GE VMIVME-7648-740 Gas turbine module

Use the period/rate mode to determine the frequency of input pulses by counting the number of internal 4MHz clock pulses over a user-specified number of input signal pulses. At the end of the specified number of pulses, the module returns the frequency and the number of internal 4MHz pulses. A channel configured for period/rate mode acts as a period rate counter. An internal 4 MHz clock is used as a frequency reference. This clock is gated by the incoming pulse train at the gate/reset input. The results of this gating action are the number of pulses or a frequency. The number of sampled gated 4MHz pulses are returned in BTR words 3 thru 10, and the frequency in words 11 thru 18. Select the period/rate mode by setting the appropriate bits in words 3 and 4 of the BTW initialization file (chapter 4). The store count features are inactive in period/rate mode.

1771-VHSC revision B and later modules

 count the total number of pulses occurring at the gate/reset pin. This function is frequency-limited. This total count is returned when you request words 19 through 26 in your BTR. You can reset this count by resetting the reset bit (bits 0-4 in BTW word 1). Rollover and preset are inactive. Refer to appendix E for additional information. 1.8 shows a diagram of the module used in the period/rate mode.

In 1.8, the incoming pulse train

 from the gate/reset terminal is used to sample pulses from the 4 MHz internal clock. As the frequency of the incoming pulse train at the gate/reset terminal increases, the number of sampled pulses from the 4 Mhz clock decreases. This relationship is shown in Table 1.A. Since accuracy is related to the number of pulses received over the sample period, the accuracy will decrease with increasing input frequencies at the Gate/Reset terminal. To some extent, the decrease in accuracy can be lessened by scaling the input frequency through the use of a scaler. A scaler value of 1 will only return an accurate input frequency if incoming pulses have a 50% duty cycle. If frequency exceeds 500KHz, the number 999,999 is returned.

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