12 2 Channel USB Relay Module – User Guide
2. Set port parameters. Most of the parameters can be left to defaults except Flow Control, which
needs to be set to “none”
3. To send a command to the module, use an API equivalent to write/writefile and pass the
buffer/string containing the command. It is important to append Carriage Return (ASCII 13) to emulate
the ENTER key.
4. If return data is expected (Eg: “ver” command), try to read a fixed number of characters (Eg: 50mS)
with timeouts so that the read call will return when 50 characters are received or the timeout occur.
Depending on the operating system and language the method implementing timeouts can be very
different. APIs equivalent to Read/ReadFile can be used to read data from the module. Please note
that the return data will include the command itself (since the module echoes everything back), the
result, carriage return and the “>” symbol.
Please visit the product page for available sample programs.
Additional Information
Analog to Digital Converter
Some products do support Analog to Digital Conversion on some of the IO terminals. A list of GPIOs
that supports analog function in this product is listed elsewhere in this document. There is no special
command is required to execute to switch between analog and digital mode. Executing “adc”
command will set the GPIO to analog mode and executing “gpio” command will set the GPIO to digital
mode on the fly. Resolution of the ADC is 10 bits unless otherwise noted. The input voltage range of
the ADC is 0 – VDD (this product uses 5V power supply, so the range will be 0 – 5V). The result will be
returned as a number starting at zero and ending at 1023. Zero indicates zero volts at the ADC input
and 1023 indicates VDD (5V for this product) at ADC input.
Using GPIOs with switches
It is possible to read the position of a switch that is connected to a GPIO. A SPST or SPDT switch is
recommended to use with GPIOs. Push switches do maintain the contacts closed for a very short time
so using them is discouraged. The fundamental idea of using a switch with GPIO is to have the switch
cause a voltage level change at the GPIO pin when pressed. Usually this is achieved by using an
external pull-up resistor along with the switch. The pull up resistor is connected between the GPIO
and VDD and the switch is connected between the GPIO and ground. When the switch is not pressed,
the pull-up resistor will cause the GPIO to stay at VDD voltage level. When the switch is pressed, the
GPIO is short circuited to ground and stays at zero voltage. This change in voltage and thus the
position of the switch can be read using “gpio read” command. Please see the recommended
connection diagram below.
