Disclaimer: This description is completely unofficial, Epson even does not know about its existance. Most of the informaion presented here is discovered by me, Eugene Crosser, while snooping the serial line and by trial and error. I never had an official protocol description, have never seen any related software source code, and have never done reverse engineering of any related software. This description may be incomplete, inaccurate or completely wrong. You are warned.
Some information is taken from `camediaplay' package by Jun-ichiro Itoh <itojun@itojun.org> and from other (open) sources and not checked by me.

Serial Protocol of Some Digital Cameras

Epson PhotoPC 500 is a digital camera that uses a serial (RS232) interface for communication with the host. Follows the description of the high-level protocol it uses over the serial line. I think that the same protocol is used with vanilla PhotoPC model. There are reports that several other models of digital cameras use the same protocol, namely Sanyo, Agfa and Olympus cameras.

Protocol Basics

The host and the camera exchange with data packets and individual bytes. Serial line paramaters used are: 8bit, no parity. No flow control is used. All arithmetic data is transmitted in the "network order" - least significant byte first.

Protocol Elements

The elementary units of the protocol are:

Initialization Byte	NUL	0x00
Action Complete Notification	ENQ	0x05
Positive Achnowledgement	ACK	0x06
Unable to Execute Command	DC1	0x11
Negative Acknowledgement, also Camera Signature	NAK	0x15
Packet	Variable length sequence of bytes
Termination Byte		0xff

Packet structure

The packet has the following structure:

Offset	Length	Meaning
0	1	Packet type
1	1	Packet subtype/sequence
2	2	Length of data
4	variable	Data
-2	2	checksum

Known packet types are:

Type Description

0x02 Data packet that is not last in sequence

0x03 Data packet that is last in sequence

0x1b Command packet

Type	Description
0x02	Data packet that is not last in sequence
0x03	Data packet that is last in sequence
0x1b	Command packet

Data packets that are sent in responce to a single command are numbered starting from zero. If all requested data fits in one packet, it has type 0x03 and sequence 0.

Command packet has subtype 0x43 or 0x53. Only the first command packet in a session has subtype 0x53.

Maximum length of data field in a packet is 2048 bytes, which yields in 2054 total packet length.

Checksum is a simple 16 bit arithmetic sum of all bytes in the data field. As already mentioned above, length and checksum values are transmitted least significant byte first.

Flow of Control

A communication session flow is as follows:

Host	Camera
Port speed set to 19200 baud
Host sends init byte 0x00
	Camera responds with signature 0x15
Host sends command packet with subtype 0x53 and "set speed" command
	Camera sends ACK 0x06
Port speed set to the new value
Host sends command
	Camera responds with either ACK plus optionally "action taken" notifier or data packet sequence
Host sends ACK to every data packet
... Command - reply cycle repeated ...
	Camera sends 0xff and resets after a few seconds (value is model-dependant) of inactivity

If the camera does not respond to a command in reasonable time, or responds with a NAK, the command can be resent. If the camera does not provide a complete data packet in reasonable time, or the data packet is corrupt (checksum does not match), the host can request resending of the packet by sending NAK instead of ACK.

Command format and codes

Command is a sequance of bytes sent in the data field of a command packet. Command format is as follows:

Offset	Length	Description
0	1	Command code
1	1	Register number or subcode
2	variable	Optional argument

Five command codes are known:

Code Argument Description

0 int32 Set value of integer register

1 none Read value of integer register

2 vdata Take action unrelated to registers

3 vdata Set value of vdata register

4 none Read value of vdata register

Code	Argument	Description
0	int32	Set value of integer register
1	none	Read value of integer register
2	vdata	Take action unrelated to registers
3	vdata	Set value of vdata register
4	none	Read value of vdata register

Commands 1 and 3 are replied with a single ACK 0x06. Command 2 is replied with an ACK 0x06 followed by an "action complete" notifier 0x05. Commands 1 and 4 are replied with a sequence of data packets, each of them must be ACK'ed by the host.

