Re: [N8VEM-S100:1037] Re: S100 VDP success

[Tom Lafleur <laf...@lafleur.us>]

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...@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