Framer

pymodbus.framer.ascii_framer module

Ascii_framer.

class pymodbus.framer.ascii_framer.ModbusAsciiFramer(decoder, client=None)

Bases: ModbusFramer

Modbus ASCII Frame Controller.

[ Start ][Address ][ Function ][ Data ][ LRC ][ End ]
1c 2c 2c Nc 2c 2c

data can be 0 - 2x252 chars

end is “\r\n” (Carriage return line feed), however the line feed character can be changed via a special command

start is “:”

This framer is used for serial transmission. Unlike the RTU protocol, the data in this framer is transferred in plain text ascii.

advanceFrame()

Skip over the current framed message.

This allows us to skip over the current message after we have processed it or determined that it contains an error. It also has to reset the current frame header handle

buildPacket(message)

Create a ready to send modbus packet.

Built off of a modbus request/response

Parameters:: message – The request/response to send
Returns:: The encoded packet

checkFrame()

Check and decode the next frame.

Returns:: True if we successful, False otherwise

decode_data(data): Decode data.

frameProcessIncomingPacket(single, callback, slave, _tid=None, **kwargs): Process new packet pattern.

getFrame()

Get the next frame from the buffer.

Returns:: The frame data or “”

isFrameReady()

Check if we should continue decode logic.

This is meant to be used in a while loop in the decoding phase to let the decoder know that there is still data in the buffer.

Returns:: True if ready, False otherwise

method = 'ascii'

pymodbus.framer.binary_framer module

Binary framer.

class pymodbus.framer.binary_framer.ModbusBinaryFramer(decoder, client=None)

Bases: ModbusFramer

Modbus Binary Frame Controller.

[ Start ][Address ][ Function ][ Data ][ CRC ][ End ]
1b 1b 1b Nb 2b 1b

data can be 0 - 2x252 chars

end is “}”

start is “{”

The idea here is that we implement the RTU protocol, however, instead of using timing for message delimiting, we use start and end of message characters (in this case { and }). Basically, this is a binary framer.

The only case we have to watch out for is when a message contains the { or } characters. If we encounter these characters, we simply duplicate them. Hopefully we will not encounter those characters that often and will save a little bit of bandwitch without a real-time system.

Protocol defined by jamod.sourceforge.net.

advanceFrame() → None

Skip over the current framed message.

This allows us to skip over the current message after we have processed it or determined that it contains an error. It also has to reset the current frame header handle

buildPacket(message)

Create a ready to send modbus packet.

Parameters:: message – The request/response to send
Returns:: The encoded packet

checkFrame() → bool

Check and decode the next frame.

Returns:: True if we are successful, False otherwise

decode_data(data): Decode data.

frameProcessIncomingPacket(single, callback, slave, _tid=None, **kwargs): Process new packet pattern.

getFrame()

Get the next frame from the buffer.

Returns:: The frame data or “”

isFrameReady() → bool

Check if we should continue decode logic.

This is meant to be used in a while loop in the decoding phase to let the decoder know that there is still data in the buffer.

Returns:: True if ready, False otherwise

method = 'binary'

pymodbus.framer.rtu_framer module

RTU framer.

class pymodbus.framer.rtu_framer.ModbusRtuFramer(decoder, client=None)

Bases: ModbusFramer

Modbus RTU Frame controller.

[ Start Wait ] [Address ][ Function Code] [ Data ][ CRC ][ End Wait ]
3.5 chars 1b 1b Nb 2b 3.5 chars

Wait refers to the amount of time required to transmit at least x many characters. In this case it is 3.5 characters. Also, if we receive a wait of 1.5 characters at any point, we must trigger an error message. Also, it appears as though this message is little endian. The logic is simplified as the following:

block-on-read:
    read until 3.5 delay
    check for errors
    decode

The following table is a listing of the baud wait times for the specified baud rates:

------------------------------------------------------------------
 Baud  1.5c (18 bits)   3.5c (38 bits)
------------------------------------------------------------------
 1200   13333.3 us       31666.7 us
 4800    3333.3 us        7916.7 us
 9600    1666.7 us        3958.3 us
19200     833.3 us        1979.2 us
38400     416.7 us         989.6 us
------------------------------------------------------------------
1 Byte = start + 8 bits + parity + stop = 11 bits
(1/Baud)(bits) = delay seconds

advanceFrame()

Skip over the current framed message.

