Howard Winter wrote:

>David,
>
>It took a bit of searching to find the board layout, so to save others the hassle: 
>http://www.dpharris.ca/index.pl/block_diagram  :-)
>  
>
Thanks...
picture of schematic at: 
http://omniport.swiki.net/.uploads/HomePage/TWTs1.jpg
picture of board: http://omniport.swiki.net/.uploads/HomePage/TWTb1.jpg

Best to sign into: http://www.dpharris.ca/

>I see you have the voltage regulators and the H-bridge chip round the edges, which is good in that it allows 
>for heatsinks, but the high output devices look a bit crammed together - a nice bit of board-packing but it 
>doesn't allow for heatsinks to be fitted if anyone wants to do so.  Does the design allow for alternate 
>transistors to be omitted to make room for heatsinks on the remainder?
>  
>
Yes, packed close -- of course you can leave out transistors, but would 
lose those outputs.  Because I was using the free Eagle, I was limited 
in surface area to 80 x 100 mm.  I suspect for most uses I had 
contemplated, heat sinking will not be a problem as the signals are 
quite transient.  . 

>I wonder if the cluster of individual resistors in the bottom-left corner can be replaced by SIL resistor 
>packs, giving you more space to play with?
>  
>
That is the opamp area and the 'cluster' includes options for caps and 
resistors.  Allows you to build two buffers, invertors, filters, max 
detectors, etc.  SILs are unlikely to be useful here because the 
resistors values are typically all different. 

>I don't see any ICSP connection anywhere, 
>
The connectors on the left edge contain the signals for ICSP, so it is 
available by making a cable for it.  (they also contain the 
RS232/485/optoisolators signals). 

>and the PIC looks difficult to remove from the board as it's closely 
>surrounded by other parts.  How do you (re-)program the PIC?
>  
>
I use a IC socket, and install the PIC/AVR into a machine socket to 
protect its pins.  I use ICSP (ICD2), usually, though.  Note, wrt the 
recent discussion on Piclist, I have allowed for use of a crystal or 
resonator. 

>Does the design allow for CAN-bus drivers instead of (say) RS485, so a CAN-compatible PIC (18F258) can be 
>used?
>  
>
No, not specifically as it predated the CAN-Pics.  However, the CAN-Pics 
will fit into the board, but would require a small amount of hacking.  I 
am planning to try it out. 

>What does the "Capacitance Discharge Unit" do?
>  
>
Model train layouts have switches/turnouts to allow selection of 
routes.  To remotely switch them, the are several methods: motorized or 
dual-coil.  The latter requires a good jolt of current to ensure 
successful operation. The capacitive discharge circuit trickle charges a 
capacitor, which is discharged through the appropriate coil to switch 
the turnout.  The circuit contains a current-limiting pass transistor. 

Thanks for the feedback...

Cheers,
David



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