Specifications

MANUAL MODE
Manual Up/Down control option provides a local operator
interface to control the analog output value independent
of the host. As shown in Figure 2, the analog output may
be moved up or down by shorting the UP* or DN* inputs
to the GND terminal. Grounding both pins at once holds
the output at its present value and inhibits any output
commands from the communications ports. The control
inputs may also be logic signals from other equipment.
The manual mode controls the output with a linear slope.
The slope rate on D3000 modules is fixed and scaled so
that a full-scale output change takes 5 seconds.
Figure 2. Manual Up/Down Control.
SLOPE CONTROL (D4000)
Most DACs provide a step function when a new output
value is desired. That is, the analog output change is in-
stantaneous subject only to DAC settling time. In many
applications this characteristic is undesirable and a grad-
ual controlled output slew rate is more appropriate. In ap-
plications where controlled output rates are needed, pre-
cious host computer time must be used to continually
monitor and step the DAC until the desired output is
reached.
The D4000 allows controlled output slopes automatically
without host computer intervention. User-programmable
output slew rates are stored in nonvolatile memory. If a
command is sent to the D4000 to change the output
value, the output will automatically slope to the new
value at the specified rate. The nonvolatile slope value is
restored each time the module is powered up.
The D4000 microprocessor controls the output slew rate
by updating the DAC at a rate of 1000 conversions per
second at precise 1ms intervals. In this manner the DAC
is smoothly stepped until the final output value is
reached. The DAC's incremental output steps and its
conversion rate combine to make the output change ap-
pear to be a linear ramp.
MANUAL SLOPE CONTROL (D4000)
The D4000 allows the user to specify the output slope
when the output is controlled by the manual UP/DOWN
inputs. The manual slope data is stored in EEPROM.
The manual slope is independent of the slopes used with
the computer controlled output. The manual rate may be
changed with the Manual Slope command.
INPUT DATA SCALING (D4000)
All D3000 and D4000 modules are factory set with data
values in millivolts or milliamps. The D4000 allows the
user to scale the input data to any desired units. In many
applications a change in input scaling makes the data
easier to read and interpret. For example, a D4000 used
to control a valve actuator may be easier to use if the
data is scaled with a range of 0-100% rather than 4-
20mA.
The input scaling may be changed by using the Maxi-
mum and Minimum commands to assign input data val-
ues corresponding to the module's maximum and mini-
mum output values.
STARTING VALUE (D4000)
When a D4000 module is powered up from a cold start,
the analog output is automatically forced to a user-pro-
grammable predetermined starting value. This feature is
useful for cold-starting systems in a controlled manner.
Usually the starting value is specified as a “safe” condi-
tion to protect equipment and material from damage.
WATCHDOG TIMER (D4000)
The D4000 contains a user-programmable software timer
to provide an orderly shutdown of the output signal in the
event of host computer or communications failure. The
timer is preset using the Watchdog Timer command to
specify a timer interval in minutes. The timer is continu-
ally incremented in software. Each time the D4000 mod-
ule receives a valid command, the timer is cleared to
zero and restarted. If the timer count reaches the preset
value, the output will automatically be forced to slew to
the starting value using the present output slope rate.
The starting value should be programmed to provide a
"safe" output value to minimize damage and disruption to
the system under control.
ANALOG READBACK (D4000)
The Read Data command in the D3000/4000 series pro-
vides a status report of the output of a module. However,
the data obtained with this command only indicates the
digital data that is being transferred from the onboard mi-
croprocessor to the DAC. It does not indicate whether the
the DAC output is correct. It cannot detect fault condi-
tions such as shorts or open circuits.
The D4000 series contains an ADC that provides true
readback of the analog output signal. The ADC is inde-
pendent of the DAC. The ADC provides true analog read-
back data to the microprocessor. While not intended to
be a highly accurate measurement of the output signal,
the ADC greatly enhances the user’s confidence that the
analog output is being produced as intended. Output fault
conditions from improper wiring or loads can be easily
detected. The ADC also provides a form of redundancy
to ensure that the module is working properly.