Teletext to Viewdata server

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Soruk
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Re: Teletext to Viewdata server

Post by Soruk » Wed Jan 08, 2020 3:43 pm

Richard Russell wrote:
Wed Jan 08, 2020 3:34 pm
Soruk wrote:
Wed Jan 08, 2020 3:25 pm
I wonder if it's a case of the bottom line being a "use with caution" one... a particularly horrible hack, I guess would be to just not send the 40th character of the bottom row (taking into account the number of ESC characters...)
Well, personally, I'm certain you shouldn't be hacking your server to fix a problem in one particular client, which might not be working for a completely different reason. Your server should be doing exactly what the original Prestel servers did, back in the day; the trouble is we don't know for sure what that is! It seems to me highly unlikely that Prestel was designed so that the last character in the bottom row can't be used.
I completely agree. I'm not going to make any further changes in that area. (Removing surplus CR and BS is fair enough, and changing the code to send HOME after the last line is also fair enough, I'm being explicit where I want the cursor to go rather than making a client assumption)

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Re: Teletext to Viewdata server

Post by tjewell » Wed Jan 08, 2020 5:24 pm

Well, personally, I'm certain you shouldn't be hacking your server to fix a problem in one particular client, which might not be working for a completely different reason.
Agreed. But equally, this is the hardware that people used back in the day to access these services, so it would make sense to try and get to the bottom of this too. What I've read implies that Viewdata screens shouldn't scroll, but the lack of a proper spec means we can't be sure there's not a circumstance under which it should. From a developers point of view, having a screen that doesn't scroll is simpler, so why have Tandata added that?

I'm a volunteer at the Centre for Computing History and I note they have the manual for the Td1600, so I'll have a look at that, see if it's an option somewhere. I also have a TD1100 and a TD1400 and it would be good to see if they show the same behaviour - but they're both dead! Again, I'll see if I can borrow a machine from the CfCH, fingers crossed they're not dead too.

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Richard Russell
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Re: Teletext to Viewdata server

Post by Richard Russell » Wed Jan 08, 2020 10:14 pm

tjewell wrote:
Wed Jan 08, 2020 5:24 pm
I note they have the manual for the Td1600, so I'll have a look at that, see if it's an option somewhere.
That would be my guess. Tandata may well have thought it would increase the potential market for the terminal if it had both a scrolling and a non-scrolling mode. We can't rule out that the genuine Prestel server sent some unique sequence which could be recognised by the terminal as a 'disable scrolling' command.

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Re: Teletext to Viewdata server

Post by Soruk » Thu Jan 09, 2020 12:18 pm

New feature (primarily of benefit to Richard's client on Android):
Press * to advance to the next available page.

(For example, from P911, pressing * will take you to P912, subpage 1)

Edit: I have swapped the * and # functionality, so # (and Enter) will advance to the next available page, and * will activate the cursor and thus the onscreen keyboard in Richard's Android client.
Last edited by Soruk on Wed Jan 15, 2020 10:34 am, edited 1 time in total.

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Re: Teletext to Viewdata server

Post by Soruk » Thu Jan 09, 2020 12:22 pm

tjewell wrote:
Wed Jan 08, 2020 10:44 am
Oh, just discovered the 'Hold' command, which makes it much easier to change pages when I'm on a carousel page.
Unless your client is sending extra characters when a page is drawn, even when a redraw appears to obliterate a partly entered page number the keypresses have NOT been lost, just continue to enter the number as if a redraw hadn't happened.

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Re: Teletext to Viewdata server

Post by tjewell » Sat Jan 11, 2020 4:34 pm

I've got my softmodem to dump out the characters as they go past, but it's not telling me much new. I note that the reason Telstar never scrolls for me seems to be more luck than anything else - all the pages have a last line that says "Select item or *page#", and don't go beyond that. So there's never a character sent to the final location on the page that might trigger a scroll.

