UVSG Satellite Data

This was the format used by United Video Satellite Group to send data out at over a satellite feed. The baud rate was initially 2400, although the speed may have been increased to 9600 baud towards the end. Many different types of information were sent out through this feed, including time/date information, system settings, channel listings, and system updates. The format was broadcast until at least 2002 (to support Sneak Prevue and remaining installations of EPG Jr.), but may even have lasted until 2005.

Tin and AriX have successfully reverse engineered parts of this format, and it is possible to send our own messages to emulated or real EPG/Prevue machines over serial. Additionally, some test files received from an ex-UV employee have helped to further the reverse engineering effort.

DATA Format

Every box on message always starts with the header "55 AA", followed by a select code, a 00, and a checksum. A command is started with a "55 AA", then a byte indicating which mode of command is being sent out, then the data of the command, which is usually followed by checksum.

For example, here are two commands that will tell an EPG machine to listen, then change its title to "PREVUE GUIDE":

55 AA 41 2A 00 94 55 AA 54 50 52 45 56 55 45 20 47 55 49 44 45 00 D0

In this example, the select code being addressed is 2A, or the ASCII code for an asterisk (*). This means every machine that is connected to the satellite feed will act on this command. Then, a 00 to terminate the string, and a checksum (94). After the 55 AA, you see the mode byte (54, or T for Title), and then an ASCII string (PREVUE GUIDE). Lastly, there is another 00 to terminate the string, and D0, the checksum for the entire string.

Initial Checksum

Most commands have a checksum that is calculated on all the bytes of the data part of the command. This is sent over the serial port and compared to the correct value by the receiving machine. If the checksum does not match, the computer discards the whole command. The checksum is calculated by bitwise XOR of each byte of data within the command excluding the command letter itself. Each command has its own seed checksum which is loaded as the initial value of the checksum as soon as the initial command letter is received. This initial checksum is calculated by XORing the mode byte with $FF.

Command Modes

Some of this is conjecture and named according to clues found in the reverse-engineered code - it's subject to change and mistakes! Some of the checksums were found from an Atari disassembly, and a few were found by running UVSG sample messages through UVSGXOR.

Command	Name	Initial Checksum	Description	Notes	Atari	Amiga
A	Box On	$BE	Addresses the machines by select code, informing them to listen for commands		Yes	Yes
C	Channel	$BC	Contains the channel lineup	Divided by the receiving machine into a list of channel numbers, call letters, and sources.	Yes	Yes
D	?	$BB	?	?	No	Yes
E	?	$BA	?	Atari L9118 (3 bytes - some kind of configuration bytes relating to CURDAY and NXTDAY?)	Yes	Yes
F	Configuration	$B9	Gives the receiving machine information about its location, local time, settings, ads, and more.	Same syntax on both Atari and Amiga, just a lot of unused values on Atari.	Yes	Yes
f	Config.dat	??	Provides some settings, like GridMR, Sport, SBS, ClockCmd, and more that are written to config.dat.	Length specified in first byte of command. Checksum is strange?	No	Yes
g	?	$98	Defines future DST information		No	Yes
H	Download	$B7	Can be used to write a file to disk, likely other Amiga maintenance tasks as well	Exact same command is also available under 'h' and '='	No	Yes
J	Order Information	$B5	For pay-per-view channels		No	Yes
j	Order Information	$95	For pay-per-view channels	(likely newer format)	No	Yes
K	Clock	$AB	Sets the time centrally	Requires ClockCmd to be 2 on Amiga	Yes	Yes
L	Ad	$B3	Centrally provides an ad or message	Seems to crash Prevue Grid if there is already ad in a slot, maybe bad syntax	Yes	Yes
M	?	$B2	?	Atari L9152 (reads 2 bytes)	Yes	Yes
N	?	$B1	?	Atari L92A4	Yes	No
O	?	$B0	?	No checksum - send $00 $B0 immediately after - Atari L91BA (Amiga accepts as valid)	Yes	Yes
P	Program	$AF	Program information		Yes	Yes
p	Program	$8F	Program information	Possibly newer format, example in NEWP test file	No	Yes
Q	?	$AE	Not sure, probably has something to do with PPV channels		No	Yes
R	Reset	$AD	Reset computer	No checksum - send $00 $AD immediately after.	Yes	Yes
T	Title	$AB	Sets the guide title	Only applicable to EPG, but recognized by Prevue Grid as a valid command	Yes	Yes
Z	?	$A5	?	Atari L9206 (12 configuration bytes read into 04E7 onwards)	Yes	No
$03	?	?	?	Atari L949F (hexadecimal 03)	Yes	No
$BB	Box Off	$44	Tells all listening boxes to stop listening	No checksum - send $BB $00 $FF immediately after	Yes	Yes

Known Command Syntax

C Command Syntax

0x43 [Julian day, byte]
0x12 [flags, byte] [6-character source, string]
0x11 [Channel #, string]
0x01 [5- to 6-character channel name, string]
0x00 [checksum, byte]

NOTE: On EPG Jr. the channel name has a limit of 5 characters

C Commmand Flags

Flag name	Hex value
None	0x01
HILITESRC	0x02
SUMBYSRC	0x04
VIDEO_TAG_DISABLE	0x08
CAF_PPVSRC	0x10
DITTO	0x20
ALTHILITESRC	0x40
STEREO	0x80

P Command Syntax

0x50 [Timeslot 1-48, hex] [Julian day, hex] [6-character source, string]
0x12 [flags, byte] [Program name, string]
0x00 [checksum, byte]

P Commmand Flags

Flag name	Hex value
None	0x01
Movie	0x02
ALTHILITEPROG	0x04
TAGPROG	0x08
SPORTSPROG	0x10
DVIEW_USED	0x20
REPEATPROG	0x40
PREVDAYSDATA	0x80

CTRL Format

In 1988, with the introduction of Prevue Guide, a second, 110-baud feed called CTRL was added.

In the CTRL format, there is no initial 55 AA to start a command. Commands are begun right away with a mode byte, which is a number from 0 to 16 (instead of an ASCII value). Next, the body of the command is sent, followed by a $0D, and then the message checksum, which uses the same format as the DATA feed. The CTRL format has not yet been fully reverse engineered, but more information can be found here.

Command	Control	Name	Length	Description
$01	A	?		Branches based on the first byte it receives - looks like it has "sub-commands"
$02	B	?
$03	C	?	0 bytes	Different than the other commands - not parsed
$04	D	Set Position	1 byte	Sets CTRLEvent_Position to 1 if byte is 'L' or 0 if 'R', if neither, CTRLEvent_Position is toggled
$05	E	Event	2 bytes	First byte corresponds to right side, second corresponds to left - seems to set a "mode" for the overlay of each side
$07	G	?	0 bytes	Sets CTRLEvent_Flag to $F
$0B	K	Clock		Slightly different syntax than DATA, only works when ClockCmd is 1
$0C	L	?		Has something to do with what program is displayed in the overlay in E mode
$0D	M	End of Command
$0F	O	?		Takes some text?
$10	P	?	0 bytes	Sets CTRLEvent_Flag to $E
$11	Q	?
$14	T	?	1 byte?	Sets some internal variable
$16	V	?