transaction — Transaction Controllers for Pymodbus

Module author: Galen Collins <bashwork@gmail.com>

Section author: Galen Collins <bashwork@gmail.com>

API Documentation

Collection of transaction based abstractions

class pymodbus.transaction.ModbusSocketFramer(decoder)

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

addToFrame(message)

Adds new packet data to the current frame buffer

Parameters:message – The most recent packet

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)

Creates 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

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

populateResult(result)

Populates the modbus result with the transport specific header information (pid, tid, uid, checksum, etc)

Parameters:result – The response packet

processIncomingPacket(data, callback)

The new packet processing 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.

Parameters:

• data – The new packet data
• callback – The function to send results to

class pymodbus.transaction.ModbusRtuFramer(decoder)

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 transmist at least x many characters. In this case it is 3.5 characters. Also, if we recieve 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

addToFrame(message)

This should be used before the decoding while loop to add the received data to the buffer handle.

Parameters:message – The most recent packet

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)

Creates 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

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

populateResult(result)

Populates the modbus result header

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

Parameters:result – The response packet

processIncomingPacket(data, callback)

The new packet processing 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.

Parameters:

• data – The new packet data
• callback – The function to send results to

class pymodbus.transaction.ModbusAsciiFramer(decoder)

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.

addToFrame(message)

Add the next message to the frame buffer This should be used before the decoding while loop to add the received data to the buffer handle.

Parameters:message – The most recent packet

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)

Creates 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

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

populateResult(result)

Populates the modbus result header

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

Parameters:result – The response packet

processIncomingPacket(data, callback)

The new packet processing 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.

Parameters:

• data – The new packet data
• callback – The function to send results to

class pymodbus.transaction.ModbusBinaryFramer(decoder)

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.

addToFrame(message)

Add the next message to the frame buffer This should be used before the decoding while loop to add the received data to the buffer handle.

Parameters:message – The most recent packet

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)

Creates a ready to send modbus packet

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

checkFrame()

Check and decode the next frame

Returns:True if we are successful, False otherwise

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

populateResult(result)

Populates the modbus result header

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

Parameters:result – The response packet

processIncomingPacket(data, callback)

The new packet processing 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.

Parameters:

• data – The new packet data
• callback – The function to send results to