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PWM Output Modules
Pulse Width Modulation DefinitionTo control the electrical current driven through a load, such as manipulating the speed of a DC motor for instance, the ideal way would be to use a variable voltage. But this requires some form of semiconductor operating in its linear region and this mode of operation can not deliver very much current before the heat that is generated becomes a major limiting factor. However, if the semiconductor is only being used to switch fully on and fully off in a saturated mode, much more current can be delivered with very little heat. So to control the current through a heavy load, pulse width modulation is used which is a series of "on" and "off" pulses switching at a fixed frequency and the current is controlled by the percentage of "on" times verses "off" times which is called duty cycle. The DAQ modules listed above incorporate a PWM mode of operation that can produce these pulses and vary the duty cycle based on commands received from the host computer. And because of the efficiency of pulse width modulation, up to
Stackable CapabilityStackable refers to the ability to connect multiple units together and share a single communications port of a host PC, laptop, or Single Board Computer (SBC). The DAQ modules are offered in two different footprints as shown above, enclosed in a case with full electrical isolation, or a
Host CommunicationsTo communicate with the DAQ modules, commands are sent to the individual units by including a header character at the beginning (the address) so that it can be routed to the appropriate unit. If using multiple units connected together, the DIP switch for each unit should be set to a different position so that it will be assigned a different header character. Each DAQ module has its own command set used to configure it, operate the functions, and to read data from it. These commands are listed in the product data sheets. A typical command string looks like this:
If using our ModCom HMI software, the carriage return is not necessary because ModCom automatically inserts this at the end of each command string. ModCom has a communications dialog box which can be used to transmit individual commands to the DAQ modules and see the data coming back. This is helpful in learning the style of the
Output CurrentThe output channels of the WTX4, WTDOT-M, and WTDIN-M series uses an open-collector configuration which can sink up to 1.0 amps per channel allowing them to directly drive relays, solenoids, DC motors, magnetic latches, flow valves, etc. To attach the outputs to a device such as a relay or solenoid, connect one side of the coil to the positive side of an appropriate power source, the other side of the coil to one of the output channels of the Digital Output module, and the COM or GND terminal of the Digital Output module to the negative side (or ground) of the power source. Then use the LOW command to pull the output channel to ground which will complete the circuit and current will flow through the coil activating the relay or solenoid. The HIGH command can be used to release the output channel from ground and deactivate the relay or solenoid.
Output Timer FunctionEach of the output channels of the Digital Output modules incorporate its own independent
This same procedure can be used to turn on an external alarm or warning light if there are any communications problems by having the host continuously transmitting a deactivation command instead of the activation command mentioned above. When using this method, the DEFAULT command should be used to make sure the output channel is set to the deactivation state upon power up.
PWM Output FunctionThe WTX4, WTDOT-M, and WTDIN-M series incorporate a special function that will produce a PWM (Pulse Width Modulation) output using the PWM command. In this mode, the output is a continuous square wave with a variable duty cycle which is controlled by the host. Duty Cycle represents the percentage of high time to low time of each pulse and can be used to control the current flow through a load such as a DC motor or flow control valve, for instance. The duty cycle can be adjusted in 0.1% increments from 0 to 1000.
Output Overload ShutdownEach of the output channels of the WTX4, WTDOT-M, and WTDIN-M series incorporate a resettable fuse which protects it from excessive current flow. If the current being sunk by an output channel exceeds 1.0 amps for an extended period of time, the output will automatically shut down to prevent damage to the output drivers. Once this happens, the current must be removed from the output channel before it will return to normal operation.