SBC Expansion Board

[RichCini]

All:

I'm working on a four-slot expansion board for Daryl's single-board computer. Actually, I should have the CAD work done this weekend and I'll probably order a prototype board or two next week.

I wanted to poll the group and see how many people, if any, would be interested in buying a bare board. For Q10, I can get the cost down to $25 or $30 per bare board. In single-unit orders it's about $70/board because of the fixed tooling charge.

Here's the feature set. It's a four-slot expansion board that's 4" x 6" in size (roughly the same size as the SBC) with its own 5v linear regulator. The backplane connectors are 50-pin female Eurocard-style connectors, so the expansion boards would be perpendicular to the backplane. It connects to the SBC with a short 50-pin IDC cable (a ribbon SCSI cable is good for this). It has pads for signal buffering logic (a topic discussed in another thread here), but you can easily jumper to bypass the buffering logic (as I will do at least initially).

So, no obligation at all on this. I'm just trying to gauge interest. I'm going to order one or two for me for testing but if there's enough interest, I'll order more and blend the price down.

[lordsteve]

What board house are you going with for this?

[RichCini]

lordsteve wrote:

What board house are you going with for this?

ExpressPCB. It's not a fancy design and I wanted an easy to work with setup. There are plenty of other board shops but several of the people on 6502.org have had good success with ExpressPCB so I'm giving them a try. It's a 2-layer FR-4 board using 0.5oz copper with no solder mask or silkscreen.

Their standard board setup is $55 plus a mathematical formula based on square inches, the number of boards and shipping. It's about $94 for two boards. Obviously running more boards enables you to spread the fixed setup charge over more units, thereby reducing the per-unit cost. A 10-board run makes it really compelling price-wise.

[RichCini]

All:

I've spent some time designing an expansion board for Daryl's SBC. It's very simple -- a separate power supply, bus buffering and four expansion connectors.

I'm using Diptrace to do the board because I found ExpressPCB too hard to use and I didn't want to manually route the board. Now, the problem I'm having is that the 6502SBC expansion connector is laid out in such a way that I'm having trouble assigning the pins to the buffer chips in an order that allows me to get a successful plot. Now, Diptrace only comes with a grid-based auto-router which is only pretty basic. I can get a Spectra autorouter but it costs about $350.

Any advice? From the other thread about bus buffering (viewtopic.php?t=1173), I don't think buffering is "required" but I was trying to build the design so that if you wanted buffering you could plug in the chips and if not, a bunch of DIP headers.

[GARTHWILSON]

Autorouters will always disappoint. They're fast, but never anywhere near as intelligent as a human. I recommend just routing by hand, which is the only way I ever could have routed our company's extremely dense boards with up to 500 parts. I say dump the autorouter.

[fachat]

RichCini wrote:

All:
I'm using Diptrace to do the board because I found ExpressPCB too hard to use and I didn't want to manually route the board.

I use Cadsoft's Eagle package and for simple boards it normally works with autorouting. Some points I have to do manually, though:

- power supply improvements (thicker traces for example)
- high-density parts of the board and
- bus structures are better routed manually before using the autorouter. here the human mind is way better :-)

André

[lordsteve]

I will have to second the motion for EagleCAD.

Also, I would like to recommend you at least check out batchpcb.com if you have not done so. I have used them three times. My latest 4"x2" board only cost 30 dollars, total ($10 setup fee, $2.50/ sq.in.).

[8BIT]

lordsteve wrote:

Also, I would like to recommend you at least check out batchpcb.com if you have not done so. I have used them three times. My latest 4"x2" board only cost 30 dollars, total ($10 setup fee, $2.50/ sq.in.).

Can you provide more details about your board setup? What density was your design (high, med low), two layer, via's and hole size options, etc??

I would love to find a cheaper source for single quantities for development. I like the quality that ExpressPCB.com provides, but having a min qty of 2 for experimenting is too expensive.

thanks!

Daryl

[lordsteve]

Um...I don't know how many vias I have. batchpcb.com's service has no restriction on number of holes or vias. I think the largest drill size is 0.5 inch or 500 mils (thous). My board is two-layer. (I understand they have a 4-layer option for $8 per square inch + $10 per order setup fee.) I am not sure how to qualify the density of the design. The board has silkscreen on the top side and solder mask on both sides.

I put a pic up at http://www.lordsteve.com if a picture of the populated board will help you.

http://www.batchpcb.com has the definitive info.

[8BIT]

Lordsteve,

Thanks for the reply and picture... that’s a nice set-up. I'm definitely going to check out batchpcb!

Daryl

[RichCini]

All:

I finished designing an expansion board for the 6502 SBC. I have provided a ZIP file on my 6502 projects page which contains the design files. I have not yet ordered these boards. First, I'd like a fresh set of eyes to look at the designs to make sure I haven't missed anything before I order prototypes. Here's the link:

http://highgate.comm.sfu.ca/~rcini/clas ... 02proj.htm

The ZIP file contains two Diptrace schematics: one for a buffer board and one for a simple straight-through board. The designs were exported to both Protel and Tango formats and then imported into the PCB123 layout program.

PCB123 is another PC board shop like ExpressPCB but their layout program has a half-way decent autorouter. I haven't done a direct price comparison but they seem similar.

I have only autorouted the simple board at this point because that's probably the one I'm going to order. The only other anomaly I've found is that the 4-pin power connector in the schematic didn't import properly even though I adjusted the assigned footprint in the schematic. So, in the board layout I put two simple pads for wires.

If anyone has any comments or suggestions, please let me know. Once I get the expansion board design done, I'm going to work on an AY38912 board and a speech board.

Thanks again for the help.

[8BIT]

Rich,

I don't have Diptrace or PCB123. Can you make some jpg images that we can look at?

thanks!

Daryl

[RichCini]

8BIT wrote:

Rich,

I don't have Diptrace or PCB123. Can you make some jpg images that we can look at?

thanks!

Daryl

The ZIP file was just updated with four PDF files. Two represent the horizontal and vertical layouts. The other two are the simple and full schematics. The only one I'm having trouble routing is the full one with the buffering. I can't seem to find a chip arrangement that I can get all nets routed.

I haven't optimized for size or anything yet. I envision about 3/4" between the cards. The connector on the card is a right-angle connector so most of the spacing is use for component height.

[smilingphoenix]

I've just checked out youe schematic and I'm afraid I've found a couple of problems

1) You've got the 74HC245 on the data bus the wrong way round. The B port should connect to the CPU and the A port to the expansion bus. Or you could keep it as it is and invert the R/W\ line that feeds its direction input. (guess who's also made this mistake before...ME!)

2) The 74HC245 data bus buffer should only be enabled for addresses that are not mapped to anything on the main SBC board, otherwise you will have bus conflicts as the '245 and something on the main SBC drive the bus simultainiously. This is my fault - in the thread where we discussed this I said you could enable the data bus buffer all the time - my brain must have been off on vacation You can only enable a data bus buffer all the time if it is sitting between the CPU and ALL the other devices on the bus. I'm surprised no-one else has spotted this. Sorry.

Good luck with the expansion board - it's looking good so far.

[RichCini]

Thanks for looking at the schematic. The flipped 245 is easy to fix. The other's going to take a little work because you have to re-decode what's happening on the main board to exclude RAM and ROM. I just need to look at the schematic again. Maybe I can pass a select signal through one of the empty connector pins using a wire mod.

Thanks again.

Rich