Protocols
Communication with devices
Here is a commented log when formatting a SSSD floppy disk with the TI disk controller. Actually, this dump was achieved using MESS and putting in some printf lines.
XModem protocol
The XModem protocol transmits data in a sequence of blocks. Each block contains a part of the file to be transmitted.
| Block | Block | Block | ... | Block |
The last block may require padding if the number of remaining bytes does not suffice to fill the block.
Each block contains a sequence number. When a block with the same number as the previous one is received, the receiver ignores the block.
| Sender sending | Block | Block | ... | Block | EOT | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Receiver sending | NAK | ACK | ACK | ... | ACK | ACK |
The receiver has a timeout of 10 seconds, after which it transmits a NAK. The message sequence starts with the receiver sending the first timeout NAK to the sender, which may be sent immediately. All errors are retried for at most 10 times.
A NAK must be sent by the receiver if the block data are invalid. If the block number is not the expected next one or a repetition of the recent one, the sequence is out of sync, and the receiver shall send a CAN.
Standard XModem
Each block is defined in this way (numbers tell the number of bytes for each field):
| 1 | 1 | 1 | 128 | 1 |
|---|---|---|---|---|
| SOH | Number | /Number | Data bytes | Checksum |
The number is a sequence number which increases by one for each successive block. The first block starts at 1. The /Number field is the one's complement of the Number field and calculates as 255-Number. When the number reaches 256 it is reset to 0.
The checksum field contains the least significant byte of the sum of the data bytes of this block.
XModem-1K
XModem-1K is a variation of XModem with 1K (1024) bytes for each data block.
| 1 | 1 | 1 | 1024 | 1 |
|---|---|---|---|---|
| STX | Number | /Number | Data bytes | Checksum |
The sender may switch between 128-byte and 1024-byte blocks after receiving an ACK from the receiver. The sender should start with 1K blocks and check for the receiver's response, either as NAK or ACK.
XModem/CRC
In this variant, a more powerful checksum algorithm is used, the CRC16 algorithm. In order to negotiate this variant for both sender and receiver, the receiver sends a "C" as the first message instead of the NAK.
| Sender sending | Block | Block | ... | Block | EOT | ||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Receiver sending | "C" | ACK | ACK | ... | ACK | ACK |
If the sender does not support XModem/CRC it keeps silent, and the receiver shall retry for some time. After that it sends a NAK to switch back to normal XModem.
The blocks have this layout:
| 1 | 1 | 1 | 128 | 2 |
|---|---|---|---|---|
| SOH | Number | /Number | Data bytes | CRC16 |
In general it is reasonable to have the sender start in CRC mode and wait for the receiver's first message.