Search found 142 matches

by SainT
Mon Feb 04, 2013 2:24 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Well, I've given the 4.7kR in / shunt 1kR out a go, and as the tpad calculator predicted, it's in the region of -22db attenuation. Dropping the ~4.9v to ~360mv and making the signal a right mess. I'm going back to the level converter and adding in the resistors for ground loop I think. Thanks for th...
by SainT
Sat Feb 02, 2013 4:43 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Thanks for the response simbo, although I'm confused as ever. I did use the calculator link you posted as it happens with 50R in and out impedance and 3.6db attenuation, which gives you 10R series in/out resisters and 120R shunt. Which as it happens does attenuate correctly, just at the source also....
by SainT
Sat Feb 02, 2013 2:13 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Twisted pair was just a quick test due to lack of 50R coax. I'm getting some coax, so i'll see how that goes.

I have no idea what impedance the solid core wire I have is. Could the impedance mismatch cause the source attenuation I am seeing?
by SainT
Sat Feb 02, 2013 9:10 am
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Hmm, a ground loop could explain some of the noise I seem to get, but would the 4.7R in the ground affect how the source signal is attenuated? I don't have any 4.7R or 33R to hand, so I'll have to wait a while to give that a try. However I see people tend to use between 10R and 100R for isolation re...
by SainT
Fri Feb 01, 2013 9:36 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Right, scope firmware patched, now it's 100Mhz. Much better for viewing the clock -- it's not square, but it ain't bad. Ok, I've wired up the t-pad with 10R and 120R and it attenuates the signal beautifully down to my target voltage. However it seems to be attenuating the signal at the source as wel...
by SainT
Fri Feb 01, 2013 9:40 am
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

I wouldn't advise to put a sine wave into a FPGA digital input. The slow edges can cause jitter, uneven duty cycle etc. /if you want to convert take a sine wave digital you might want to use schmitt trigger gate. Or just use a level converter and do it digitally with impedance controlled lines and ...
by SainT
Tue Jan 29, 2013 10:46 am
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Thanks for the clarification simbo. I've ordered some suitable resistors for a t-pad setup to drop the needed 3.6db.

And thanks to everyone else for their comments, I'm sure they'll come in handy once I get the clock stable. Undoubtedly there will be some issues with the signal lines as well... :D
by SainT
Mon Jan 28, 2013 1:29 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

I think I understand simbo's load resistor method now. (??) The Cyclone II (which I am using) has a PCI clamping diode to protect the input from overvoltage upto 25ma. So the series resistor restricts the current from the clock line as to not burn out the clamping diode then the diode restricts the ...
by SainT
Mon Jan 28, 2013 11:02 am
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Looking at the input / output characteristics a bit more, the 5v TTL and 3.3v LVTTL have the same VIL / VIH, just a different VOH. http://www.interfacebus.com/voltage_LV_threshold.html Is it possible to clamp a signal? Attenuating the signal would work, but just curious if you can clamp the TTL out ...
by SainT
Sun Jan 27, 2013 6:02 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

After reading lots of things, I understand impedance matching is important. I just about understand impedance now I think, previously I had no clue. The 74s257 which is used to select the clock on the STE and drive MC seems to have a 50R output impedance from the datasheet (I think this is right). I...
by SainT
Sun Jan 27, 2013 2:37 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Thanks for all the info. I'm still a bit puzzled how a 250R load with a 50R to ground suitably lowers the voltage. If this is acts as a voltage divider with 250/50 then this would drop it to 1v...?? Sorry for all the questions, this really isn't my area of expertise. I'm trying my best to read up on...
by SainT
Sat Jan 26, 2013 9:32 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Putting a scope on the 32mhz line (input to clock select) and MC line (output from clock select) you're looking at ~4.7v and ~5v vpp. The 32mhz line is straight from the PLL syncing the video carrier and main clock, so that's as good as it gets level wise. Now to figure out quite what the T pad does...
by SainT
Sat Jan 26, 2013 4:27 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

Cheers simbo - I'll check the clock. The place I took it from the motherboard was on the output of a ttl chip as I remember (to do with genlock clock select) and was 5v, but I'll have a poke around with the scope and see if there's a more suitable place to take it from where the level is lower. As y...
by SainT
Sat Jan 26, 2013 1:44 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

I have two power connections to the board, 5v fed from the st and 3.3v fed from the fpga, which is powered from the pc via USB. Both power connections have a 100nf cap placed between gnd and power. I have wondered if the two power sources are an issue, and also connecting the USB ground to st ground...
by SainT
Sat Jan 26, 2013 8:41 am
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