This allows us to skip over the current message after we have processed it or determined that it contains an error. It also has to reset the current frame header handle

buildPacket(message)

Create a ready to send modbus packet.

Parameters:: message – The populated request/response to send

checkFrame()

Check if the next frame is available.

Return True if we were successful.

Populate header
Discard frame if UID does not match

decode_data(data): Decode data.

frameProcessIncomingPacket(single, callback, slave, _tid=None, **kwargs): Process new packet pattern.

getFrame()

Get the next frame from the buffer.

Returns:: The frame data or “”

getFrameStart(slaves, broadcast, skip_cur_frame): Scan buffer for a relevant frame start.

get_expected_response_length(data)

Get the expected response length.

Parameters:: data – Message data read so far
Raises:: IndexError – If not enough data to read byte count
Returns:: Total frame size

isFrameReady()

Check if we should continue decode logic.

This is meant to be used in a while loop in the decoding phase to let the decoder know that there is still data in the buffer.

Returns:: True if ready, False otherwise

method = 'rtu'

populateHeader(data=None)

Try to set the headers uid, len and crc.

This method examines self._buffer and writes meta information into self._header.

Beware that this method will raise an IndexError if self._buffer is not yet long enough.

populateResult(result)

Populate the modbus result header.

The serial packets do not have any header information that is copied.

Parameters:: result – The response packet

recvPacket(size)

Receive packet from the bus with specified len.

Parameters:: size – Number of bytes to read
Returns:

resetFrame()

Reset the entire message frame.

This allows us to skip over errors that may be in the stream. It is hard to know if we are simply out of sync or if there is an error in the stream as we have no way to check the start or end of the message (python just doesn’t have the resolution to check for millisecond delays).

sendPacket(message)

Send packets on the bus with 3.5char delay between frames.

Parameters:: message – Message to be sent over the bus
Returns:

pymodbus.framer.socket_framer module

Socket framer.

class pymodbus.framer.socket_framer.ModbusSocketFramer(decoder, client=None)

Bases: ModbusFramer

Modbus Socket Frame controller.

Before each modbus TCP message is an MBAP header which is used as a message frame. It allows us to easily separate messages as follows:

[         MBAP Header         ] [ Function Code] [ Data ]         [ tid ][ pid ][ length ][ uid ]
  2b     2b     2b        1b           1b           Nb

while len(message) > 0:
    tid, pid, length`, uid = struct.unpack(">HHHB", message)
    request = message[0:7 + length - 1`]
    message = [7 + length - 1:]

* length = uid + function code + data
* The -1 is to account for the uid byte

advanceFrame()

Skip over the current framed message.

This allows us to skip over the current message after we have processed it or determined that it contains an error. It also has to reset the current frame header handle

buildPacket(message)

Create a ready to send modbus packet.

Parameters:: message – The populated request/response to send

checkFrame()

Check and decode the next frame.

Return true if we were successful.

decode_data(data): Decode data.

frameProcessIncomingPacket(single, callback, slave, tid=None, **kwargs)

Process new packet pattern.

This takes in a new request packet, adds it to the current packet stream, and performs framing on it. That is, checks for complete messages, and once found, will process all that exist. This handles the case when we read N + 1 or 1 // N messages at a time instead of 1.

The processed and decoded messages are pushed to the callback function to process and send.

getFrame()

Return the next frame from the buffered data.

Returns:: The next full frame buffer

isFrameReady()

Check if we should continue decode logic.

This is meant to be used in a while loop in the decoding phase to let the decoder factory know that there is still data in the buffer.

Returns:: True if ready, False otherwise

method = 'socket'