Need some help fixing CPC6128 not booting

[gerald]

Now, on cheap logic analyser, I would back Bryce on limited usablility of a 8 channel logic analyser to debug a CPC when you've run out of your multimer and finger sensing bullets.
A bare minimum is 24 channel, which allow you to see at the address bus and control signal of the Z80, ideally a 32/34 way is the sweet spot for a CPC/8bit.

While I understand the budget point, the (re)usability is also a strong point.
Remember that USB stuff always requires a driver, which you may miss in few years, because the manufacturer does not provide an updated one for your new OS, or because the original manufacturer found a way to detect counterfeit HW (FDTI nuked counterfeit RS232 adapter by driver update).

[Bryce]

Obviously impedance is much more critical on a scope, but 100K on an analyser can have a negative effect on rise times. On low frequency equipment this isn't usually an issue, but on high frequency equipment this can be critical. It all depends on what frequency range you are working in. Maybe just due to the type of stuff I do (did) at work, I tend to go for as high an impedance and as low a capacitance as possible. They don't even mention the probe capacitance on cheap devices.
The analysers at work (way outside my price limit - ie: in a different galaxy) are 10M / 0.7pf. My hobby analyser 1M / 5pf.

(FDTI nuked counterfeit RS232 adapter by driver update).

Oops, wasn't aware of that. Must check whether my cheap copy still works!!

Bryce.

[pelrun]

My analysers are fully open source, so no issues there


I spent my monies on an original HP 16550 logic pod and high quality micro test hooks. There's a mod for the OBLS that properly terminates it for the isolation network in the pod and biases it for various trigger levels. Even without that the inputs are buffered and have a pretty high input impedance (far higher than the 1Mohm of the saleae.) But yeah, it's really only for under 100MHz.
This is where it's beneficial to have a friend with a lot of expensive test equipment in the odd instance I need to work on something more demanding

[Bryce]

Then you're sorted Mixing the HP Pod with the low cost analyser is an interesting idea.

Bryce.

[RobertM]

Bit of an update -

I installed a 3.5" floppy as drive A. It's hard to make them fit lol. I had to remove a fair amount of plastic.

I noticed the screw spacing on the side of the CPC is the same as the screw spacing on a 3.5" floppy but the 3.5" sits a little inward of the front of the CPC. It works and I copied the system disks with a floppy in a PC with CPCDiskXP. I couldn't get CPCDiskXP to do *anything* with a USB floppy so I gave up on that.

I have ordered another drive and 50 Disks.

I noticed that the edge connector for the external drive has a couple of +5 Volt pins so I am going to try an external 3.5" disk from the edge connector that uses power from the same connector.

I have been using the composite monochrome output from the video connector with a monitor that has VGA, s-Video and Composite inputs so it can scan down to the low 50Hz PAL frame rate.

I tried connecting the signals from the CPC Video connector to the VGA input by connecting the CPC composite sync to both the vertical and horizontal sync signals going into the monitor. That almost worked but the Hsync was out at the top of the screen as the Vsync was interfering.

I then tried collecting all the signals directly from the gate array and then the image was stable but the active area of the screen started right on the left edge of the screen. It looks like the gate array delays the Hsync signal in some way.

So now I will make a box with a RGBs to colour composite and s-Video converter. I will run the power plug to the box and have a second lead that then conveys power (5 Volts) to the CPC. I will also have a 6 Pin Din lead to connect the box to the CPC. On the output side I will have a composite colour output and an s-Video output for the monitor.

I am going to build the circuit with an AD722 (not AD724) as it has directly coupled inputs. I will be making it the worst possible way to test, ie vero type board in a plastic unshielded box.

I am hoping to get a better picture with the directly coupled inputs but at same time it will be far more sensitive to RF induced into any possible ground loop. The AD722 has both a digital and analog grounds and I haven't yet worked out how would be best to attempt to isolate the two. My concern is that because the circuit in the box shares the same power supply as the CPC then I have to worry about any RF that might be induced into the power cable between the box and CPC, even the earth itself.

