Datasheet

6-Channel Intelligent Fan Controller
MAX31785
20
Interpreting Received
DIRECT Format Values
The host system uses the following equation to convert
the value received from the PMBus device—in this case,
the MAX31785—into a reading of volts, degrees Celsius,
or other units as appropriate:
X = (1/m) x (Y x 10
-
R
- b)
where X is the calculated, real world value in the appro-
priate units (V, NC, etc.); m is the slope coefficient; Y is
the 2-byte, two’s complement integer received from the
PMBus device; b is the offset; and R is the exponent.
Sending a DIRECT Format Value
To send a value, the host must use the below equation
to solve for Y:
Y = (mX + b) x 10
R
where Y is the 2-byte, two’s complement integer to be
sent to the unit; m is the slope coefficient; X is the real
world value, in units such as volts, to be converted for
transmission; b is the offset; and R is the exponent.
The following example demonstrates how the host can
send and retrieve values from the device. Table 4 shows
the coefficients used in the following parameters.
If the host received a value of 0D89h on a READ_VOUT
command, this is equivalent to:
X = (1/1) x (0D89h x 10
-(-0)
- 0) = 3465mV = 3.465V
All voltage-related parameters of PMBus devices are
reported as positive values. It is up to the system to know
that a particular output is negative if that is of interest to
the system. All output voltage-related commands use 2
data bytes.
Table 3. PMBus Command Code Coefficients
Table 4. Coefficients for DIRECT Format
Value
PARAMETER COMMANDS UNITS RESOLUTION MAX m b R
Voltage
VOUT_OV_FAULT_LIMIT
VOUT_OV_WARN_LIMIT
VOUT_UV_WARN_LIMIT
VOUT_UV_FAULT_LIMIT
READ_VOUT
MFR_VOUT_PEAK
MFR_VOUT_MIN
mV 1 32,767 1 0 0
Voltage Scaling VOUT_SCALE_MONITOR — 1/32,767 1 32,767 0 0
Temperature
OT_FAULT_LIMIT
OT_WARN_LIMIT
READ_TEMPERATURE_1
MFR_TEMPERATURE_PEAK
NC
0.01 327.67 1 0 2
Fan Speed
READ_FAN_SPEED_1
FAN_COMMAND_1
MFR_FAN_FAULT_LIMIT
MFR_FAN_WARN_LIMIT
RPM 1 32,767 1 0 0
FAN_COMMAND_1
MFR_READ_FAN_PWM
MFR_FAN_FAULT_LIMIT
MFR_FAN_WARN_LIMIT
MFR_FAN_PWM_AVG
% 0.01 327.67 1 0 2
COMMAND
CODE
COMMAND NAME m b R
8Bh READ_VOUT 1 0 0