This may sound a silly question, but do you just get crosstalk on connected signals? If I unplug the ribbon from the converter then everything is stable without the resistor. I assume the signal has to be flowing somewhere to get a problem. Also if I connect just MC and a signal from the other side ...
by SainT
Fri Jan 25, 2013 11:10 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Re: Signal Noise

I've also just realised while the 70R resistor is stopping noise issues on the ST side of things, the clock on the FPGA side is getting corrupted such that it is unusable with more than just the MC clock hooked up to the voltage converter. On the converter board itself both 5v and 3.3v power are dec...
by SainT
Fri Jan 25, 2013 10:58 pm
Forum: Hardware
Topic: Signal Noise
Replies: 45
Views: 5358

Signal Noise

I've tapped off the MC clock, digital RGB and h/v sync from various places on the motherboard and I'm feeding this into an 74ALVC164245 to convert it down to 3.3v before feeding into an FPGA. Now, I'm a bit of a n00b at electronics, so I'd just like to get some advice on what I'm seeing -- and there...
by SainT
Wed Jan 23, 2013 9:24 am
Forum: Hardware
Topic: Atari ST Schematics
Replies: 5
Views: 2104

Re: Atari ST Schematics

Thanks krupkaj, that's a lovely scan! The STE schematics are very iffy in comparison.

-- James
by SainT
Tue Jan 22, 2013 11:32 am
Forum: Hardware
Topic: Atari ST Schematics
Replies: 5
Views: 2104

Atari ST Schematics

Does anyone have the schematics for the original 260 and/or 520 ST / STM ?

I've looked in the "usual places", but cant find them.

ta!
by SainT
Fri Dec 21, 2012 10:24 pm
Forum: Hardware
Topic: STE Microwire Read / Write?
Replies: 10
Views: 2199

Re: STE Microwire Read / Write?

as i say this is already available via the dma port and simple id hog setting on the scsi protocol simple microwire is a very simple thing to encode to a scsi ID why the shifter comes into it escapes me~?? why people dont use DMA control and the cart port for 16 bit i/o i dont know???? True, I'd no...
by SainT
Fri Dec 21, 2012 7:08 pm
Forum: Hardware
Topic: STE Microwire Read / Write?
Replies: 10
Views: 2199

Re: STE Microwire Read / Write?

There are certainly plenty of other ways to do i/o. Use STE joypad ports for 16 bit bi-directional i/o, hook up to the parallel port, use the serial port, use the DMA... But the fact the microwire is on a multi-device bus, and very much unused on the STE, it seems like an interesting option for comm...
by SainT
Thu Dec 20, 2012 7:33 pm
Forum: Hardware
Topic: STE Microwire Read / Write?
Replies: 10
Views: 2199

Re: STE Microwire Read / Write?

I did wonder if you'd respond me old mate, you seem to have the answer to most things. :D If microwire is usually a 4 wire, then I do tend to agree that the shifter only has the data out, clock and enable lines. It makes sense really as they only needed write access over microwire for the LMC, not r...
by SainT
Thu Dec 20, 2012 5:12 pm
Forum: Hardware
Topic: STE Microwire Read / Write?
Replies: 10
Views: 2199

STE Microwire Read / Write?

I'm starting to tinker with the old ST hardware, and wanted a way to talk to another device. Doing anything memory mapped is tricky as all that is handled by the MCU, and accessing the bus requires lots of wiring, which is a bit of a pain. Then I remembered the good old microwire. It's easy to tap t...
by SainT
Tue Feb 12, 2008 4:29 pm
Forum: FPGA Chat
Topic: FPGA 68000
Replies: 77
Views: 44914

Re: FPGA 68000

Yeah, the Suska project is what got me interested in the first place. There is also some interesting work on old arcade games, C64, Amstrad CPC and so on... FGPA's are interesting bits of kit.

I'll drop him a line...

-- James
by SainT
Tue Feb 12, 2008 9:59 am
Forum: FPGA Chat
Topic: FPGA 68000
Replies: 77
Views: 44914

FPGA 68000

I'm thinking about getting an FPGA board to have a mess around with some 68000 based hardware hacking, does anyone have any suggestions for something fairly cheap which has enough gates to run something like an ST? I was looking at the Spartan-3 with a XC3S400 (http://www.enterpoint.co.uk/store/erol...

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