UVSG Satellite Data

This was the format used by United Video Satellite Group to send data out at 2400 baud over a satellite feed. Many different types of information were sent out through this feed, including time/date information, system settings, channel listings, and system updates.

Tin has successfully reverse engineered parts of this format, and it is now 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.

Overview

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 hardcoded seed checksum which is loaded as the initial value of the checksum as soon as the initial command letter is recieved.

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
A	Box On	$BE	Addresses the machines by select code, informing them to listen for commands
C	Channel	$BC	Contains the channel lineup	Divided by the receiving machine into a list of channel numbers, call letters, and sources.
E	?	$BA	?	Atari L9118
F	Configuration	$B9	Gives the receiving machine information about its location, local time, settings, ads, and more.	Completely different syntax between Atari and Amiga commands, but same command letter and initial checksum
H	Download	$B7	Can be used to write a file to disk, likely other Amiga maintenance tasks as well	Works only on Amiga platforms
J	Order Information	$B5	For pay-per-view channels
j	Order Information	$95	For pay-per-view channels	(likely newer format)
K	Clock	$AB	Sets the time centrally	The known syntax of this command does not work on post-1997 Prevue Grid software
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
M	?	$B2	?	Atari L9152
N	?	$B1	?	Atari L92A4
O	?	$B0	?	No checksum - send $00 $B0 immediately after - Atari L91BA (Amiga accepts as valid)
P	Program	$AF	Program information
p	Program	$8F	Program information	Possibly newer format, example in NEWP test file
Q	?	$AE	Not sure, probably has something to do with PPV channels
R	Reset	$AD	Reset computer	No checksum - send $00 $AD immediately after. Works on both Atari and Amiga.
T	Title	$AB	Sets the guide title	Only applicable to EPG, but recognized by Prevue Grid as a valid command
Z	?	$A5	?	Atari L9206
$03	?	?	?	Atari L949F (hexadecimal 03)
$BB	Box Off	$44	Tells all listening boxes to stop listening	No checksum - send $BB $00 $FF immediately after