TPDD Base Protocol

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General Reference

  • Disks are single sided 3.5 inch standard disks
  • PDD1 - 40 tracks 2 sectors 1280 bytes/sector (100K/disk)
  • PDD2 - 80 tracks 2 sectors 1280 bytes/sector (200K/disk treated as two banks of 100k each)
  • Maximum file size = 64k.
  • Maximum number of directory entries (files) is 40 for PDD1 and 80 for PDD2.
  • File names are maximum 24 characters (padded with trailing blanks) although Tandy always used 6 for filename and 2 for filetype, with period seperator (i.e. XXXXXX.TT)
  • All communications with drive are at 19,200 bps. PDD1 has dip switches so this can drop to 9600bps. PDD2 is auto sensing down to 1200bps (??? - can anyone confirm this?).

Command Format

All commands are in a request/return format (half-duplex)

General request format:

preamble type length data checksum

General return format:

type length data checksum

Command Type

(all values are in HEX)

 command  request  return
 directory ref  00  11 12
 open file  01  12
 close file  02  12
 read file  03  10 12
 write file  04  12
 delete file  05  12
 format disk  06 *  12
 drive status  07 *  12
 drive condition  0C *  15
  rename file  0D  12

* PDD2 treats the disk as two banks of 100k each. All commands except these two must specify the bank number as part of the request. The above commands reference BANK 0. To reference BANK 1 you must add 40 HEX to these values (bit 6 = bank number). E.G. 'open file' becomes 41 for BANK 1.

Request Command details

Type 00 - Directory Reference

    2   1  1  24             1           1           1      bytes
 +----+--+--+--------------+----------+------------+------+
 |5a5a|00|1a| filename     |attribute |search form |chksum| 
 +----+--+--+--------------+----------+------------+------+
  • Preamble - always 'ZZ'
  • request - type 00h*
  • length - length of data 1ah (26 decimal)
  • filename - padded with blanks
  • attribute - specify 'F' (not used normally)
  • search form
  • 00h - reference file for open or delete
  • 01h - request first directory block
  • 02h - request next directory block
  • 03h - request previous directory block
  • 04h - end directory reference

checksum - see below for calculating

Type 01 - Open file

   2   1  1  1     1      bytes
 +----+--+--+------+------+
 |5a5a|01|01| mode |chksum|
 +----+--+--+------+------+
  • Preamble - always 'ZZ'
  • request - type 01h
  • length - length of data 01h (1 decimal)
  • mode -
  • 01h - open new file for WRITE
  • 02h - open existing file for APPEND
  • 03h - open existing file for READ

checksum - see below for calculating

Type 02 - Close file

   2   1  1  1        bytes
 +----+--+--+------+
 |5a5a|02|00|chksum|
 +----+--+--+------+
  • Preamble - always 'ZZ'
  • request - type 02h
  • length - length of data 00h (no data field)
  • checksum - see below for calculating

Type 03 - Read file

   2   1  1  1        bytes
 +----+--+--+------+
 |5a5a|03|00|chksum|
 +----+--+--+------+
  • Preamble - always 'ZZ'
  • request - type 03h
  • length - length of data 00h (no data field)
  • checksum - see below for calculating

Type 04 - Write file

   2   1  1   1-128  1       bytes
 +----+--+---+------+------+
 |5a5a|04|01-|data  |chksum|
 |    |  | 80|      |      |
 +----+--+---+------+------+
  • Preamble - always 'ZZ'
  • request - type 04
  • length - length of data 01h-80h (actual length of data)
  • checksum - see below for calculating

Type 05 - Delete file

   2   1  1  1        bytes
 +----+--+--+------+
 |5a5a|05|00|chksum|
 +----+--+--+------+
  • Preamble - always 'ZZ'
  • request - type 05h
  • length - length of data 00h (no data field)
  • checksum - see below for calculating

Type 06 - Format Disk

   2   1  1  1        bytes
 +----+--+--+------+
 |5a5a|06|00|chksum|
 +----+--+--+------+
  • Preamble - always 'ZZ'
  • request - type 06h
  • length - length of data 00h (no data field)
  • checksum - see below for calculating

Type 07 - Drive Status

   2   1  1  1        bytes<
 +----+--+--+------+
 |5a5a|07|00|chksum|
 +----+--+--+------+
  • Preamble - always 'ZZ'
  • request - type 07h
  • length - length of data 00h (no data field)
  • checksum - see below for calculating

Type 0C - Drive Condition

   2   1  1  1        bytes
 +----+--+--+------+
 |5a5a|0C|00|chksum|
 +----+--+--+------+
  • Preamble - always 'ZZ'
  • request - type 0Ch
  • length - length of data 00h (no data field)
  • checksum - see below for calculating