I haven't managed to get into the Museum yet to look at their terminals and manuals. I do note we've got a document called 'British Telecom teletex service technical guide' - has anyone seen that? If it's not well known, I'll try and pull it out of the archive.

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Re: Teletext to Viewdata server

Post by Soruk » Sat Jan 11, 2020 4:41 pm

tjewell wrote:
Sat Jan 11, 2020 4:34 pm
I've got my softmodem to dump out the characters as they go past, but it's not telling me much new. I note that the reason Telstar never scrolls for me seems to be more luck than anything else - all the pages have a last line that says "Select item or *page#", and don't go beyond that. So there's never a character sent to the final location on the page that might trigger a scroll.

I haven't managed to get into the Museum yet to look at their terminals and manuals. I do note we've got a document called 'British Telecom teletex service technical guide' - has anyone seen that? If it's not well known, I'll try and pull it out of the archive.
Trying not to be the "choosing beggar" type, do they happen to have the Prestel Terminal Specification manual? It seems the answer, if anywhere, would be in there. Unfortunately there isn't a copy of this online anywhere that I can find.

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Re: Teletext to Viewdata server

Post by Richard Russell » Sat Jan 11, 2020 5:38 pm

Soruk wrote:
Sat Jan 11, 2020 4:41 pm
It seems the answer, if anywhere, would be in there. Unfortunately there isn't a copy of this online anywhere that I can find.
There is certainly evidence online to support the belief that Prestel terminals do not scroll. In the main Wikipedia article (and several other places having identical text) it states "Pages did not scroll, but could effectively be extended by the use of frames, which required alphanumeric suffixes to be appended to the numeric page numbers". In the Australian Journal of Educational Technology it says "Surprisingly, videotex cannot 'scroll up' as most microcomputers can". And in ITU-T V.18 'Data Communication Over The Telephone Network' the only reference to scrolling is in connection with the French Minitel system.

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Re: Teletext to Viewdata server

Post by tjewell » Tue Feb 18, 2020 4:33 pm

Quick update - I still haven't got to the bottom of why my Tandata 1616 is scrolling. All my other Tandata terminals are dead sadly and waiting to be fixed. But some good news - Teefex works 99% fine on my latest toy, a Philips NMS3000 'Telematico'. It seems to be a cut down and hardwired MSX machine, made into a Viewdata terminal for the Italian Videotel network.
IMG_20200218_160650.jpg

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Re: Teletext to Viewdata server

Post by Soruk » Tue Feb 18, 2020 4:52 pm

tjewell wrote:
Tue Feb 18, 2020 4:33 pm
Quick update - I still haven't got to the bottom of why my Tandata 1616 is scrolling. All my other Tandata terminals are dead sadly and waiting to be fixed. But some good news - Teefex works 99% fine on my latest toy, a Philips NMS3000 'Telematico'. It seems to be a cut down and hardwired MSX machine, made into a Viewdata terminal for the Italian Videotel network.
IMG_20200218_160650.jpg
Nice!

I'm curious about the 1%...

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Re: Teletext to Viewdata server

Post by tjewell » Tue Feb 18, 2020 5:18 pm

Ha, apologies for putting that 1% of doubt in your mind! Look under the line starting 'MATRIX NETWORK' - I think that should be a row of dashes, but on the Telematico it's U-grave. I'm guessing there's a few differences in the 'standard' to support the characters it would need on the continent. Not sure if all the accents are in the UK viewdata spec?

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Re: Teletext to Viewdata server

Post by guesser » Tue Feb 18, 2020 5:26 pm

Just a different character ROM. I bet if you open it it contains an SAA5053 :)

Find the engineering test page, all the fractions etc will be replaced with Italian national characters
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Re: Teletext to Viewdata server

Post by Richard Russell » Tue Feb 18, 2020 6:12 pm

tjewell wrote:
Tue Feb 18, 2020 5:18 pm
I think that should be a row of dashes, but on the Telematico it's U-grave.
I'm not sure that Viewdata (i.e. delivered over a telephone line) ever acquired the ability, which level 1.5 and later teletext had, for the character set to be specified in the transmitted signal. Instead it probably had to be hard-wired, and since your terminal is of Italian origin it's not surprising that it uses one of the non-English variants (such as the SAA5053 character generator as previously suggested; see Figure 14 here).

