PRELIMINARY TECHNICAL DATA
ADM1026
– 2 1 –
REV. PrL
PRELIMINAR
Y
TECHNICAL
DA
TA
pulses are counted from the second rising edge to the
fourth rising edge. This is repeated for the other six fan
channels.
To accommodate fans of different speed and/or different
numbers of output pulses per revolution, a pre-scaler (di-
visor) of 1, 2, 4 or 8 may be added before the counter.
Divisor values for Fans 0 to 3 are contained in the Fan 0-
3 Divisor Register (address 02h) and those for Fans 4 to 7
in the Fan 4-7 Divisor Register (address 03h). The de-
fault value is 2, which gives a count of 153 for a fan run-
ning at 4400 RPM producing two output pulses per
revolution.
The count is calculated by the equation:
Count = (22.5 x 10
3
x 60) /(RPM x 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 situation will be different.
Table 4 shows the relationship between fan speed and
time per revolution at 60%, 70% and 100% of nominal
RPM for fan speeds of 1100, 2200, 4400 and 8800 RPM,
and the divisor that would be used for each of these fans,
based on two tacho pulses per revolution.
TABLE 4. FAN SPEEDS AND DIVISORS
Divisor Nominal Time per 7 0 % Time per 6 0 % Time per
R P M
rev
R P M rev (70%) R P M rev (60%)
( m s )
( m s )
( m s )
÷
1
8800
6.82
6160
9.74
5280
11.36
÷
2
4400
13.64
3080
19.48
2640
22.73
÷
4
2200
27.27
1540
38.96
1320
45.45
÷
8
1100
54.54
770
77.92
660
90.9
LIMIT VALUES
Fans in general will not overspeed if run from the correct
voltage, 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 reg-
isters 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 measurement exceeds the limit value.
FAN MONITORING CYCLE TIME
The fan speeds are measured in sequence from 0 to 7.
The monitoring cycle time depends on the fan speed, the
number of tacho output pulses per revolution and the
number of fans being monitored.
If a fan is stopped or running so slowly that the fan speed
counter reaches 255 before the second tach pulse after ini-
tialization, or before the fourth tach pulse during mea-
surement, the measurement will be terminated. This will
also occur if an input is configured as GPIO instead of
fan. Any channels so connected will time out after 255
clock pulses.
The worst-case measurement time for a fan-configured
channel occurs when the counter reaches 254 from start to
the 2nd tach pulse and reaches 255 after the second tach
pulse. Taking into account the tolerance of the oscillator
frequency, the worst-case measurement time is:
509
⫻
D
⫻
0.047 milliseconds
where:
509 is the total number of clock pulses.
D is the divisor, 1,2, 4 or 8.
0.047 is the worst-case oscillator period in ms.
The worst-case fan monitoring cycle time is the sum of
the worst case measurement time for each fan.
Although the fan monitoring cycle and the analog input
monitoring cycle are started together, they are not
synchronised 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
Model
Frame Size
Airflow
CFM
2408NL
2.36 in sq. X 0.79 in (60mm sq. X 20mm)
9-16
2410ML
2.36 in sq. X 0.98 in (60mm sq. X 25mm)
14-25
3108NL
3.15 in sq. X 0.79 in (80mm sq. X 20mm)
25-42
3110KL
3.15 in sq. X 0.98 in (80mm sq. X 25mm)
25-40
Mechatronis Inc.
P.O. Box 20
Mercer Island, WA 98040
800-453-4569
Models - Various sizes available with tach output option.
Sanyo Denki/Keymarc Electronics
2310 205th, Suite 101
Torrance, CA 90501
