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Old 23rd Dec 2017, 5:35 pm   #1
MeerKat
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Default Connecting a BenQ BL912 to a BBC Master computer

Hi All,

Anybody had any experience of doing this?
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Old 23rd Dec 2017, 5:47 pm   #2
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

For anyone unaware, this is a fairly modern LCD monitor: https://www.amazon.co.uk/BenQ-BL912-.../dp/B00DZG2IWE
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Old 23rd Dec 2017, 7:28 pm   #3
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Probably better to use a LCD TV with SCART, the output of the BBC RGB output being RGB with composite sync just at TTL levels iirc (so some resistors on the RGB lines should work), and an appropriate adaptor cable as unless the screen has composite sync as an option and even if it does it will need circuitry to get it to work, not just a passive adaptor.

This is how my S330 digital sampler that has RGB video out is connected, and the picture is as crisp as you like.
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Old 23rd Dec 2017, 8:36 pm   #4
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

I used to run my BBC B into the SCART socket of a TV using resistors to drop from TTL levels to SCART. I don't remember off-hand what values, but I can dig out the cable I made up to check - if that would be helpful.
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Old 23rd Dec 2017, 8:43 pm   #5
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

The BenQ is likely not to work directly with a BBC Micro. The BBC's 15.625kHz horizontal scan rate is lower than most LCD monitors claim to support (they generally don't go lower than 31.5kHz VGA) though I have seen some that work by happy accident. Even if you were to wire a cable from the Beeb's RGB output socket to the monitor's VGA input you'd probably still need a sync separator and even then the monitor might not produce a picture. Not worth the effort of trying unless you're really curious, desperate or already have the wiring set up. The most effective approach is to use a SCART-VGA converter and a BBC RGB to SCART-VGA lead, which are readily available. The performance of the SCART-VGA converters can be somewhat variable though.

Unless you're really attached to the BenQ monitor you're probably better off doing as dglcomp says and finding an LCD or CRT TV with a SCART socket that supports RGB input (the vast majority do).

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Old 23rd Dec 2017, 11:21 pm   #6
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Thanks for all your answers guys. The BenQ is known to be able to get down to
the scan rates used by the BBC and other computers of that era. Its VGA input needs horizontal and vertical sync, the BBC only outputs composite at its RGB socket. It's possible though to delve inside and pickup the H & V syncs separately. A small interface will be needed that I can make but I was just wondering if anybody had done anything similar before, save reinventing the wheel so to speak. I have ideas for a circuit and will make one up and publish it in due course. I have tried one of those upscalers you see advertised on Ebay and elsewhere but couldn't stop it twitching, even without that the quality was very so so. The commercial seller claimed it was perfect when they tested it so obtained a refund.
Happy Chrismas all and if anybody does have a circuit known to work it would be speedily built.
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Old 24th Dec 2017, 2:44 pm   #7
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Hi,
I think with some monitors it is possible to use two resistors and feed the composite sync into both horizontal and vertical sync inputs ....
But I seem to remember that with one monitor ( using the vga connector ) I tried it with it turned out that one of the sync signals needed to be of the opposite polarity to the other one.... this needed a bit more circuitry.
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Old 24th Dec 2017, 3:26 pm   #8
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

OK, if the BenQ is known to work at low frequencies that's 90% of the problem solved!

A sync separator doesn't have to be complicated. The monitor will be quite happy with composite sync fed to the horizontal sync input, so that leaves just the vertical to sort out. If you want a simple building block, the LM1881 chip needs hardly any external components and will give you the vertical sync output you need. Or, from the junk box, a simple RC filter (time constant about 50us or so) followed by a transistor or logic gate Schmitt trigger will do the job. I built one like this to run a Beeb into an EGA monitor years ago and it worked fine.

You'll want resistors in series with the RGB video outputs from the Beeb to feed to the VGA input. I'd start with 330 ohms and see how it looks.

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Old 24th Dec 2017, 7:10 pm   #9
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

The nice thing about the LM1881 is that it takes in a video-level composite sync signal but outputs sync pulses at whatever voltage level the IC is running on, so for 0V-5V sync pulses you run the IC on a 5V supply. I think it needs just three external components. (It is an 8-pin IC).
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Old 27th Dec 2017, 10:35 pm   #10
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

I'm about to go through the same exercise, although in my case with one of the pile of old HP L1502 displays I've picked up.

SCART TVs are not common here - I do have an old Goldstar with one, but would need to track down a SCART plug and make up a cable, and then have no idea if it even accepts RGB through it! It's also a pain having to lug around a 20" TV.

I do have a nice little 13" NEC Autocolor TV, but with no RGB in and only a VHF tuner it's not helpful here.

Instead, I was going to try a GBS8200 that I purchased a while ago, mainly to use with my various SORD computers and an Apple IIe with RGB card.