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Re: Teletext to Viewdata server

Post by guesser » Tue Feb 18, 2020 8:22 pm

Alternative national option characters can be selected using three of the control bits in a teletext page header. These bits were unallocated in the 1976 broadcast teletext specification but were introduced before the "levels" were formalised and so are part of what was retrospectively defined as level 1 as I understand it.

As three bits are not sufficient to identify all the character sets required, level 2.5 and above includes more character set addressing bits in the new enhancement packets. The character sets are split regionally so that in a level 1/1.5 system receivers can be sold with the appropriate character sets installed for that market. (e.g. teletext chips with Western European language support).

Which level introduced which feature is complicated somewhat by the original levels 2 and 3 being obsoleted and redefined into the levels 1.5, 2.5, and 3.5 we have now.
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Re: Teletext to Viewdata server

Post by tjewell » Sat Feb 22, 2020 4:20 pm

Just got this lovely little machine working, and it works 99.9% perfectly with Teefax. Why a missing 0.1%? Again, not really Teefax's fault, but because I've got no keyboard, just a number pad, I can't hit 'H' to hold the screen. I wonder if this machine might have come with an optional full keyboard? There's a socket on the back for 'keyboard 2', and it's got a different number of pins to the socket for the keyboard, so I can only presume it had a different function.

(Although the machine is branded 'Ace-Telcom', I note it's actually made by Radofin).
IMG_20200222_155918.jpg

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Re: Teletext to Viewdata server

Post by John_Newcombe » Sat Mar 14, 2020 4:51 am

Richard Russell wrote:
Sat Jan 11, 2020 5:38 pm
Soruk wrote:
Sat Jan 11, 2020 4:41 pm
It seems the answer, if anywhere, would be in there. Unfortunately there isn't a copy of this online anywhere that I can find.
There is certainly evidence online to support the belief that Prestel terminals do not scroll. In the main Wikipedia article (and several other places having identical text) it states "Pages did not scroll, but could effectively be extended by the use of frames, which required alphanumeric suffixes to be appended to the numeric page numbers". In the Australian Journal of Educational Technology it says "Surprisingly, videotex cannot 'scroll up' as most microcomputers can". And in ITU-T V.18 'Data Communication Over The Telephone Network' the only reference to scrolling is in connection with the French Minitel system.
Just been speed reading this thread so hope I haven't misunderstood the discussion but my understanding is that videotex terminals should never scroll. Prestel made use of this fact when using 'roll over' (i.e. using a DOWN char on the bottom row to get to the top row quickly) Telstar does the same in that it calculates the shortest route to a specific row thereby saving transmission time, although typically a HOME followed by an UP is more common than a DOWN on the last line (please note that this is currently switched off on Currer and Ellis to help the NXTEL guys).

There may be another factor that helps the Tandata terminal in that pages on Prestel were limited to 960 characters (including esc and ctrl chars) which meant that with non-visible escapes on the page, I don't think the last line would ever be fully populated so the 40th char on the last line would typically never be present.

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Re: Teletext to Viewdata server

Post by John_Newcombe » Sat Mar 14, 2020 5:44 am

Soruk wrote:
Thu Mar 28, 2019 11:56 am
The Viewdata server is at pegasus.matrixnetwork.co.uk port 6502.
I have just added a gateway to this service on Telstar, hope thats OK.

