520ST stuck in MONO mode

[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:

[derkom]

The mono monitor grounds the mono pin. It's connected to the 68901, so you might investigate there.

[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

[PhilC]

Thanks Both, gives me something to work with.

[derkom]

Screenshot 2021-04-17 at 18.12.33.png

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.

[PhilC]

@derkom that's what's likely missing, it's not seeing it go high to select colour mode then.

[derkom]

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.

[rubber_jonnie]

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.

[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

[rubber_jonnie]

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?