Rainbow-electronics DS1780 Manual de usuario descargar pdf (Pagina 11)

DS1780

11 of 28

maximum possible input up to V

for best noise immunity. Alternatively, use a shunt reference or Zener

diode to clamp the input level.

Tables 7 and 8 describe the format of the data stored in the FAN reading registers (Internal Address

Registers 28h and 29h).

FAN TACHOMETER INPUT OPTIONS Figure 3

VOLTAGE/DATA RELATIONSHIPS FOR FAN INPUTS (FAN1, FAN2) UNDER

DEFAULT (÷2) MODE Table 7

RPM Timer Per Revolution Counts for “Divide by 2” (default mode) in decimal Comments

4400 13.64 ms 153 Typical RPM

3080 19.48 ms 219 70% RPM

2640 22.73 ms 255 (maximum) 60% RPM

VOLTAGE/DATA RELATIONSHIPS FOR FAN INPUTS (FAN1, FAN 2) Table 8

MODE

SELECT

NOMINAL

RPM

TIME PER

REVOLUTION

COUNTS FOR THE

NOMINAL

SPEED IN DECIMAL

70% RPM

TIME PER

REVOLUTION FOR

70% RPM

Divide by 1 8800 6.82 ms 153 6160 9.74 ms

Divide by 2 4400 13.64 ms 153 3080 19.48 ms

Divide by 4 2200 27.27 ms 153 1540 38.96 ms

Divide by 8 1100 54.54 ms 153 770 77.92 ms

In general, the value stored in the FAN registers (in decimal) follows the equation:

Count (RPM, Divisor) ≅

Divisor RPM

10 x 1.35

If fans can be powered while the power to the DS1780 is off, the DS1780 inputs will provide diode

clamping. Limit input current to the Input Current at Any Pin specification shown in the ABSOLUTE

MAXIMUM RATINGS section. In most cases, open collector outputs with pullup resistors inherently

limit this current. If this maximum current could be exceeded, either a larger pullup resistor should be

used or resistors connected in series with the fan inputs.

The Fan Inputs gate an internal 22.5 kHz oscillator for one period of the Fan signal into an 8-bit counter

(maximum count = 255). The default divisor, located in the Fan Divisor/RST_Register, is set to 2

(choices are 1, 2, 4, and 8) providing a nominal count of 153 for a 4400 rpm fan with two pulses per

revolution. Typical practice is to consider 70% of normal RPM a fan failure, at which point the count will

be 219.

1 2 ... 6 7 8 9 10 11 12 13 14 15 16 ... 27 28

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