Data Translation DT9857, Руководство пользователя, Страница: 90 / 192

Chapter 5
90
Tachometer Input Features
You can connect a tachometer signal with a range of ±30 V to a DT9857 or DT9857E module.
On the DT9857 module, you connect this signal to a BNC connector; on the DT9857E module,
you connect this signal to pin 11 of the Digital connector. This signal has a maximum
frequency of 1 MHz and a minimum pulse width of 0.4

s. The threshold voltage is fixed at
+2 V with 0.5 V of hysteresis.
You can measure the frequency or period of the tachometer input signal to calculate the
rotation speed for high-level (±30 V) tachometer input signals. An internal 12 MHz counter is
used for the measurement, yielding a resolution of 83 ns (1/12 MHz).
You can read the number of counts between two consecutive starting edges of the tachometer
input signal by including channel 8 for the DT9857-8 or DT9857E-8 module, or channel 16 for
the DT9857-16 or DT9857E-16 in the analog input channel list. The starting edge is
programmable (either rising or falling).
Using software commands, you can specify the following parameters for the tachometer
input:
• The starting edge of the tachometer input signal to use for the measurement (rising or
falling edge).
• A flag (called Stale) indicating whether or not the data is new. If the Stale flag is set as
Used (the default value), the most significant bit (MSB) of the value is set to 0 to indicate
new data; reading the value before the measurement is complete returns an MSB of 1. If
the Stale flag is set to Not Used, the MSB is always set to 0.
When the operation is started, the internal 12 MHz counter starts incrementing when it detects
the first starting edge of the tachometer input and stops incrementing when it detects the next
starting edge; at that point, the counter stores and resets the count. The stored count is
maintained until it is read as part of the input data stream or until a new count is stored. The
next tachometer measurement operation is started automatically.
If the sample rate of the input subsystem is faster than the tachometer input frequency, then
the stored count retains the current value when the count is read by the input subsystem. The
operation of the Stale flag in this case is described as follows:
• If another input subsystem sample occurs before another measure completes and the Stale
flag is enabled, then the Stale flag is set and the stale measure count is written into the
input data stream.
• If another input subsystem sample occurs before another measure completes and the Stale
flag is disabled, then the Stale flag is not set and the stale measure count is written into the
input data stream.
If the input sample rate is slower than the tachometer input frequency, then as each period
measurement completes, a new count value is stored. When the input subsystem sample
occurs, the most recently stored measure count is written into the input data stream.