ADM9240 datasheet(13/22 Pages) AD

13 / 22 page

ADM9240

–13–

REV. 0

To accommodate fans of different speed and/or different num-

bers of output pulses per revolution, a prescaler (divisor) of 1, 2,

4 or 8 may be added before the counter. The default value is 2,

which gives a count of 153 for a fan running at 4400 rpm pro-

ducing two output pulses per revolution.

The count is calculated by the equation:

Count = (22.5

× 103 × 60) /(rpm × Divisor)

For constant speed fans, fan failure is normally considered to

have occurred when the speed drops below 70% of nominal,

which would correspond to a count of 219. Full scale (255)

would be reached if the fan speed fell to 60% of its nominal

value. For temperature controlled variable speed fans the situa-

tion will be different.

Table III shows the relationship between fan speed and time per

revolution at 60%, 70% and 100% of nominal rpm for fan

speeds of 1100 rpm, 2200 rpm, 4400 rpm and 8800 rpm, and

the divisor that would be used for each of these fans, based on

two tacho pulses per revolution.

Table III. Fan Speeds and Divisors

Time per

Nominal

70% Rev

60% Rev

Divisor

rpm

(ms)

rpm

(ms)

rpm

(ms)

8800

6.82

6160

9.74

5280

11.36

4400

13.64

3080

19.48

2640

22.73

2200

27.27

1540

38.96

1320

45.45

1100

54.54

770

77.92

660

90.9

Note that Fan 1 and Fan 2 Divisors are programmed into Bits 4

to 7 of the VID0–VID3/Fan Divisor Register.

LIMIT VALUES

Fans in general will not overspeed if run from the correct volt-

age, so the failure condition of interest is underspeed due to

electrical or mechanical failure. For this reason only low speed

limits are programmed into the limit registers for the fans. It

should be noted that, since fan period rather than speed is being

measured, a fan failure interrupt will occur when the measure-

ment exceeds the limit value.

MONITORING CYCLE TIME

The monitoring cycle time depends on the fan speed and num-

ber of tacho output pulses per revolution. Two complete periods

of the fan tacho output (three rising edges) are required for each

fan measurement. Therefore, if the start of a fan measurement

just misses a rising edge, the measurement can take almost three

tacho periods. In order to read a valid result from the fan value

registers, the total monitoring time allowed after starting the

monitoring cycle should therefore be three tacho periods of

FAN1 plus three tacho periods of FAN2 at the lowest normal

fan speed.

Although the fan monitoring cycle and the analog input moni-

toring cycle are started together, they are not synchronized in

any other way.

FAN MANUFACTURERS

Manufacturers of cooling fans with tachometer outputs are

listed below:

NMB Tech

9730 Independence Ave.

Chatsworth, California 91311

818-341-3355

818-341-8207

Airflow

Model

Frame Size

CFM

2408NL

2.36 in sq.

× 0.79 in (60 mm sq. × 20 mm) 9–16

2410ML 2.36 in sq.

× 0.98 in (60 mm sq. × 25 mm) 14–25

3108NL

3.15 in sq.

× 0.79 in (80 mm sq. × 20 mm) 25–42

3110KL

3.15 in sq.

× 0.98 in (80 mm sq. × 25 mm) 25–40

Mechatronis Inc.

P.O. Box 613

Preston, WA 98050

800-453-4569

Models—Various sizes available with tach output option.

Sanyo Denki/Keymarc Electronics

468 Amapola Ave.

Torrance, CA 90501

310-783-5400

Models—109P Series

CHASSIS INTRUSION INPUT

The Chassis Intrusion (CI) input is an active high input/open-

drain output intended for detection and signalling of unautho-

rized tampering with the system. An external circuit powered

from the system’s CMOS backup battery is used to detect and

latch a chassis intrusion event, whether the system is powered up

or not. Once a chassis intrusion has been detected and latched,

the CI input will generate an interrupt when the system is pow-

ered up.

The actual detection of chassis intrusion is performed by an

external circuit that will, for example, detect when the cover has

been removed. A wide variety of techniques may be used for the

detection:

– Microswitch that opens or closes when the cover is removed.

– Reed switch operated by magnet fixed to the cover.

– Hall-effect switch operated by magnet fixed to the cover.

– Phototransistor that detects light when cover is removed.

The chassis intrusion interrupt will remain asserted until the

external detection circuit is reset. This can be achieved by set-

ting Bit 6 of the Configuration Register, or Bit 7 of the Chassis

Intrusion Clear Register to one, which will cause the CI pin to be

pulled low for at least 20 ms. These register bits are self-clearing.

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