Data Translation DT9837, User Manual

Page: 82 / 150

Chapter 5
82
Tachometer Input Features
You can connect a tachometer signal with a range of ±30 V to the DT9837, DT9837A, and
DT9837B modules. (The DT9837C does not support a tachometer input.) On the DT9837, this
signal has a maximum frequency of 380 kHz and a minimum pulse width of 1.3
μ
s. On the
DT9837A and DT9837B modules, this signal has a maximum frequency of 1 MHz and a
minimum pulse width of 0.4
μ
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 using tachometer
counter 0. On the DT9837A and DT9837B modules, you can also measure the phase of the
tachometer input signal in relation to the A/D sample using tachometer counter 1. These
measurements are described in more detail in the following subsections.
Frequency or Period Measurements – Tachometer Counter 0
Use frequency or period measurements to calculate the rotation speed for high-level (±30 V)
tachometer input signals. An internal 12 MHz counter (tachometer counter 0) 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 4 in the analog input channel list. On the DT9837 module,
the starting edge is always rising; on the DT9837A and DT9837B modules, the starting edge is
programmable (either rising or falling).
You can specify the following parameters for tachometer counter 0 using the Open Layers
Control Panel applet, described on
page 36 , by using software commands, or by writing to the
registers of the module, as described in Appendix C starting on page 135 :
• The starting edge of the tachometer input signal to use for the measurement: rising edge
or falling edge. On the DT9837 module, the starting edge is always the rising edge.
• The value read between measurements: either zero or the previous measurement value.
On the DT9837 module, this value is always the previous measurement value.
• A flag (called Stale) indicating whether or not the data is new. If the Stale flag is set as
Used, 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.
On the DT9837 module, the MSB is always 0 (Not Used).
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 of the tachometer input. When the measurement is complete, the counter/timer
remains idle until it is read. On the next read, either 0 or the current value of the tachometer
input (from the previous measurement operation) is returned depending on the module and
the Control Panel settings, described above, and the next operation is started automatically.