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)

tjewell
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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.

Soruk
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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.

Soruk
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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.

tjewell
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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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Richard Russell
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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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