RE: [N8VEM-S100:1039] Re: S100 VDP success

["Andrew Lynch" <LYN...@YAHOO.COM>]

Hi Tom!  Thanks!  Well, I am hoping findchips.com is right...

http://www.findchips.com/avail?part=LM1881

It is why we prototype though.  Sometimes designs that look good on paper
really stink in reality!  :-)

Thanks and have a nice day!

Andrew Lynch

> -----Original Message-----
> From: n8vem...@googlegroups.com [mailto:n8vem-
> s1...@googlegroups.com] On Behalf Of Tom Lafleur
> Sent: Friday, August 10, 2012 2:52 PM
> To: n8vem...@googlegroups.com
> Subject: Re: [N8VEM-S100:1039] Re: S100 VDP success
> 
> Just a note:  last time I needed an LM1881, they were VERY hard to find!
> 
> tom...
> 
> On Fri, Aug 10, 2012 at 8:55 AM, Andrew Lynch <LYN...@yahoo.com>
> wrote:
> > Hi Leon!  I was wondering if you ever got the RAMDAC to respond during
> > build and test?  By itself the RAMDAC would not be sufficient for VGA
> > display but with some additional circuitry in the prototyping area I
> > think it may be possible with the V1 prototype.
> >
> >
> >
> > The key would be to configure the V9938 to output color information on
> > its Cx port.  Then the RAMDAC could decode that information and produce
> ?almost?
> > VGA compatible signals.  It would still need the final stage circuitry
> > and the HD15 connectors.  One signal that is absolutely missing though
> > is VSYNC* and on the V2 prototype it will be (hopefully) derived from
> > the composite video signal using the LM1881 circuit.
> >
> >
> >
> > Even just establishing the V9938 can output color information on the
> > Cx port would be big progress though.  I am confident the RAMDAC will
> > work since it does so beautifully on the ECB uPD7220 circuit.  Once
> > programmed, the RAMDAC gives a *lot* of flexibility over the display
> > palette and is really a neat device.
> >
> >
> >
> > Also, the V9938 could be tweaked to produce sort of VGA signals using
> > a set of 390 ohm resistors on R, G, and B.  Also the HSYNC* comes
> > directly from the V9938 but VSYNC* would again have to be derived
> > using the LM1881 or other means.
> >
> >
> >
> > I appreciate your help on this neat board!  Thanks and have a nice day!
> >
> > Andrew Lynch
> >
> >
> >
> > From: n8vem...@googlegroups.com [mailto:n8vem-
> s1...@googlegroups.com]
> > On Behalf Of Elsid
> >
> >
> > Sent: Thursday, August 09, 2012 6:08 PM
> > To: n8vem...@googlegroups.com
> > Cc: 'Elsid'; Pontus Oldberg
> > Subject: Re: [N8VEM-S100:1036] Re: S100 VDP success
> >
> >
> >
> > Hi Andrew,
> >
> >
> >
> > Sure, I can check things look OK on the new board design.
> >
> > I don't think I ever got the VGA output going.
> >
> > Did anyone else have any success with this board?
> >
> >
> >
> > Best regards
> >
> > Leon
> >
> > On Thursday, August 9, 2012 10:42:23 PM UTC+10, lynchaj wrote:
> >
> > Hi Leon and Pontus!  Thanks!
> >
> >
> >
> > Remember the S-100 VDP project?  I?ve finally found some time to
> > update the circuit to include the AY-3-8910 sound generator, VGA
> > monitor interface, and do some other clean ups.  Would you please
> > check out the schematic and PCB layout and compare against your notes
> > from the last prototype build and test?
> >
> >
> >
> > I would like to do another round of the S-100 VDP prototype boards and
> > would like to make sure all the lessons we learned from the first
> > prototype are fixed.
> >
> >
> >
> > Thanks and have a nice day!
> >
> > Andrew Lynch
> >
> >
> >
> > From: n8ve...@googlegroups.com [mailto:n8ve...@googlegroups.com] On
> > Behalf Of Elsid
> > Sent: Sunday, April 01, 2012 12:37 AM
> > To: n8ve...@googlegroups.com
> > Subject: [N8VEM-S100:784] Re: S100 VDP success
> >
> >
> >
> > Hi Andrew,
> >
> > The additional caps across the V9938 power pins are an attempt to make
> > the video less noisy. They didn?t seem to have much effect. I suspect
> > the noise I?m seeing on the video signal on the CRO may be due to the
> > routing of the power lines for the video amp through the dynamic RAM
> > circuit. (lots of hash from the refresh?) Maybe the video output could
> > be routed away from the RAM and separate traces for the video amp power
> from the regulator provided.
> >
> > The extra pull up on DHCLK p3 of the V9938 (and cut trace to R53) is
> > there because I was worried that the 5.37MHz output would be doing
> > something to the LightPen inputs LPS and LPD.
> >
> > The 470 ohm pull downs on R, G and B outputs  (p22, p23 and p24) on
> > the
> > V9938 are there because I suspected I may have damaged the COMP_VIDEO
> > output
> > (p21) and needed the pull downs to get the internal transistors to
> > drive ? the output was OK as it turned out.
> >
> > The reset circuit needs some changes. The V9938 is not receiving a
> > ?power on? reset because BRD-RESET* is being gated by SYNTMS* at U27C.
> > (SYNTMS* is reset at turn on by BRD-RESET*). The V9938 has to the