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Re: Teletext to Viewdata server

Post by Soruk » Sat Mar 14, 2020 9:47 am

John_Newcombe wrote:
Sat Mar 14, 2020 5:44 am
Soruk wrote:
Thu Mar 28, 2019 11:56 am
The Viewdata server is at pegasus.matrixnetwork.co.uk port 6502.
I have just added a gateway to this service on Telstar, hope thats OK.
That's absolutely fine! Thank you :D

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Re: Teletext to Viewdata server

Post by tjewell » Sat Mar 14, 2020 8:48 pm

IMG_20200314_175056.jpg
Just got another Tandata terminal working, and this one behaves perfectly. So the scrolling behaviour seems limited it to the Td1616. Perhaps, as John Newcombe says, the fact that Prestel pages were unlikely to reach the last character of the page means this 'feature' wasn't encountered.

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Re: Teletext to Viewdata server

Post by John_Newcombe » Sun Mar 15, 2020 8:52 am

I made a slight error, the max page size was 920 bytes including esc chars etc. anything larger was truncated. After August 1979, a message would appear on line 24 to warn the user, prior to this it was truncated without warning. As I remember, line 24 couldn't be used by an information provider anyway as it was a status line for such warnings so the 920 byte limit is a mute point as the status line would, I am sure, always be less than 40 chars long.

Great to see more Tandatas running well done. Once I get the dial up working I would love to see them displaying Telstar data. I keep getting distracted though. Ho Hum..

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Re: Teletext to Viewdata server

Post by Richard Russell » Sun Mar 15, 2020 11:58 am

John_Newcombe wrote:
Sun Mar 15, 2020 8:52 am
I made a slight error, the max page size was 920 bytes including esc chars etc.
Since this was a Prestel-specific limitation (rather than something imposed by the Videotex specification or transmission protocol) it would have been unwise for a terminal manufacturer to rely on it, because it wouldn't necessarily have applied to other services with which it might get used. But it does seem likely that the TD1616 did exactly that, perhaps by accident (e.g. having only ever been tested with Prestel).

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Re: Teletext to Viewdata server

Post by tjewell » Sun Mar 15, 2020 4:09 pm

IMG_20200315_155226.jpg
Just for you John, here's my Td1616 hooked up to Telstar!

It's still odd that the Td1616 scrolls - as someone would actually have had to sit down and write that code (and given the Td1000 doesn't, it's not a fault across all their machines). But indeed, it was probably never an issue, as these machines probably only connected to Prestel.

I guess it's a point to consider when designing a Viewdata gateway - whether it's a Teletext one like we're discussing here, or the Minitel and Bildschirmtext ones I've been tinkering with. Do you stick to the letter of the Videotex specification, or do you allow for the slight differences in Prestel (given that's what most terminals here in the UK were designed for)? It's a fairly esoteric point, given that the only machine I've discovered with this issue is the Tandata Td1616. Only problem is that the Td1616 is the only terminal I have with composite, so I tend to use it the most. Maybe I need to build myself a little Viewdata to Prestel gateway that imposes the 920 char limit!

John - just to clarify ... So when I see a character 12 (0C) go by, I start counting - and if we get to 920, I stop sending characters? Another 12 resets the counter, obviously?

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Re: Teletext to Viewdata server

Post by Richard Russell » Sun Mar 15, 2020 5:14 pm

tjewell wrote:
Sun Mar 15, 2020 4:09 pm
I guess it's a point to consider when designing a Viewdata gateway
Hacking my telstar.bbc terminal to count the number of received characters, the Engineering Test Page (page 793 on Teefax) totals 1367, which is nearly 50% more than the 920 'limit'!
So when I see a character 12 (0C) go by, I start counting - and if we get to 920, I stop sending characters? Another 12 resets the counter, obviously?
You might want to reset on 30 (0x1E) too.

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Re: Teletext to Viewdata server

Post by John_Newcombe » Sun Mar 15, 2020 6:41 pm

The 920 characters was a storage limit on the old GEC 4040 machines (I think) and I am thinking that this never became part of the videotex standard. Telstar does not have this limit.

