Python Serial Read Timeout Example

Read (length, timeout=None) source ¶. Read up to length number of bytes from the serial port with an optional timeout. Timeout can be positive for a blocking read with a timeout in seconds, zero for a non-blocking read, or negative or None for a blocking read that will block until length number of bytes are read. This ensures that a sensible timeout is set even if the developer forgets to add the timeout=1 parameter to his individual call, but allows for overrides on a per-call basis. Below is an example of a custom Transport Adapter with default timeouts, inspired by this Github comment. We override the constructor to provide a default timeout when.

UART implements the standard UART/USART duplex serial communications protocol. Atthe physical level it consists of 2 lines: RX and TX. The unit of communicationis a character (not to be confused with a string character) which can be 8 or 9bits wide.

UART objects can be created and initialised using:

Bits can be 7, 8 or 9. Parity can be None, 0 (even) or 1 (odd). Stop can be 1 or 2.

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  • Read (length, timeout=None) source ¶. Read up to length number of bytes from the serial port with an optional timeout. Timeout can be positive for a timeout in seconds, 0 for a non-blocking read, or negative or None for a blocking read that will block until length number of bytes are read. Default is a blocking read. For a non-blocking or timeout-bound read, read may return data whose.

Note: with parity=None, only 8 and 9 bits are supported. With parity enabled,only 7 and 8 bits are supported.

A UART object acts like a stream object and reading and writing is doneusing the standard stream methods:

Individual characters can be read/written using:

To check if there is anything to be read, use:

Note: The stream functions read, write, etc. are new in MicroPython v1.3.4.Earlier versions use uart.send and uart.recv.


class pyb.UART(bus, ...)

Construct a UART object on the given bus. bus can be 1-6, or ‘XA’, ‘XB’, ‘YA’, or ‘YB’.With no additional parameters, the UART object is created but notinitialised (it has the settings from the last initialisation ofthe bus, if any). If extra arguments are given, the bus is initialised.See init for parameters of initialisation.


The physical pins of the UART busses are:

  • UART(4) is on XA: (TX,RX)=(X1,X2)=(PA0,PA1)
  • UART(1) is on XB: (TX,RX)=(X9,X10)=(PB6,PB7)
  • UART(6) is on YA: (TX,RX)=(Y1,Y2)=(PC6,PC7)
  • UART(3) is on YB: (TX,RX)=(Y9,Y10)=(PB10,PB11)
  • UART(2) is on: (TX,RX)=(X3,X4)=(PA2,PA3)

The Pyboard Lite supports UART(1), UART(2) and UART(6) only. Pins are as above except:


UART.init(baudrate, bits=8, parity=None, stop=1, *, timeout=1000, flow=0, timeout_char=0, read_buf_len=64)

Initialise the UART bus with the given parameters:

  • baudrate is the clock rate.
  • bits is the number of bits per character, 7, 8 or 9.
  • parity is the parity, None, 0 (even) or 1 (odd).
  • stop is the number of stop bits, 1 or 2.
  • flow sets the flow control type. Can be 0, UART.RTS, UART.CTSor UART.RTS|UART.CTS.
  • timeout is the timeout in milliseconds to wait for writing/reading the first character.
  • timeout_char is the timeout in milliseconds to wait between characters while writing or reading.
  • read_buf_len is the character length of the read buffer (0 to disable).

This method will raise an exception if the baudrate could not be set within5% of the desired value. The minimum baudrate is dictated by the frequencyof the bus that the UART is on; UART(1) and UART(6) are APB2, the rest are onAPB1. The default bus frequencies give a minimum baudrate of 1300 forUART(1) and UART(6) and 650 for the others. Use pyb.freqto reduce the bus frequencies to get lower baudrates.

Note: with parity=None, only 8 and 9 bits are supported. With parity enabled,only 7 and 8 bits are supported.


Turn off the UART bus.


