Date   : Tue, 24 Apr 2007 12:23:01 +0100
From   : RS432@... (Mick Champion)
Subject: BBC to VGA monitor converter

Robin Commander wrote:
> If you leave Pin 20 of the MC1377 chip unconnected it will switch to PAL
> mode. The P suffix usually just denotes a plastic chip packaging.
>
> I think that the crystal may also need to be changed for a 4.43Mhz one as
> well.
>
> HTH
>
> Cheers
> Robin
>   

I should have read the manual / instructions before I asked about the 
crystal as it does state 4.43 Mhz for PAL as I found out hours after you 
first pointed it out. Thanks for that.  Looking at the device specs 
posted by David Hunt, P means case 738. From the picture, it looks like 
a DIL (dual in line) chip. There is also a DW suffix (case 751D). This 
looks much smaller. I suspect it to be SMD (sub miniature device???). 
Yuk! I'll have a "P" please Bob!


David Hunt wrote ;

> Have a look at http://www.datasheetarchive.com/datasheet.php?article=224762
>   
> I believe the Beeb outputs composite sync already, so you don't need to use
> a sync separator to use this IC.
Ahh! This is excellent! Many thanks. It seems that pin 2 of the MC1377 
accepts composite sync. Directly from the RGB port hopefully? I should 
have worked out that the 74LS08 (being an AND gate) would only send a HI 
pin 2 when both horizontal and vertical sync are HI at the same time, as 
composite sync already does I presume. No need for the 7408 then.
> There's a circuit on page five for RGB->PAL/NTSC and it explains the
> component differences for NTSC/PAL too.
>   
I will be using the Motorola circuit you posted. It has a trimer on the 
oscillator circuit giving some adjustment. It's got to be better than 
the double inverter setup on the diagram link I posted. So this means 
only one chip is required, the MC1377. At under a fiver, it has got to 
be worth a go.

There's one thing I don't understand on the circuit. There's a component 
between pins 6 and 8 that looks like a cross between a resistor and a 
capacitor. Written underneath it is 400ns Y delay. I understand this 
must put a 400 nano second delay in, but any ideas what the component 
that does this actually is? I'm not familiar with filters either. 
There's one between pins 13 and 10 but I think they are easy enough to 
get. Heres hoping I wire it up correctly.
> Unless you're going to use a DC-DC converter, you're going to need to take
> 12v from the aux port.
>   
I use an (AT) PC case to power my BBC, hard and floppy drives so I will 
power device from the 12v supply there.


Cheers,

Mick