I am not sure that the engineering test page was ever part of Prestel but if it was, I can only assume it was stored differently to an IPs page and didn't have the same restrictions, someone here may have the answer.

I guess that counting characters from a '12' maybe a way forward but wouldn't it be better to simply take control of line 24 and truncate it to 39 characters?

I am very interested in how you access Telstar with these terminals, presumably they are dial up?

J

P.S. Can I use the image on the GlassTTY website as an example of a terminal connected to Telstar.

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Re: Teletext to Viewdata server

Post by guesser » Sun Mar 15, 2020 10:56 pm

A lot of the characters on that test page come from the odd numbered clock cracker lines which have 38 control codes. That becomes 76 characters in viewdata. Then the hold mosaics stuff on either side of the engineering and test page labels is similarly solid with control codes.
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Re: Teletext to Viewdata server

Post by colonel32 » Sun Mar 15, 2020 11:02 pm

In NXtel I escape the control codes on the fly for transmission, and I unescape them on the fly for receipt. I also have a fixed 960 byte buffer at both the server and client end, but it can hold a complete page of 8-bit characters.

My understanding is that 7-bit escaping is part of the transmission protocol that’s necessary due to modems of the era, rather than something intrinsic to the characters or control codes themselves.
Last edited by colonel32 on Sun Mar 15, 2020 11:48 pm, edited 1 time in total.

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Re: Teletext to Viewdata server

Post by Richard Russell » Sun Mar 15, 2020 11:31 pm

colonel32 wrote:
Sun Mar 15, 2020 11:02 pm
My understanding is that 7-bit escaping is part of the transmission protocol that’s necessary due to modems of the era, rather than something intrinsic to the characters or control codes themselves.
Yes, but the point is that (as I understand it) Prestel had a 920 character limit including the control and escape characters. I agree that one can consider the 7-bit escaping as being part of the transmission protocol, but it seems that in practice it was done in the editor and consumed memory.

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Re: Teletext to Viewdata server

Post by colonel32 » Sun Mar 15, 2020 11:55 pm

Ah, I see. That is unfortunate.

I wonder if we can invent something where pages are limited to 920 chars by default, but modern clients could opt into not having their pages not being truncated.

Similar to the IAC DO SUPPRESS-TYPEAHEAD codes (or whatever they actually are, I typed that from memory).

Would that cause more problems than it solved?

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Re: Teletext to Viewdata server

Post by Richard Russell » Mon Mar 16, 2020 2:48 am

colonel32 wrote:
Sun Mar 15, 2020 11:55 pm
Would that cause more problems than it solved?
I'm not sure that there is actually a 'problem' that needs solving. Since the 920-character limit was specific to Prestel, and not a guarantee made by the Viewdata specification, no terminal (whether ancient or modern) should be relying on it. Even with the limit I can't see that it necessarily prevents the TD1616 from scrolling, since only a few control characters are needed to move into the bottom row.

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Re: Teletext to Viewdata server

Post by tjewell » Mon Mar 16, 2020 10:00 pm

John_Newcombe wrote:
Sun Mar 15, 2020 6:41 pm
I am very interested in how you access Telstar with these terminals, presumably they are dial up?

P.S. Can I use the image on the GlassTTY website as an example of a terminal connected to Telstar.
Of course you can use the picture! I've also got pictures of Telstar on the Italian MSX terminal and the Telcom Ace if you'd like?

I keep meaning to write up how I'm using these terminals, but we could all die of old age waiting for that to happen. In summary, I'm using this software - https://github.com/irrelevantdotcom/asterisk-Softmodem - which emulates an 1200/75 modem - on a Pi running Asterisk v13 (note, it doesn't work on the latest version). This talks to a Grandstream HT502 ATA, which handles the incoming call from the terminal. I use this ATA specifically because it's one of the few that understand pulse dialling. It's quite expensive compared with the much more common Linksys PAP2t, which is fine if you only care about tone dial. If anyone wants help setting up a system like this, drop me a line.

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