520ST stuck in MONO mode

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PhilC
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520ST stuck in MONO mode

Post by PhilC »

Hi All,

Can someone please remind me how the ST detects a mono monitor is connected. I'm using Exxos's VGA adaptor which works fine with other STSs but this particular one boots to MONO regardless of monitor selection jumper position.

I'm presuming it sends 5v back on Mono pin for Mono monitor but I'm getting no changes between colour or mono selection. I think I have another dead component somewhere. So far two transistors and a number of diodes just to get this far :roll:
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derkom
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Re: 520ST stuck in MONO mode

Post by derkom »

The mono monitor grounds the mono pin. It's connected to the 68901, so you might investigate there.
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rubber_jonnie
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Re: 520ST stuck in MONO mode

Post by rubber_jonnie »

@PhilC Taken from a useful website:

Pin 4 - Monochrome detection (ST/STE):
This pin is connected to pin 17 of the MFP68901 circuit and is used to set the Atari in Hi-resolution when grounded and to Med/Low-resolution when left open. A transition on this pin generates a reset of the system.

So when pin 4 of the 13 pin plug is grounded, the resolution is set to high.

Taken from here: https://info-coach.fr/atari/hardware/interfaces.php
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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PhilC
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Re: 520ST stuck in MONO mode

Post by PhilC »

Thanks Both, gives me something to work with.
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derkom
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Re: 520ST stuck in MONO mode

Post by derkom »

Screenshot 2021-04-17 at 18.12.33.png
Screenshot 2021-04-17 at 18.12.33.png (28.9 KiB) Viewed 2784 times

The mono detect pin connects to the 68901 thusly. It's pulled up via RP1, and the mono monitor grounds it when connected.

This excerpt is from the 1040ST schematic, but I assume the 520 is identical.
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PhilC
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Re: 520ST stuck in MONO mode

Post by PhilC »

@derkom that's what's likely missing, it's not seeing it go high to select colour mode then.
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derkom
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Re: 520ST stuck in MONO mode

Post by derkom »

rubber_jonnie wrote: Sat Apr 17, 2021 4:12 pm Pin 4 - Monochrome detection (ST/STE):
This pin is connected to pin 17 of the MFP68901 circuit
This is not correct. I think whoever wrote this probably misread the "I7" on the schematic I clipped from above. The mono detect pin is connected to pin 29 on MFP.
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Re: 520ST stuck in MONO mode

Post by rubber_jonnie »

derkom wrote: Sat Apr 17, 2021 4:17 pm
rubber_jonnie wrote: Sat Apr 17, 2021 4:12 pm Pin 4 - Monochrome detection (ST/STE):
This pin is connected to pin 17 of the MFP68901 circuit
This is not correct. I think whoever wrote this probably misread the "I7" on the schematic I clipped from above. The mono detect pin is connected to pin 29 on MFP.
Yes, I have just looked at the schematic and agree it should be pin 29 and not 17.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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PhilC
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Re: 520ST stuck in MONO mode

Post by PhilC »

One step forward, one sideways.

With no monitor connected pin 29 stays high, with the monitor adaptor plugged in and with it set to colour it still shows mono mode.

Disconnect pin 4 on the monitor socket and I get low resolution. I've tried playing around with the 5v pullup value with inconsistent results. Even disconnecting the original pullup and getting 5v from elsewhere doesn't make a difference.

I can reliably reconnect pin 4 and get mono, then disconnect it and get low res colour.

#baffled
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rubber_jonnie
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Re: 520ST stuck in MONO mode

Post by rubber_jonnie »

PhilC wrote: Sat Apr 17, 2021 7:33 pm One step forward, one sideways.

With no monitor connected pin 29 stays high, with the monitor adaptor plugged in and with it set to colour it still shows mono mode.

Disconnect pin 4 on the monitor socket and I get low resolution. I've tried playing around with the 5v pullup value with inconsistent results. Even disconnecting the original pullup and getting 5v from elsewhere doesn't make a difference.

I can reliably reconnect pin 4 and get mono, then disconnect it and get low res colour.

#baffled
Yet the cable etc is fine on other STs?
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...
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