Appendix R

R.1   ASYNC PROTOCOL

R.1  Data Block

Async transmits data in 133-byte blocks with headers and trailers 
as follows.  All byte values are in hexadecimal.

 _______________________________________________________________
| header | block # | block # | 128 data bytes | checksum | NAK  |
| byte   | 1 to FFh| XOR FFh |                |          |      |
|  01h   | then    |         |                |          | 15h  |
|        | 0 to FFh|         |                |          |      |
|________|_________|_________|________________|__________|______|

Notes:

1.   The  first byte in the block is a header byte.   A value  of 
     01h indicates that this block is a data block.

2.   The  second byte in the block is the block number which is 1 
     for  the first block transmitted,  2 for the  second  block, 
     etc.   When  the  value passes FFh the block  numbers  start 
     again at zero.

3.   The  third byte is the value of block number  exclusive-or'd 
     with  FFh.   This  provides a check on the integrity of  the 
     block number.

4.   Following the third byte is 128 bytes of data.

5.   Following  the  data is a checksum byte.   The  checksum  is 
     formed  by taking the algebraic sum of all the  data  bytes, 
     any carry or overflow is discarded.

6.   The last byte in the block is the NAK character (15h).

7.   The receiver commences by sending NAK characters.   When the 
     transmitter receives a NAK character it starts transmitting.

8.   The receiver responds with the following single characters:

     ACK (06h) - block received correctly
     NAK (15h) - incorrect frame, checksum not correct
     CAN (18h) - receiver wishes to abort

9.   The transmitter may send the following single characters:

     CAN (18h) - abort
     EOT (04h) - end of transmission


R.2  File Name Transmission Blocks

Multiple  files may be transmitted by the sender,  in which  case 
the data is preceded by a file name block.

There are two types of file name blocks:

1.   Block containing a file name.

 _______________________________________________________
| header | file name | file name       | checksum | EOT |
| byte   | indicator |                 | on file  |     |
|        |           | 8-byte name plus| name     |     |
| 02h    |   24h     | 3-byte extension| 1 byte   | 04h |
|________|___________|_________________|__________|_____|


2.   Block indicating no more files.

 ______________________________
| header | no-more-files | EOT |
| byte   | byte          |     |
|        |               |     |
| 02h    |   25h         | 04h |
|________|_______________|_____|


Notes:

1.   A header byte of 02h indicates a file name block.

2.   The  second  byte has the value 24h if the block contains  a 
     file name or the value 25h if there are no more files  names 
     to come.

3.   The  file  name is an 11-byte string containing 8 bytes  for 
     the file name (packed with spaces if necessary) and 3  bytes 
     for the extension.

4.   The checksum is on the 11-byte file name.

5.   The last byte is the EOT character (04h).     

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Last revision 29/01/2005