Type 0D - Rename file

   2   1  1  24     1       1       bytes<
 +----+--+--+-------+------+------+
 |5a5a|0D|19|newname|attrib|chksum|
 +----+--+--+-------+------+------+
  • Preamble - always 'ZZ'
  • request - type 0D
  • length - length of data 19h (25 decimal)
  • newname - new name for the file
  • attribute- not used (specify 'F')
  • checksum - see below for calculating

Return Command Details

Type 10 - Read file Return

 1  1   0-128       1      bytes
 +--+---+----------+-----+
 |10|00-| file data|cksum|
 |  | 80|          |cksum|
 +--+---+----------+-----+
  • return - type 10
  • length - length of data 00h-80h (0-128 decimal)
    • if length is equal to 80h there may be more data - you must issue another read command
    • if length is less than 80h then this is last block
  • file data - data read from file
  • checksum - see below for calculating

Type 11 - Directory reference return

 1  1   24       1      2    1    1       bytes
 +--+--+--------+------+----+----+-----+
 |11|1c|filename|attrib|size|free|cksum|
 +--+--+--------+------+----+----+-----+
  • return - type 11h
  • length - length of data 1Ch (28 decimal)
  • filename - file name - if no name specied is 00H. If at end of directory is 00H.
  • attribute - not used
  • size - size of file, big endian format
  • free - number of free sectors (multiply by 1280 for bytes)
  • checksum - see below for calculating

Type 12 - Normal Return

 1  1  1      1      bytes
 +--+--+-----+-----+
 |12|01|error|cksum|
 +--+--+-----+-----+
  • return - type 12h
  • length - length of data 01h
  • error code-
 00 - normal (no error)  10 - file does not exist  11 - file exists
 30 - no filename  31 - dir search error  35 - bank error
 36 - parameter error  37 - open format mismatch  3f - end of file
 40 - no start mark  41 - crc check error in ID  42 - sector length error
 44 - format verify error  46 - format interruption  47 - erase offset error
 49 - crc check error in data  4a - sector number error  4b - read data timeout
 4d - sector number error  50 - disk write protect  5e - un-initilized disk
 60 - directory full  61 - disk full  6e - file too long
 70 - no disk  71 - disk change error  
  • checksum - see below for calculating

Type 15 - Drive Condition Return

 1  1  1          1      bytes
 +--+--+---------+-----+
 |15|01|condition|cksum|
 +--+--+---------+-----+
  • return - type 15h
  • length - length of data 01h
  • condition- bit values
         7 6 5 4 3 2 1 0    bit
        +-+-+-+-+-+-+-+-+
    MSB |0|0|0|0|x|x|x|x| LSB
        +-+-+-+-+-+-+-+-+
                 | | | |
                 | | | +--power (0=normal 1=low)
                 | | +----write protect (0=not prot 1=prot)
                 | +------disk out (0=disk in 1=disk out)
                 +--------disk change status (0=not changed 1=changed)
  • checksum - see below for calculating

Sequence of Events

 Get directory req 00 search form 01

req 00 search form 02 (repeat as needed)

 Write file req 00 search form 00

req 01 mode 01 or 02
req 04 (repeat as needed)
req 02

 Read file req 00 search form 00

req 01 mode 03
req 03 (repeat as needed)
req 02

 Rename file req 00 search form 00

req 0d

 Delete file  req 00 search form 00

req 05


Calculating Checksum

The check sum is "the one's complement of the least significant byte of the number

of bytes from the block format through the data block". Most people (me included) don't understand what that involves if you have to calculate it. Fortunately I found an

example of how to do this and so I'm passing it on to you.

Checksum=(bytes MOD 256) XOR 255

where bytes = number of bytes including the Request Type, Length and all Data fields (but not including the preamble).


Using this Information

A lot of the above commands can be set up in advance since there is no variable part to

calculate. Some commands must have the length and checksum calculate as the data is built

but the others don't. Here's how I set up some of the commands in my program:

Close$   = "ZZ"+Chr$(2)+Chr$(0)+Chr$(253)
Dir1$    = "ZZ"+Chr$(0)+Chr$(26)+Space$(24)+"F"+Chr$(1)+Chr$(158)
Dir2$    = "ZZ"+Chr$(0)+Chr$(26)+Space$(24)+"F"+Chr$(2)+Chr$(157)
Status$  = "ZZ"+Chr$(7)+Chr$(0)+Chr$(248)+Chr$(13)
Format$  = "ZZ"+Chr$(6)+Chr$(0)+Chr$(249)+Chr$(13)
Erase$   = "ZZ"+Chr$(5)+Chr$(0)+Chr$(250)
Seek$(1) = "ZZ"+Chr$(1)+Chr$(1)+Chr$(1)+Chr$(252)
Seek$(2) = "ZZ"+Chr$(1)+Chr$(1)+Chr$(2)+Chr$(251)
Seek$(3) = "ZZ"+Chr$(1)+Chr$(1)+Chr$(3)+Chr$(250)

The above commands can be sent directly without any calculating to speed up your program.