning of the execution of the Table containing the Pulse Count Instruction, the total in the 16 bit accumulator is transferred to a temporary RAM buffer. The 16 bit accumulator is then zeroed. When the table execution reaches the Pulse Count Instruction, the value in the RAM buffer is multiplied by the multiplier and added to the otfset and placed into the designated input location. CAUTION: The RAM butfer does NOT accumulate counts; it is zeroed each time the table is executed regardless of whether or not the pulse instruction is executed. lf allcounts are necessary, it is imperative that the Pulse Count Instruction be executed (not branched around) every time the table is executed. lf a table execution was skipped because the processor was executing the previous table (Section 2.1) or if the user resets the time, the value in the 16 bit accumulator is the result of a longer than normal interual. This value can either be used or it can be discarded. lf pulse counts are being totalized, a missing count could be significant and the value from the erroneously long interval should NOT be discarded. lf the pulse count is being processed in a way in which the resultant value is dependent upon the sampling interval (e.9., speed, RPM), the value from the excessive interval should be discarded. lf the value is discarded the value in the RAM buffer from the previous measurement will be used. 9-2