Once I get the right DIN plug and make up a cable I'll report back here, but in the meantime it may be a good option to look at.
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Old 28th Dec 2017, 3:24 am   #11
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Most monitors will accept a composite sync on the horizontal sync input, but you might need to invert it (an EOR gate with the other input switchable to 0V or +5V can be used as a "programmable inverter-or-not"). You could even just use the BBC's composite video output as a negative-going sync signal.

One of the BBC's RGB out pins is carrying +5V, which you could connect to pin 16 of a SCART socket to switch it into RGB mode.

I think SCART sockets were required to be RGB-capable on any TV larger than 35cm. My first SCART-equipped TV was an early 1990s-vintage 51cm. CRT set, which certainly accepted RGB as I used to run my first-generation Sony PlayStation through it.
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Old 28th Dec 2017, 5:59 am   #12
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Quote:
Originally Posted by julie_m View Post
I think SCART sockets were required to be RGB-capable on any TV larger than 35cm.
If that's referring to my comment, then it is probably true in the UK, but anything is possible here where SCART is very uncommon.

This thread has prompted me to get the GBS8200 up and running though - at least as far as discovering that the hunk of multicore shielded cable I am sure was in the workshop is AWOL. After a trip to Jaycar in the morning to grab some more (and a 3x AA holder) I'll know if the BBC Master I have works though it (assuming it is alive, but I'm hopeful!)
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Old 28th Dec 2017, 12:01 pm   #13
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

The ASTEC switched mode power packs used by Acorn would definitely benefit from some new capacitors. You might be able to bodge in a standard PC power pack -- the BBC motherboard needs +5V, +12V and -5V.
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Old 28th Dec 2017, 4:53 pm   #14
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

I would also look at the GBS8200 as arjoll mentioned. I use one with one of my A1200s into an LCD VGA monitor and it works brilliantely.
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Old 29th Dec 2017, 3:50 pm   #15
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Hi All,
Thanks for all your tips and suggestions. I have gone down the LM1881 route as I have 3 BEEBs. The BBC RGB DIN connector has 0v, 5v, red, green, blue and composite sync, all except 0v & 5V are at TTL levels via 68 ohm resistors. So that I can use it with any of my BEEBs I have made up a DIN to D-sub lead with a small interface box in it with a small circuit board for the IC etc. The LM1881 needs 0.5 - 2v p-p composite video signal input so I have potted it down with a 6.8k/3.3k divider. 330 ohm resistors are in each of the RGB feeds. It works well with no jitter at all, it's not as good as a good CRT monitor but is more than adequate for my pensioner eyes even in Mode 0.
I've recapped all three of my ASTEC PSUs, there are three caps that fail after many years and the mains filter ones make an awful stink when they go. Another mod I have made is put a low speed 8cm fan in the lid, even after hours of use the PSU metal work is cold to the touch, should extend its life some. I have also just purchased John Kortincs ReCo6502Mini, quote; "ReCo6502Mini is a remake, using modern parts, of the Acorn 65C102 Co-Processor. ReCo6502Mini is faithful to the original specification. But it offers one major enhancement. It will run at the original 4 MHz, but also at 2, 8 and 16 MHz.". Not installed it yet but if this performs as promised it will be a steal at less than £50 delivered.
Just had a look at the GBS8200, the circuit board looks identical to one in the the £70 jobbie I rejected and got a refund for, as i mentioned before the commercial sellers said it tested fine but I had tried it with 3 different BEEBs and 3 different modern monitors, one being a DELL U2412M, no matter what I tried I couldn't stop the random about 2 per second jitters. I see you can get them on ebay for circa £10!
Happy New Year to you all!
P.S. If anybody wants the exact circuit let me know.
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Old 3rd Jan 2018, 11:46 pm   #16
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Job done now so Mods please close and thanks to all who contributed.

Andrew
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Old 4th Jan 2018, 1:17 am   #17
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Just for interest's sake, how did you wire it up in the end?

It might help some future reader with a similar problem .....
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Old 4th Jan 2018, 4:00 pm   #18
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

As requested, pictures not the best but you get the idea.

Andrew.

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Old 6th Jan 2018, 11:50 pm   #19
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

Oooops, just to clarify the IC is an LM1881.

Andrew.
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Old 8th Jan 2018, 5:57 pm   #20
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Default Re: Connecting a BenQ BL912 to a BBC Master computer

I've got the full BBC setup buried somewhere in the depths of the loft. It's complete with an original monitor and printer. It's got the proper matching colour coded (cream with brown framework) matching work station trolley with shelves and mounted power sockets on the side, however, I have this in use with more modern equipment at the moment. I'll have to dig it all out sometime and see if it still works, but certainly not this time of year, perhaps a job for this coming summer. I'll perhaps have to resist my usual practice of a quick visual check and then applying mains and see what pops as there's probably some filter capacitors across the mains input waiting to weld switch contacts together and causing a lot of smoke!
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