I was going to put 100nF caps in the DC input and output of the box and run the digital rail of the AD722 from the 5 Volts. I was then going to connect the Video signal ground to the analog ground of the AD722 and use an inductor to decouple the 5 Volt rail that would otherwise be directly shared with the digital side of the AD722.

Does this sound like it could work or am I lining myself up for failure by sharing the 5 Volts power supply with the CPC and s-Video converter Any comments

[gerald]

I noticed that the edge connector for the external drive has a couple of +5 Volt pins so I am going to try an external 3.5" disk from the edge connector that uses power from the same connector.

Are you sure these are connected ? The 6128 schematic show them on the disk side of the connector, but not on the main board.
I think this is a left over from the DDI1 schematic where the drive indeed powered the interface.

As for powering the drive from the CPC trough that connector, there are no real issue. I have one CPC configured that way.
You will have to patch the drive and the CPC. I am using pin 6 of the connector, which is usually unconnected on drives.

[RobertM]

Are you sure these are connected ? The 6128 schematic show them on the disk side of the connector, but not on the main board.
I think this is a left over from the DDI1 schematic where the drive indeed powered the interface.

As for powering the drive from the CPC trough that connector, there are no real issue. I have one CPC configured that way.
You will have to patch the drive and the CPC. I am using pin 6 of the connector, which is usually unconnected on drives.

Good point. I though that was just a convention in the schematic but now I look again. I see that all the other 5 Volt rails are lined in and the FDD connector 5 Volt rails are NOT. So I will check before I get too carried away.

[RobertM]

As for the ground loop issue ... I cheated and just looked at the schematic for the monitor. They just short the grounds and hope for the best lol. I will do the same and just decouple the 5 Volt rails. If I do have trouble then I I can make changes then.

[Bryce]

The pins marked 5V on the 6128 floppy port aren't connected to anything. These are the pins that the FD-1 uses to send power to the DDI-1 so an incoming 5V would be expected if an FD-1 was connected.
As for the AD722 Analogue / digital supplies, there's no need to isolate them so much from each other. If you connect a 100nf capacitor directly across the power pins of each power input it will work fine. See my schematic for the AD724: RGB SVideo - CPCWiki  What's more critical is the crystal start-up. Make sure to use a capacitor less than 7pf across the crystal and that the leads between the crystal and the AD aren't too long or parallel or you will get a really shitty picture. The AD722 is also very sensitive to stray capacitance generally, using a stripboard may cause problems that you don't get on a properly laid out board.
Btw: The 724 is an improvement over the 722, the three capacitors on the RGB Inputs improve the picture over the 722 solution.

Bryce.

[ikonsgr]

Finally i found a friend who can burn the 27C256 eeprom with fw/basic! 
Now i was thinking to make a new rom for amsdos too. Can i use another 27c256 eeprom chip ,and if yes,where should i put the 16kb amsdos rom,to upper or lower 16k segment?

[Bryce]

Ideally you should use a 27C128. If you use a 27C256 then you have to either:

A) Program AmsDOS to the lower portion.

B) Program AmsDOS to the upper portion and connect A14 (pin 27) to 5V.

Alternatively

C) You could program AmsDOS to the lower portion and ParaDOS to the upper portion and add a switch to switch pin 27 between GND and 5V so that you can select the DOS you want to use.

Pin 27 is normally connected to VCC (Pin 28) on the CPC mainboard, so you'll have to cut the connecting track on the mainboard for options B or C.

Bryce.

[CraigsBar]

I have several (hundred) amd 27c128 chips if you need some.

[Munchausen]

What's more critical is the crystal start-up. Make sure to use a capacitor less than 7pf across the crystal and that the leads between the crystal and the AD aren't too long or parallel or you will get a really shitty picture. The AD722 is also very sensitive to stray capacitance generally, using a stripboard may cause problems that you don't get on a properly laid out board.

+1. I had exactly this problem with the AD724 and strip board, and it took me forever to figure out what was going wrong. Make those tracks to the crystal as short as you possibly can because they are really sensitive!

[ikonsgr]

Ideally you should use a 27C128. If you use a 27C256 then you have to either:

A) Program AmsDOS to the lower portion.

B) Program AmsDOS to the upper portion and connect A14 (pin 27) to 5V.