> > manually reset by toggling SYNTMS*.
> >
> > I have tried the FMS6141 video amp configuration and it works fine.
> >
> > Parts of the board working/not working:
> >
> > 1. Address decoding ? working but could do with changes to make more
> > MSX compatible ? as you?ve said.
> >
> > 2. Reset circuit ? only working manually ? see comment above.
> >
> > 3. Composite video output ? working but noisy ? power routing?
> >
> > 4. Video RAM ? working ? I tried adding in 100 ohm resistor in the
> > RAS,CAS and R/W* lines (like some MSX2 designs I?d seen) to reduce
> > ringing. It didn?t reduce the video noise but it may be a good idea
anyway.
> >
> > 5. BT478 ? I haven?t tried yet but maybe we can route the RED, GRE and
> > BLU outputs to a header for a DBS15 connector to drive a 15kHz VGA
> monitor.
> >
> > Best regards
> >
> > Leon Byles
> >
> > On Sunday, 1 April 2012 00:00:57 UTC+11, lynchaj wrote:
> >
> > Hi Leon!  Thanks!  I am updating the S-100 VDP design and would like
> > some clarifications.
> >
> > I see the data lines on the V9938 are reversed.  That's a carry over
> > from the N8VEM SCG board.  Apparently the TMS9918 *did* flip the data
> > lines and I carried that over but it wasn't necessary.
> >
> > So far, I've fixed the V9938 data bus but I see from your photos there
> > were several other changes.  Additional capacitors and what appear to
> > be pull up resistors on the V9938.  Also the data lines going into the
> > BT478 and also picking off signals from the U7 data bus latches. I
> > presume those are data lines?
> >
> > Have you had any luck accessing the BT478?  John Coffman was able to
> > get it to work on the uDP7720 board.  I think the first step is to see
> > if its registers are responding.  In theory at least the BT478 RAMDAC
> > would allow the VDP to produce VGA like signals.  They would still be
> > the lower NTSC like dot clock but should still work on modern LCD
> > monitors.
> >
> > My plan is to add in the AY-3-8910 sound generator and the joystick
> > interface from the N8 board.
> >
> > Also to rearrange the IO decoding so the V9938 can appear at x8H
> > addresses.
> >
> > Are there any other changes we need on this board?
> >
> > Would it be possible to list out what parts of the board work and
> > which don't?
> >
> > It seems the the V9938 is working so that implies the VRAM subsystem
> > is working.  Also the IO decode is at least partially working.
> >
> > Thanks and have a nice day!
> >
> > Andrew Lynch
> >
> >
> >
> > On Mar 19, 7:57 pm, Elsid <le...@swiftdsl.com.au> wrote:
> >> I?ve finally had some success getting video out of the prototype
> >> S100- VDP board.
> >>
> >> The first thing I found was the data bus on the V9938 swapped around.
> >> That is, D7 was wired to CD0 not CD7, D6 was wired to CD1 not CD6 etc
> >> down to D0. I think this may have been an early error on one of the
> >> prototype N8VEM VDU boards also.
> >>
> >> I correct the Data bus swap around but still no video output on p21
> >> COMP_VIDEO or P22,23or 24 (RGB). I should say there was actually a
> >> composite video signal complete with color burst and synchs but there
> >> was no video component on the top half of the waveform. I tried
> >> another V9938 ? same result. I pulled out the BT478 to just try and
> >> get the V9938 part going on its own. Still no luck.
> >>
> >> Finally last night Wayne posted that he?d found some problems were
> >> caused on the N8VEM Color VDU board when the VT82C42 is not reset. I
> >> looked at the VDP reset circuit and found that the V9938 is not
> >> receiving a reset because BRD-RESET* is being gated by SYNTMS* at
> >> U27C. SYNTMS* is reset at turn on by BRD-RESET*. ? Thanks for the
> >> inspiration Wayne.
> >>
> >> I?m not sure what the function of SYNTMS* is. If it?s to enable a
> >> programmable reset for the V9938 we may be better off inverting it
> >> and eliminating the inverter U11F after U27C. This should give us a
> >> reset from either SYNTMS or  BRD-RESET.
> >>
> >> I was using a modified version of HW9918.COM to test the VDP board so
> >> I added a couple of lines to toggle the SYNTMS* signal to reset the
> >> V9938 in the initialization section and Bingo ? ?Hello World? popped
> >> up on the screen.
> >> I?ve put some photos and the test program on the N8VEM site
> >> at:http://n8vem-sbc.pbworks.com/w/browse/#view=ViewFolder¶m=VDP
> >>  for anyone who is interested.
> >> I set the address base to 90H but maybe it would be nice to change
> >> this to 98H (start of the V9938 registers) for MSX2 compatibility. I
> >> think it will need some rewiring in the address decoding.
> >>
> >> Next step is checking out the BT478 and maybe VGA output.
> >>
> >> How?s everyone else going with their VDPs
> >>
> >> Leon Byles
> 
> 
> 
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> 
> Tom Lafleur
> (858) 759-9692