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Author Topic: Resistor values for SCART cable (Read 14194 times)

Offline BruceTanner

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Resistor values for SCART cable
« on: 2013.March.23. 22:40:27 »
I wanted to make myself a SCART cable to connect my Enterprise to a monitor, and used the standard pinout diagram at http://enterprise.iko.hu/pinouts.htm. But then I started wondering if the resistor values were calculated or just a convenient round number (100!). My own calculations suggested that 100R would result in a signal that is too big, so I probed around a bit with an oscilloscope. (Monitors must be quite tolerant because 100R has been used by lots of people for many years!)

The SCART spec is that the RGB signals should be 0-0.7V with an input impedance of 75R and the composite sync should be 0-1V again with an input impedance of 75R.

I wrote a little BASIC program that turned off the status line, turned the border red and cleared the screen. The top channel is ~CSYNC (B5) and the scope triggers off this. The second channel is the red signal (B3). You can see the red border on each side of the video line. The dotted lines nearly 1/4 way down and just over 3/4 way down are the 0V levels of the channels.


8968-0
This is the output from the Enterprise ie. before the resistors. ~CSYNC signal is displayed at 2V/div and the red signal is at 1V/div. Both the ~CSYNC and red outputs are around 3V.

8970-1
The SCART side of the RGB resistors, 100R. ~CSYNC 2V/div, red 0.5V/div. ~CSYNC is just over 2V, red 2.5V. 



8972-2
The SCART side of the RGB resistors, 330R. ~CSYNC 2V/div, red 0.5V/div. Red now just under 1.5V.

8974-3
The SCART side of the RGB resistors, 470R. ~CSYNC 2V/div, red 0.5div. Red now about 1V.


The output of the Enterprise is supposed to be 4V but mine seems to be more like 3V. Although with 470R resistors it is nearer the SCART spec, the red border is noticeably darker and I think I will go for 330R, both for the RGB resistors and the ~CSYNC resistor. In other words, replace all 4 100R resistors with 330R.

Any hardware engineers care to comment?

Online Zozosoft

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Re: Resistor values for SCART cable
« Reply #1 on: 2013.March.23. 22:56:03 »
Background information (what we talked with Bruce):
The 100R comes from a Hungarian magazine (page 25). I think all Hungarian users used these values.

Now I have found in a Dutch magazine (page 10) very different resistors. 470R and also 82R between the signals and GND!
« Last Edit: 2013.March.24. 10:59:00 by szipucsu »

Online Zozosoft

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Re: Resistor values for SCART cable
« Reply #2 on: 2013.March.23. 23:05:32 »
Quote from: Zozosoft
 and also 82R between the signals and GND!
Bruce! Did you try this version?

Offline BruceTanner

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Re: Resistor values for SCART cable
« Reply #3 on: 2013.March.23. 23:31:13 »
Quote from: Zozosoft
Bruce! Did you try this version?
Yes but with the 82R to ground at the Enterprise end of the cable rather than in the SCART plug (I can't see that moving a resistor from one end of the cable to the other would make a huge difference!) The result was a much darker red (too dark) - signal about 0.3V.

Perhaps the Dutch magazine was for going to a monitor/TV that had a high impedance input? (I can't read Dutch any more than Hungarian :(.) Then the 82R resistor would be approximately equivalent to the 75R input impedance of a standard SCART monitor/TV.

Edit: I have typed in the text on that page and passed it through google translate and it says it works well with a Sony television!
« Last Edit: 2013.March.23. 23:50:27 by BruceTanner »

Offline BruceTanner

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Re: Resistor values for SCART cable
« Reply #4 on: 2013.March.24. 00:04:07 »
Something else I have noticed: the red, green and blue signals all have separate grounds, which are all connected to the Enterprise 0V at the monitor connector. (In the "classic" diagram some grounds are connected to B2 and some to A2, but they are both the same!)

But the composite sync input also has a separate ground on pin 17...but that is shown unconnected!

They may well all be connected together within the monitor anyway but just for correctness it looks to me as though pin 17 should also be connected to A2 or B2.

