The DEC VT520 manual states:
The VT520 supports up to four sessions using virtual terminals. Each virtual terminal maintains the full keyboard and display state of a real physical terminal but shares a single keyboard and display with other virtual terminals.To use a fourth session or reduce the number of serial comm lines needed, more than one session must be connected through a single comm port. The VT520 allows this using a session management protocol, called TD/SMP (Terminal Device/Session Management Protocol). TD/SMP allows two or more sessions to share a single comm line, as long as the other end of the comm line supports the protocol. ...
TD/SMP is standard on most terminal servers. If you are using a terminal server, check with its documentation to determine if it can run TD/SMP.
By "standard on most terminal servers", they mean "only available on DECservers".
The DEC VT520 Programmer manual states:
If your VT520 is connected directly to a host computer without a terminal server, use [sic] can still use TD/SMP by running the Session Support Utility (SSU) on your host computer. On a system that has SSU software, such as OSF or OpenVMS, open the first session as follows:The install manual for OpenVMS's SSU software says:$ mcr ssu <return>On an UNIX-style system, open the first using:% ssu
SSU software lets you log in to and maintain multiple independent sessions on a single terminal line on an OpenVMS system. SSU software is for use with Digital's VT330, VT340, and VT420 video terminals. SSU software communicates with the single-line, multiple-session firmware in the terminal.
An HP support request regarding OpenVMS SSU clarifies:
The Session Support Utility (SSU) product is no longer available, and was retired in 1998 — the OpenVMS Wizard is not aware of any currently-available means to acquire the license, and is not in a position to determine of [sic] the release is contractually possible.
The key question is: How do you get TD/SMP working server-side, using modern software / hardware? (eg. Linux)
Newsgroups: comp.os.vms Path: cs.utk.edu!gatech!howland.reston.ans.net!europa.eng.gtefsd.com!uunet !organpipe.uug.arizona.edu!CS.Arizona.EDU!zippy.Telcom.Arizona.EDU !mvb.saic.com!info-vax Message-ID: <17923857@MVB.SAIC.COM> X-Gateway-Source-Info: Mailing List Organization: Info-Vax<==>Comp.Os.Vms Gateway Date: Sat, 27 Nov 1993 09:16:35 EDT From: Jerry Leichter <leichter@lrw.com> Subject: RE: VT420/server multi-session handshake esc seq ?Can someone please tell me the escape sequences used by a VT420 and SSU or a terminal terminal to perform the multi-session handshaking?They are not "escape sequences," a term with a specific meaning defined in ANSI X.34 (I think that's the number). The commands use an entirely different format with a different introducer character; they exist at a different level in the protocol stack. (The protocol involved is called TDSMP, for Terminal Device Session Management Protocol. TDSMP commands can actually appear in the middle of an escape sequence; when the session is resumed, parsing of the escape sequence takes up where it left off.)I suspect they are documented in the VT420 programming reference manual, but I would like to avoid the expense and lead time of ordering it for this "simple" need. [EK-VT420-RM is list $C 173]Don't bother ordering it for this purpose; the TDSMP commands are not documented.I am trying to tailor a PC communications package to permit it to use a TCP/IP DECserver connection with ANSI emulation to one host, and VTxxx emulation to another host.Be aware that TDSMP is protected by patents. (Yes, not just in the US; in Canada and much of the rest of the world, too.) While it's true that a number of third parties appear to have reverse-engineered the protocol and are selling products based on it without public challenge from DEC, that doesn't mean DEC won't change its mind - it's got a history of aggressively defending its intellectual property rights. -- Jerry [Co-inventor of TDSMP, who may not necessarily agree with how DEC manages it, but is telling you the facts as he knows them.]
(emphasis mine)
The patents involved are:
------------------------------------------------------------------------------- Sequence: DC4 (^T) Used by the TDSMP (Terminal Device Session Management Protocol) on DEC VT4xx terminals. The protocol is patented (US 4791566 & 5165020). Commands seems to end with ST. Source: <URL:http://www.cs.utk.edu/~shuford/terminal/dec_vt220_codes.txt> [paraphrased] Status: DEC private -------------------------------------------------------------------------------That lists this email as its source:
Newsgroups: comp.terminals Message-ID: <3618F862.38DB@smarts.com> References: <6v670o$iov$1@callisto.clark.net> Date: Mon, 05 Oct 1998 12:48:35 -0400 From: Jerry LeichterTo: Keith Lynch Subject: Re: DEC Multisessions Does anyone know the protocol used for multisessions on the VT420 and similar terminals? I think it involves sequences that begin with Control-T and end with Control-\. Thanks.The protocol is called TDSMP (Terminal Device Session Management Protocol). You're correct that CTRL/T is the introducer for these control sequences. I'm not sure if CTRL-\ always terminates them - the sequences use a kind of bit encoding, and that might just be a coincidence of the examples you looked at. TDSMP is covered by two US patents, 4791566 and 5165020, and correspond- ing patents in most of the industrialized world. (I'm one of the inventors, and recall such fun things as being FAXed a document in Portugese - with no translation - that I had to sign and FAX back for filing in Brazil within a few hours to avoid missing a deadline.) While DEC made terminals, it never licensed these patents, as far as I know. The protocol was never published. DEC sold its terminals business a couple of years ago; presumably, at least the rights to use the patents went along with the sale. The basic encoding method for the protocol was borrowed from some existing ISO standards; I'm afraid I can't give you a reference after all these years, however. Much of the protocol is pretty easy to reverse-engineer. A couple of DEC competitors did so, years back. I know DEC looked into suing them at the time; they may have instead reached some kind of negotiated settlement. (I recall that we evaluated one competitor's terminal and reached the conclusion that it "would infringe on the patent--if it actually worked".) You can get a pretty good picture of the overall design by reading the patents (though you have to work your way through the "patentese" they are written in). Except for pure research purposes, I would not recommend that you implement TDSMP without getting permission from the current owner of the patent. Legal action can be expensive and messy.... -- Jerry