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YMODEM.md

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YMODEM Batch Transmission Protocol

YMODEM BatchTransmission Protocol(YMODEM Protocol) is based on XMODEM-1K Protocol and added batch transmisson feature.

There are serveral common versions about YMODEM implementation, including complete version which with file size(Hyper Terminal) and no-file-size one(SecureCRT). But it doesnt matter and the cores are the same.

Symbol

In the following introduction, we will use some symbols to define special hex data.

Symbol Hex
SOH \x01
STX \x02
EOT \x04
ACK \x06
NAK \x15
CAN \x18

Data Format

Sender

File information package(First package)

Type Seq Seq-OC File Data CRC-high8 CRC-low8
1 Byte 1 Byte 1 Byte n Bytes m Bytes 1 Byte 1 Byte
SOH / STX 00 FF NUL[m]
  • Type: The size of raw data can be 128 bytes or 1024 bytes. SOH in Type column means the packet taking 128 bytes(including File and Data, n+m == 128) and STX for 1024 bytes
  • Seq：Sequence number of packet, starting from 0
  • Seq-OC：One's complement of sequence number
  • File：The name of file for transporting
  • Data: First packet's Data part is NUL. In complete version of YModem, the header of Data part will take the file size and the rest filling by NUL
  • CRC-high8：16 bits CRC high-byte
  • CRC-low8：16 bits CRC low-byte

File data package

Type Seq Seq-OC Data CRC-high8 CRC-low8
1 Byte 1 Byte 1 Byte n Bytes 1 Byte 1 Byte
SOH / STX

Procedure

Stage one: Preparing

Receiver send char 'C' to sender

Receiver->>Sender: Char 'C'

Stage two: First packet

Sender received char 'C' from receiver and start to send first packet and expect ACK and 'C'. After sender get ACK and 'C' from receiver, sender will send raw data of file.

Sender->>Receiver: Packet 0
Receiver->>Sender: ACK
Receiver->>Sender: Char 'C'

Stage three: File packet

Sender send a file packet and expect ACK.

Sender->>Receiver: Packet 1
Receiver->>Sender: ACK
Sender->>Receiver: Packet 2
Receiver->>Sender: ACK

Handling exception

If sender received NAK from receiver during Stage three, last packet will be resend constantly until ACK comming or exit.

Sender->>Receiver: Packet 1
Receiver->>Sender: NAK
Sender->>Receiver: Packet 1
Receiver->>Sender: ACK
Sender->>Receiver: Packet 2

If sender received CAN() from receiver during Stage three, sender will stop packet sending and exit.

Sender->>Receiver: Packet 1
Receiver->>Sender: CAN

Stage four: Finish

At the end of transmission, sender send EOT to receiver and expect NAK. After NAK arrived, sender send EOT immediately and expect ACK. The ACK means the task was finish successfully.

Sender->>Receiver: EOT
Receiver->>Sender: NAK
Sender->>Receiver: EOT
Receiver->>Sender: ACK
Receiver->>Sender: Char 'C'
...(next round)