# Framer

## pymodbus.framer.FramerAscii module

Modbus ASCII Frame Controller.

Layout::
  [ Start ][ Dev id ][ Function ][ Data ][ LRC ][ End ]

    1c 2c 2c N*2c 1c 2c

  - data can be 1 - 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.

## pymodbus.framer.FramerRTU module

Modbus RTU frame type.

Layout:

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

Note

due to the USB converter and the OS drivers, timing cannot be quaranteed neither when receiving nor when sending.

Decoding is a complicated process because the RTU frame does not have a fixed prefix only suffix, therefore it is necessary to decode the content (PDU) to get length etc. There are some protocol restrictions that help with the detection.

For client:

  - a request causes 1 response !

  - Multiple requests are NOT allowed (master-slave protocol)

  - the server will not retransmit responses

this means decoding is always exactly 1 frame (response)

For server (Single device)

  - only 1 request allowed (master-slave) protocol

  - the client (master) may retransmit but in larger time intervals

this means decoding is always exactly 1 frame (request)

For server (Multidrop line –> devices in parallel)

  - only 1 request allowed (master-slave) protocol

  - other devices will send responses

  - the client (master) may retransmit but in larger time intervals

this means decoding is always exactly 1 frame request, however some requests will be for unknown slaves, which must be ignored together with the response from the unknown slave.

Recovery from bad cabling and unstable USB etc is important, the following scenarios is possible:

> - garble data before frame

> - garble data in frame

> - garble data after frame

> - data in frame garbled (wrong CRC)

decoding assumes the frame is sound, and if not enters a hunting mode.

The 3.5 byte transmission time at the slowest speed 1.200Bps is 31ms. Device drivers will typically flush buffer after 10ms of silence. If no data is received for 50ms the transmission / frame can be considered complete.

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

Danger

Current framerRTU does not support running the server on a multipoint rs485 line.

## pymodbus.framer.FramerSocket module

Modbus Socket frame type.

Layout:

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

length = uid + function code + data

## pymodbus.framer.FramerTLS module

Modbus TLS frame type.

Layout:

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

length = uid + function code + data