Communication Specification

This document specifies the communication between the host and a controller with the AYAB firmware.

Serial Communication

115200 baud

Line Ending: \n\r (10 13) Each message ends with a Line Ending.

Sequence Chart

sequence diagram for the communication between host and controller

The host waits for a indState(true) message before requesting to start the knitting. On startup, the Arduino continuously checks for the initialization of the machine (carriage passed left hall sensor). When this happens, it sends an indState(true) to tell the host that the machine is ready to knit. After receiving this message, the host sends a reqStart message, which is immediately confirmed with a cnfStart message. When reqStart was successful, the Arduino begins to poll the host for line data with reqLine, the host answers with cnfLine. This reqLine/cnfLine happens each time the carriage moves passed the borders given by the Start/StopNeedle parameters in reqStart. When the host does not have any more lines to send, it marks the last line with the lastLine flag in its last cnfLine message.

To see an example implementation, see the states of the communication module.

Message Identifier Format

Messages start with a byte that identifies their type. This byte is called “id” or “message id” in the following document. This table lists all the bits of this byte and assigns their purpose:

Bit Value Name Description and Values
7 128 message source
  • 0 = the message is from the host
  • 1 = the message is from the controller
6 64 message type
  • 0 = the message is a request
  • 1 = the message is a confirmation of a request
5 32 reserved must be zero
4 16
3 8 message identifier

These are the values that identify the message.

2 4
1 2
0 1

Message definitions (API v4)

The length is the total length with id and parameters. Note that the two characters \r\n following the message are not included in the length.

source name id length parameters


0x01 3

0xaa 0xbb

  • aa = left end needle (Range: 0..198)
  • bb = right end needle (Range: 1..199)

Start and



0xC1 2


  • a = success (0 = false, 1 = true)


0x82 2


  • aa = line number (Range: 0..255)


0x42 29

0xaa 0xbb[24, 23, 22, ... 1, 0] 0xcc 0xdd

  • aa = line number (Range: 0..255)
  • bb[24 to 0] = binary pixel data
  • cc = flags (bit 0: lastLine)
  • dd = CRC8 Checksum


0x03 1  


0xC3 4

0xaa 0xbb 0xcc

  • aa = API Version Identifier
  • bb = Firmware Major Version
  • cc = Firmware Minor Version


0x84 8

0x0a 0xBB 0xbb 0xCC 0xcc 0xdd 0xee

  • a = ready (0 = false, 1 = true)
  • BBbb = int left hall sensor value
  • CCcc = int right hall sensor value
  • dd = the carriage
    • 0 = no carriage detected
    • 1 = knit carriage “Strickschlitten”
    • 2 = hole carriage “Lochmusterschlitten”
  • ee = the needle number currently in progress


0x23 var A debug string. The id is the character #. The length is variable and can be determined by the end \r\n'.


0x04 1 put the controller into test mode


0xC4 2


  • a = success (0 = false, 1 = true)

The reqStart Message

The host starts the knitting process.

The cnfStart Message

The controller indicates the success of The reqStart Message.

The reqLine Message

The controller requests a new line from the host.

More than 256 lines are supported. There are three possibilities for the next line based on the last line:

  1. the new line is greater than the last line
  2. the new line is lower than the last line
  3. the new line is the last line

We choose the line closest to the last line. This is trivial for (3). In case two lines are equally distant from the last line, we choose the smaller line.

This is computed by the function AYABInterface.utils.next_line() which is tested and can be seen as a reference implementation for other languages.

The cnfLine Message

The host answers The reqLine Message with a line configuration.

This table shows the message content without the first byte that identifies the message:

Byte Name Description
0 line number These are the lowest 8 bit of the line. They must match the line number in The reqLine Message.
1 needle positions

Each bit of the bytes represents a needle position.

  • 0 = “B”
  • 1 = “D”

For the exact mapping of bits to needles see the table below.

26 flags

Bits: 0000000L

  • L - “LastLine” (0 = false, 1 = true)
27 crc8 checksum This checksum is computed from bytes 0 to 26, including byte 26. The controller may use this checksum to check the result and if the checksum does not match, it can send The reqLine Message anew.

In the following table, you can see the mapping of bytes to needles.


  • The Needles are counted from the leftmost needle on the machine.
  • The Needle count starts with 0.
  • The Byte numbering is taken from the table above.
  • The Bit numbering is consistent with Message Identifier Format. The highest bit has the number 7 and the lowest bit has number 0.
Byte 1 2   24 25
Bit 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 ... 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
Needle 0 1 2 3 4 5 6 7 8 9 ... 198 199

The reqInfo Message

The host initializes the handshake.

The cnfInfo Message

The controller answers The reqInfo Message with the API version.

The indState Message

This is sent when the controller indicates its state. When ready it is

The debug Message

This message ends with a \r\n like evey message. It contains debug information from the controller.

The reqTest Message

This message puts the controller in a test mode instead of a knitting mode.

The cnfTest Message

This messsage confirms whether the controller is in the test mode. If success is indicated, the controller sends The indState Message messages periodically, containing the sensor and position values.


See also