Returns the number of bytes waiting (may be 0).[nbytes])

Read characters. If nbytes is specified then read at most that many bytes.If nbytes are available in the buffer, returns immediately, otherwise returnswhen sufficient characters arrive or the timeout elapses.

If nbytes is not given then the method reads as much data as possible. Itreturns after the timeout has elapsed.

Note: for 9 bit characters each character takes two bytes, nbytes mustbe even, and the number of characters is nbytes/2.

Return value: a bytes object containing the bytes read in. Returns Noneon timeout.


Receive a single character on the bus.

Return value: The character read, as an integer. Returns -1 on timeout.

UART.readinto(buf[, nbytes])

Read bytes into the buf. If nbytes is specified then read at mostthat many bytes. Otherwise, read at most len(buf) bytes.

Return value: number of bytes read and stored into buf or None ontimeout.


Read a line, ending in a newline character. If such a line exists, return isimmediate. If the timeout elapses, all available data is returned regardlessof whether a newline exists.

Return value: the line read or None on timeout if no data is available.


Write the buffer of bytes to the bus. If characters are 7 or 8 bits widethen each byte is one character. If characters are 9 bits wide then twobytes are used for each character (little endian), and buf must containan even number of bytes.

Return value: number of bytes written. If a timeout occurs and no byteswere written returns None.


Write a single character on the bus. char is an integer to write.Return value: None. See note below if CTS flow control is used.


Send a break condition on the bus. This drives the bus low for a durationof 13 bits.Return value: None.



to select the flow control type.

Flow Control¶

On Pyboards V1 and V1.1 UART(2) and UART(3) support RTS/CTS hardware flow controlusing the following pins:

  • UART(2) is on: (TX,RX,nRTS,nCTS)=(X3,X4,X2,X1)=(PA2,PA3,PA1,PA0)
  • UART(3) is on :(TX,RX,nRTS,nCTS)=(Y9,Y10,Y7,Y6)=(PB10,PB11,PB14,PB13)

On the Pyboard Lite only UART(2) supports flow control on these pins:

In the following paragraphs the term “target” refers to the device connected tothe UART.

When the UART’s init() method is called with flow set to one or both ofUART.RTS and UART.CTS the relevant flow control pins are configured.nRTS is an active low output, nCTS is an active low input with pullupenabled. To achieve flow control the Pyboard’s nCTS signal should be connectedto the target’s nRTS and the Pyboard’s nRTS to the target’s nCTS.

CTS: target controls Pyboard transmitter¶

If CTS flow control is enabled the write behaviour is as follows:

If the Pyboard’s UART.write(buf) method is called, transmission will stall forany periods when nCTS is False. This will result in a timeout if the entirebuffer was not transmitted in the timeout period. The method returns the number ofbytes written, enabling the user to write the remainder of the data if required. Inthe event of a timeout, a character will remain in the UART pending nCTS. Thenumber of bytes composing this character will be included in the return value.

If UART.writechar() is called when nCTS is False the method will timeout unless the target asserts nCTS in time. If it times out OSError116will be raised. The character will be transmitted as soon as the target asserts nCTS.

Python Serial Read Time Out Examples

RTS: Pyboard controls target’s transmitter¶

If RTS flow control is enabled, behaviour is as follows:

If buffered input is used (read_buf_len > 0), incoming characters are buffered.If the buffer becomes full, the next character to arrive will cause nRTS to goFalse: the target should cease transmission. nRTS will go True whencharacters are read from the buffer.

Python serial read time out example free

Python Serial Readline Timeout

Note that the any() method returns the number of bytes in the buffer. Assume abuffer length of N bytes. If the buffer becomes full, and another character arrives,nRTS will be set False, and any() will return the count N. Whencharacters are read the additional character will be placed in the buffer and willbe included in the result of a subsequent any() call.

Pyserial Read Timeout

If buffered input is not used (read_buf_len 0) the arrival of a character willcause nRTS to go False until the character is read.

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