Command 0 must be issued with a 4 byte argument containg the new value for the register (bytes in network order). Command 2 typically is issued with a single zero byte as an argument. Command 3 is issued with an argument of variable number of bytes. If this is a printable string, it should not include the trailing zero byte.

Camera replies to the command 1 with a single data packet containing 4 bytes of a 32bit integer (in network order). Camera replies to the command 4 with a sequence of data packets with variable number of data bytes. Note that if a printable string is returned, it is terminated with a zero byte, and thus may be safely printed or otherwise treated as a normal C language character string.

Registers

The following registers are known (read/writablity info is inaccurate):

No.	Type	R/W	Description
1	int32	R/W	Resolution: 1 - Std, 2 - Hi, 3 - Ext
2	int32	R/W	Clock in UNIX time_t format
3	int32	R/W	Shutter speed (microseconds)
4	int32	W	Current frame number
5	int32	R/W	Unknown (5)
6	int32	R/W	Color mode: 1 - Color, 2 - B/W
7	int32	R/W	Flash mode: 0 - Auto, 1 - Force, 2 - Off, 3 - Anti RedEye
8	int32	R/W	Unknown (128)
9	int32	R/W	Unknown (128)
10	int32	R	No. of frames taken
11	int32	R	No. of frames left
12	int32	R	Length of current frame *
13	int32	R	Length of current thumbnail *
14	vdata	R	Current frame JFIF data *
15	vdata	R	Current thumbnail JFIF data *
16	int32	R	Battery capacity percentage
17	int32	W	Communication speed 1 - 9600 .. 5 - 115200
18	int32	R	Unknown (1)
19	vdata	R/W	Unused
20	vdata	R/W	Unused
21	vdata	R/W	Unused
22	vdata	R/W	Camera I.D.
23	int32	R/W	Autoshut on host timer (seconds)
24	int32	R/W	Autoshut in field timer (seconds)
25	vdata	R/W	Serial No. (string)
26	vdata	R	Version
27	vdata	R/W	Model
28	int32	R	Available memory left
29	vdata	R/W	Upload image JFIF data to this register
30	vdata	R	Unknown ("")
31	vdata	R	Unknown ("\0")
32	int32	R	Put "magic spell" 0x0FEC000E here before uploading image data
33	int32	R/W	Lens mode: 1 - Macro, 2 - Normal
34	int32	R	Unknown (0)
35	int32	R	LCD brightness 1 to 7
36	int32	R	Unknown (3)
37	vdata	R	Unknown ("\0")
38	int32	R	LCD autoshut timer (seconds)
39	int32	R	Protection state of current frame *
40	int32	R	Unknown (0)
41	int32	R/W	Date format (?)

* Note: registers 12 thru 15 and 39 only become useful for reading after setting register 4. If value of 0 assigned to register 4, subsequent retrieval of picture data gives the "live preview".

For command 2, the second byte is action code not register number. The following action codes are known:

Code Argument Description

1 single zero byte Erase all picures

2 single zero byte Take picture

4 single zero byte Finish session immediately

7 single zero byte Erase current frame *

9 single byte Set protection state of current frame to the value of parameter (binary 0 or 1)*

11 single zero byte Store freshly uploaded image into NVRAM

* Note: actions 7 and 9 only useful after setting register 0x04.

Code	Argument	Description
1	single zero byte	Erase all picures
2	single zero byte	Take picture
4	single zero byte	Finish session immediately
7	single zero byte	Erase current frame *
9	single byte	Set protection state of current frame to the value of parameter (binary 0 or 1)*
11	single zero byte	Store freshly uploaded image into NVRAM

Unknown things

Format of "live preview" data sent by the camera

Please mail your corrections/additions to <crosser@average.org>
See http://www.average.org/digicam/ for possible updates.