This is what I found out about the Minolta Pagepro 1[23]xxW ESC codes:
(The information contained in this document may be wrong or inaccurate,
this is only what I guessed by looking at the output of the windoof
driver and by monitoring the traffic to and from the printer I have)

ESC sequence layout
	1b cmd sq lw ~cmd [cmd data bytes] ck [further data bytes]

	cmd	printer command
	sq	sequence number for all ESC sequences in print job, starting
		with 00
	lb	word: length of command data in bytes (a word is 2 bytes!)
	ck	byte: sum over all bytes of the ESC sequence modulo 256
		(... or AND the result with 0xff if you like that better)

note: ~cmd denotes the bitwise complement of cmd
note: words and dwords are always transmitted least significant byte first,
      most significant byte last

known commands:
	40 - start-of-printer-commands
	41 - end-of-printer-commands
	50 - select resolution and paper type
	51 - new page and set paper format
	52 - send raster data
	55 - eject-current-sheet-of-paper (that's what it seems to be)
	60 - read printer register (?)
	6a - alternate read printer register (?)

note: things which are still unclear are marked with an asterisk (*)

------------------------------------------------------------------------------
command 40 (start-of-printer-commands)
------------------------------------------------------------------------------
	model byte		81 - Minolta PagePro 12xxW series
				83 - Minolta PagePro 13xxW series
 *	data 			00 (this data is required!)

------------------------------------------------------------------------------
command 41 (end-of-printer-commands)
------------------------------------------------------------------------------
 *	data			00 (seems to be don't care data)

------------------------------------------------------------------------------
command 50 (select resolution and paper type)
------------------------------------------------------------------------------
	resolution byte		00 -  300 dpi (seen only with 12xxW)
				01 -  600 dpi (12xxW, 13xxW series)
				02 - 1200 dpi (12xxW, 13xxW series)

				it would be great if someone could test
				300 dpi printouts on 13xxW printers - may or
				may not work

				when printing with different horizontal and
				vertical resolutions, this gives the vertical
				resolution

	horizontal modifier	00 - horizontal and vertical resolution equal
				01 - horizontal twice the veritcal resolution

				12xxW models drivers from other OSs seem to
				support only equal resolution modes - however,
				tests show that 12xxW models have no problem
				printing with different resolutions...

 *	data 			00 (seems to be don't care data)

	paper type byte		00 - normal paper
				01 - thick paper
				02 - transparency
				03 - envelope/postcard

 *	data			04 00 00 00 (for Minolta PagePro 12xxW)
				04 00 04 00 (for Minolta PagePro 13xxW)

				12xxW models seem to accept 04 00 04 00...

------------------------------------------------------------------------------
command 51 (start new page and set paper format)
------------------------------------------------------------------------------
 *	data 			00 01 (seems to be don't care data)

	page width dword	(in dots at current resolution)

	page height dword	(in dots at current resolution)

 *	data			08 00 08 00 (seems to be don't care data)

	tray byte		ff - auto
				00 - tray 1
				01 - tray 2
				80 - manual feed

	paper byte		04 - a4
				06 - b5
				08 - a5
				0c - J-Post
				0d - cor. post
				10 - JIS Y6
				11 - JIS Y0
				13 - Chinese 16K
				15 - Chinese 32K
				19 - legal
				1a - G.legal
				1b - letter
				1d - G.letter
				1f - executive
				21 - half letter
				24 - Env Monarch
				25 - Env #10
				26 - Env DL
				27 - Env C5
				28 - Env C6
				29 - Env B5
				2d - Choukei-3Gou
				2e - Choukei-4Gou
				31 - Custom, Env B6, Folio, JIS Y1, JIS Y2,
				     quad post

 *	data			?? ?? ?? (seems to be don't care data)

 *	data			00 (seems to be don't care data)
	flag byte		(c0 for 300 dpi printouts, 00 otherwise)
 *	data			00 (seems to be don't care data)

	note: command 51 seems to allow setting a different sequence number
	      by simply specifying one out of sequence in the sequence...

------------------------------------------------------------------------------
command 52 (send raster data)
------------------------------------------------------------------------------
	dword			bytes in further data bytes section

	word			number of lines in data section

	raster data is appended in the further data section
	Adam Bocim <beetman@seznam.cz> came up with an algorithm to compress
	the raster data in a format that the minolta understands, and the
	implementation in this program is heavily based upon it
	(see http://sweb.cz/beetman for the original source)
	I tried to understand what his code does and why it does things that
	way. My implementation differs in some aspects, but seems to produce
	correct results. See the source for details on how the raster data
	compression works (this is not difficult, but using the source,
	things may be clearer than my explanations). A short outline of the
	method is given below to give the general idea and to make reading
	the source easier.
 
	note: there always seem to be eight ESC 52 commands per page
	      the data seems to be divided in eight stripes with the first
	      one starting at the top of the page; the last one may be short
	      by a number of scanlines

	Compression is done on a per-scanline basis. It uses RLE encoding as
	well as a special table driven encoding. This table contains up to
	16 bytes which can be compressed using the index into the table. The
	table length and its nonempty entries are transmitted at the start of
	each scanline. Compression principle works as follows:

	while (bytes in scanline left)
		three of more repeating bytes?
			encode using RLE coding
		next four or more bytes in table (or is space for them)?
			encode using table indices (possibly filling the
				table on our way) unless we have three or
				more repeating bytes ahead, where we do
				RLE-compression instead
		we can't use RLE- or table-compression, so we have to send
			literal bytes in chunks of up to ten bytes (Minolta
			knows why exactly ten bytes - I don't) until one
			of the other methods becomes feasible again
	end compress scanline

------------------------------------------------------------------------------
command 55 (seems to be eject-current-sheet-of-paper command)
------------------------------------------------------------------------------
 *	data			00 (seems to be don't care data)

	note: this command is only used after the last page has been printed

------------------------------------------------------------------------------
command 60 (read printer register (?))
------------------------------------------------------------------------------
	register byte		i have seen queries for registers 0x02, 0x04,
				0x05, 0x53, and 0x81 (this last one on 1350W)
*	data			00

	see below for reading data procedure
	see the source for data items that have been identified

------------------------------------------------------------------------------
command 6a (alternate read printer register (?))
------------------------------------------------------------------------------
*	data			78 00 04 (on 1200W)
				1c 00 04 (on 1350W)

	very little is known about this command. reading two chunks on the
	1200W will have register 0x53 in the second chunk (page counter!)

------------------------------------------------------------------------------
procedure used for reading data
------------------------------------------------------------------------------
Just fgetc from the device, the linux kernel does the right thing. The printer
answers in packets of the following layout:

	register byte (the register the printer sends - if the register is
		unknown to the printer it might send a different one instead)

	length byte (number of bytes in the packet remaining to be read)

	remaining bytes (packet data section)

You should always read all bytes of a packet's data section.


------------------------------------------------------------------------------
layout of a print job:
------------------------------------------------------------------------------
ESC 0x40
ESC 0x50
while pages left {
	ESC 0x51
	while bitmap data for this page left {
		ESC 0x52
	}
}
ESC 0x55
ESC 0x41