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Re: Resistor values for SCART cable
« Reply #5 on: 2013.March.24. 10:51:12 »
Quote from: BruceTanner
The output of the Enterprise is supposed to be 4V but mine seems to be more like 3V.
Next time when you open the machine check if the output DAC resistors are same as the schematics.
I will check some machines (low and high serial numbers).
« Last Edit: 2013.March.24. 11:00:32 by szipucsu »

Offline BruceTanner

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Re: Resistor values for SCART cable
« Reply #6 on: 2013.March.24. 13:15:16 »
Quote from: Zozosoft
Next time when you open the machine check if the output DAC resistors are same as the schematics.
I will check some machines (low and high serial numbers).
I have just discovered the output is 4V after all...when not connected to anything. When connected to the 75R input of a monitor, it drops to 3V.  :oops:

Online Zozosoft

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Re: Resistor values for SCART cable
« Reply #7 on: 2013.April.09. 13:03:16 »
Bruce! Finally you use 330R resistors? What do you think, change the connection diagram to this values?

Offline BruceTanner

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Re: Resistor values for SCART cable
« Reply #8 on: 2013.April.09. 14:26:04 »
Quote from: Zozosoft
Bruce! Finally you use 330R resistors? What do you think, change the connection diagram to this values?
Yes I am using 330R, and looking on a 'scope the RGB max voltages are nearer the SCART spec max.

The picture looks fine to me and there is no flare or smudging even on a screen full of adjacent bright colours but I suspect you would find it a little dark if you have been using 100R for years/decades!

I had been hoping to play around a bit more but I was ill for a while and now I have a backlog of work to do :cry:

I hesitate to overrule years/decades of people using 100R with no problems just on the strength of my one Enterprise and a couple of TV/monitors. Maybe add a note to the diagram to say use 330R if the picture is too bright? (Or the other way round.)

Do you use a SCART monitor? Have you tried 330R yourself?

Online Zozosoft

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Re: Resistor values for SCART cable
« Reply #9 on: 2013.April.09. 14:51:51 »
Quote from: BruceTanner
Do you use a SCART monitor?
Phillips CM8833 and LG M2362D

Quote
Have you tried 330R yourself?
Not yet, but soon!

Offline Z80System

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Re: Resistor values for SCART cable
« Reply #10 on: 2013.April.09. 15:00:51 »
I have a monitor which can handle 15Khz signal of the EP. (Unfortunately only 200 raster line on screen at once... ((I am so unhappy because of this))) It's a tft, with multiple various input connectors.

I usually test my EP's video signal and nick behaviour with this monitor, because that 200 raster line is brilliant clear, "like printed paper".

With this monitor I use the analog VGA input connector (D-Sub 15) with the EP.

With that connector I do not use resistors at all.

On scart inputs, I have to use resistors too. (I use the "hungarian standard" values of this, I don't remember the exact values.)

So, what is this difference between the two connector types ? Why don't I have to use resistors on D-Sub input too ?
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Offline ikduncan

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Re: Resistor values for SCART cable
« Reply #11 on: 2013.April.09. 15:32:10 »
ill agree with Bruce. Depends on your enterprise and your monitor.

On my 128 the 100R is toooo bright adn the 330R much better, but when i move to another screen the 100R is ok and 330R too dark
On my 64 the 100R is ok and 330R too dark, on the other screen they are both too dark!!!

I think marking the 330R as an option if your picture is too bright is the correct way to go.


whoooah! wire up through analogue vga? whats the pinouts for that?

Offline Z80System

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Re: Resistor values for SCART cable
« Reply #12 on: 2013.April.09. 15:51:26 »
there are many places with d-sub pinout on the net, like this:

http://www.trash.net/~luethi/microchip/datasheets/pinout/pinout.html#vga

just google d-sub 15 pinout

and on that, there are all the EP's signals, like R,G,B,Sync signals an ground(s)

But very-very few modern monitor can handle the 15KHz type signal of the EP ...

Most of the modern monitors say: "Out of frequency" or display blank screen,

because modern monitors can only display signals with 30Khz or above ...
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Offline BruceTanner

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Re: Resistor values for SCART cable
« Reply #13 on: 2013.April.09. 16:58:59 »
Of course the choice isn't just between 100R and 330R - there are values available in between! Eg 180R or 220R might be a good compromise.

A VGA TV/monitor that can display a TV picture is more likely to work than a plain VGA monitor (PAL uses a 15kHz line frequency like the Enterprise). In the UK at the moment unwanted but modern LCD analogue TVs with SCART sockets can be picked up very cheaply as the whole country has just switched over to digital TV.

Offline ikduncan

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Re: Resistor values for SCART cable
« Reply #14 on: 2013.April.09. 18:11:55 »
id also add that in my experience cheap unbranded supermarket tvs are more friendly/accomodating than branded tvs e.g. sony, toshiba, samsung when it comes to old computer equipment and scart inputs