Alternatively

C) You could program AmsDOS to the lower portion and ParaDOS to the upper portion and add a switch to switch pin 27 between GND and 5V so that you can select the DOS you want to use.

Pin 27 is normally connected to VCC (Pin 28) on the CPC mainboard, so you'll have to cut the connecting track on the mainboard for options B or C.

Bryce.

That's a very nice idea bryce, dual booting cpc! I will give it a try and see what happens! 

[Bryce]

I did a proper tutorial on what you need to do here: DualOS - CPCWiki

Bryce.

[RobertM]

The pins marked 5V on the 6128 floppy port aren't connected to anything. These are the pins that the FD-1 uses to send power to the DDI-1 so an incoming 5V would be expected if an FD-1 was connected.
As for the AD722 Analogue / digital supplies, there's no need to isolate them so much from each other. If you connect a 100nf capacitor directly across the power pins of each power input it will work fine. See my schematic for the AD724: RGB SVideo - CPCWiki  What's more critical is the crystal start-up. Make sure to use a capacitor less than 7pf across the crystal and that the leads between the crystal and the AD aren't too long or parallel or you will get a really shitty picture. The AD722 is also very sensitive to stray capacitance generally, using a stripboard may cause problems that you don't get on a properly laid out board.
Btw: The 724 is an improvement over the 722, the three capacitors on the RGB Inputs improve the picture over the 722 solution.

Bryce.

Wow I fell off the planet there for a while - found this place called 2000's

The resolution is excellent but the game play sux.

Anyway - I recently got back to this PAL baseband modulator.

I had two chips that were like AD722 and AD724. I used the one with capacitive coupled inputs as the datasheet for the other had obselete across the page.

The baseband modulator works for composite but not s-video and I can't seem to tune the coulor burst freq. I see that Brice recomends a 7pF trimmer and I have been using a larger one so I will drop that and see.

I made the cable for the s-video so maybe I functioned that up.

[RobertM]

I did a proper tutorial on what you need to do here: DualOS - CPCWiki

Bryce.

Thanks Bryce - I will have to get back to this.

[RobertM]

Ideally you should use a 27C128. If you use a 27C256 then you have to either:

A) Program AmsDOS to the lower portion.

B) Program AmsDOS to the upper portion and connect A14 (pin 27) to 5V.

Alternatively

C) You could program AmsDOS to the lower portion and ParaDOS to the upper portion and add a switch to switch pin 27 between GND and 5V so that you can select the DOS you want to use.

Pin 27 is normally connected to VCC (Pin 28) on the CPC mainboard, so you'll have to cut the connecting track on the mainboard for options B or C.

Bryce.

Hi Bryce - I will assume that the original question was about a duel operaqting system because I don't remember.

Since I was here last I have learnt VHDL so I might see what I can do with a CPLD / FLASH on the expansion port.

[Munchausen]

The baseband modulator works for composite but not s-video and I can't seem to tune the coulor burst freq. I see that Brice recomends a 7pF trimmer and I have been using a larger one so I will drop that and see.

I built one based on an AD724 on some veroboard in the past, and had exactly the same issue. I eventually fried it and my CPC by connecting 12V instead of 5V and just bought one from Bryce later, but that's a different story! Anyway, the issue with tuning it turned out to be that the tracks to the crystal were too long - make sure they are as short as possible because parasitic capacitance can really screw this up. In fact - keep all the tracks as short as possible!

[RobertM]

I built one based on an AD724 on some veroboard in the past, and had exactly the same issue. I eventually fried it and my CPC by connecting 12V instead of 5V and just bought one from Bryce later, but that's a different story! Anyway, the issue with tuning it turned out to be that the tracks to the crystal were too long - make sure they are as short as possible because parasitic capacitance can really screw this up. In fact - keep all the tracks as short as possible!

lol well mine was probably even worse.

I was trying (for the first time) a minium etch board and I couldn't find how to make a keep out zone for the crystal tracks in the CAD so it ended up being 20mil tracks with 20mil clearenece.

That was one of several problems lol. The tracks are long to put the trimmer cap near some color ballance pots.

Anyway - it's a proto so no biggie.