Standard serial, typically referred to as RS-232 or UART (Universal Asynchronous Receiver Transmitter) is a moderate-speed, reliable, old-school, digital protocol used to communicate with a single device, using two wires:
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Depending on the hardware involved, serial can be used in extremely noisy industrial environments, over long distances; up to 60 meters (200 feet).
Hardware
Standard serial uses two lines for communication; a transmit (TX) line for sending messages to the peripheral, and a receive (RX) for listening to messages sent from the peripheral.
Meadow TX => Peripheral RX, Meadow RX => Peripheral TX
It’s important to note that the TX pin on the Meadow board should connect to the RX pin on the peripheral, and the RX pin on the Meadow board should connect to the TX pin on the peripheral.
Ground and Power
In addition to TX and RX, serial devices will also have ground and power pins/leads. Ground will need to be connected to GND on the Meadow board. The power lead will need to be connected either to the Meadow board’s 3.3V rail or 5V rail, depending on what the device needs are.
If the device is powered by an external power supply, you must make sure the external power’s ground is connected to the Meadow GND so that their voltages are the same.
UART/TTL vs RS-232
The RS-232 specification is a complete specification that includes hardware designs, electrical characteristics, and protocol specifications.
However, within that specification are generally two different flavors that describe the electrical characteristics of the protocol; UART/TTL & RS-232.
Serial has been around for a long time; before USB, it used to be a standard way for computers to talk to various peripherals such as keyboards and mice and connected to the serial port on a computer via an RS-232 cable with a connector like this:
And many industrial peripherals that use standard serial communications still use RS-232 connectors.
Voltage Difference
RS-232 specifies that the voltages that signify 0 can be anywhere from 3V to 15V, and voltages that signify a 1 can range from -3V to -15V.
However, most modern microcontrollers have UART chips that operate at a TTL (Transistor-Transistor Logic) level of voltages that typically use OV to signify 0, and either 3.3V or 5V to signify 1.
Attempting to interface an RS-232 voltage level with a TTL level microcontroller will usually lead to the quick destruction of that port on the microcontroller, or even the entire microcontroller.
TTL/RS-232 Converters
Fortunately, many modern serial peripherals operate on TTL voltage levels. However, most industrial serial peripherals still use RS-232, which allows them to operate in noisy environments.
When using an RS-232 peripheral, the signal voltages must be level-shifted and inverted (high voltage in RS-232 signifies 0, whereas low-voltage at TTL signifies 0) in order to communicate. Fortunately there are low cost ICs (Integrated Circuits) that do this, such as the MAX232 chip.
Additionally, Sparkfun has an RS-232 to TTL Shifter breakout board that not only converts RS-232 to TTL levels, but also includes an onboard RS-232 connector:
Meadow Serial Ports
The Meadow F7 Micro has two exposed serial ports, named COM4 and COM1 with the following pinout:
- COM4 -
D00=RX,D01=TX - COM1 -
D13=RX,D12=TX
Using the Meadow Serial API
Creating and Opening a Serial Port
To use serial in Meadow, first create an ISerialPort from the IIODevice you’re using, using the port name, and the desired speed (baud rate), and then call Open() to establish a connection with a peripheral:
var serialPort = Device.CreateSerialPort(Device.SerialPortNames.Com4, 115200);
serialPort.Open();
Optionally, you can also specify the number of data bits in a message frame, parity, and number of stop bits. The datasheet on your serial peripheral should specify what those values should be. The most common is 8n1, which means 8 data bits, no parity check, or Parity.None, and one stop bit, or StopBits.One. These are the defaults for the SerialPort constructor, but you can specify something different as well:
var serialPort = Device.CreateSerialPort(Device.SerialPortNames.Com4, 9600, 7, Parity.Even, StopBits.Two);
serialPort.Open();
Reading and Writing
Once the SerialPort is opened, communication is done via reads and writes which pass byte[] messages.
Writing to the Peripheral
To write, call the Write(byte[] buffer) method and pass in the bytes to send to the peripheral:
var buffer = new byte[] { 0x00, 0x0F, 0x01 };
serialPort.Write(buffer);
Reading from the Receive Buffer
When data from the peripheral is received, it’s placed in an internal circular recieve buffer. The simplest way to read the data from that buffer is to call the Read(byte[] buffer, int offset, int count) method, passing in a buffer to read the bytes into, as well as the start index and the number of bytes to read.
For example, the following code will read 7 bytes from the buffer:
byte[] response = new byte[7];
this.serialPort.Read(response, 0, 7);
Read will also remove (dequeue) those bytes from the buffer. If you want to read from the buffer without removing the data, you can use the Peek() method.
ReadToToken()
If the received data is separated by a known token, such as a new line indicator (/n), you can use the ReadToToken(byte token) method to automatically read the bytes until the token is found.
Receive Buffer
By default, the SerialPort is created with a 4,096 byte receive buffer, but that size can be set during port construction.
Event Notifications
The SerialPort also has a DataReceived event that is raised when data is present.
However, that functionality is currently broken, so for now, you must manually poll the buffer to see if there is data.
Additional APIs
There are a number of other APIs available on serial ports, please see the ISerialPort API documentation for more details.