Documentation for the AT&T Sceptre Videotex terminal

I've just gotten my hands on a working AT&T Sceptre system, which is a 1983 Videotex terminal that implements the NAPLPS standard for vector graphics transmission (the same sort of graphics that was used by the Prodigy online service, if you remember that). It powers up fine and generates the welcome screen, which includes a "Press [Data] to login" message. Once you press [Data], the screen blanks, and I presume that it's trying to establish some sort of connection over the RS-232 port. I've just started poking at the thing and haven't gotten far, as I need to grab a null modem from elsewhere, but I thought there might be a chance that somebody out there has the documentation for one of these things. Through searching about, I'm fairly certain that it's hardwired to speak 1200/7E1 on the serial port, but what it's expecting to hear is the question. Just a stream of NAPLPS-encoded data? Or is there more negotiation to be done?

I have at least found an online source for the NAPLPS encoding itself: http://netghost.narod.ru/gff/vendspec/naplps/naplps.txt