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DEC used H851 top connectors to interconnect board sets for the CPU, disk controllers, memory and other subsystems. Buying board sets recently, I found that H851s are rare and pricey. Here is what you can do about it.
The Problem
From the looks of the interior of some PDP-8 Omnibus systems (right), it seems like DEC was in love with the H851 top connector. Having managed to restore my formerly bedraggled PDP-8/m to operating condition, I went on a quest for the three-board controller for the companion RK05 disk drive. Acquiring the boards one-by-one, they came of course, without the four H851s needed to interconnect them. Similarly, the three H851s for the newly purchased Extended Arithmetic Element boards. I searched all over for the top connectors but it seems thay have ended up in the same black hole where extension cords and left socks go. I sent scores of RFQs and followed lots of false listings on the Web, to no avail. Actually, there was one dealer in the UK who quoted the quantity-7 for $127 each plus $129 for shipping ($1018 total). Couldn’t afford that, so I decided to make some replacements. [Note: Long after completing this project, four H851s appeared on eBay and were sold for $100, including shipping.]
Fabrication Alternatives
As seen at right, Each H851 consists of two paralleled edge connector slots. These slots are the same as the ones used in the Omnibus, except that the bus has four of them in a row. In fact, when a card has tabs for four top connectors, it can be mistakenly plugged into the Omnibus, if turned upside down. (That’s not a good thing.) Each card tab handles 36-connections (18 per side). Contact spacing is 0.125-inches. Unfortunately, I could not find a 2x18, 0.125” edge connector at Digi-Key, Mouser or Newark Electronics.
Cutting DEC Backplane Blocks
Jim Gonzales at Omnicorp suggested that I could cut backplane blocks with a band saw, to make top connectors. One such block is shown below, with a red line to indicate where the cut would be. (There would also be vertical cuts.) As I don’t have access to a band saw, I would have to use a table saw.
 The kerf (width of the cut), however, is about 1/8”, which is too much to preserve both sides of the block. Instead, it would have to be offset, sacrificing half of the block.
The connections to the block are in the form of wirewrap pins. One would need to either make a PCB to connect the two slots or solder connections using wires.
Douglas Extender Board Connectors
 While cutting backplane blocks was a possibility, I looked for other options. One idea was to contact Douglas Electronics. They produce extender cards (at right) that have Omnibus compatible connectors. However, those only come in dual and quad slot versions, not a single-tab version. In the dual-width product, a single connector services both PCB tabs. It has a plastic separator inserted in the middle, so this wasn’t a connector which would help with an H851 remake.
Cutting Larger Connectors
The only other solution I could think of was to buy larger connectors and cut them down. The problem had been with finding the specific 2x18 length. For example, the Digi-Key 346-100-555-802-ND is an S100 extender board connector (2x50-position). That was priced at just $4 and could be cut to make two 2x18-position connectors. The drawback would be that one side of the connector would not have a closed end to guide the board tab. It would still be usable, though.
Finding the EDAC Connector
 Not happy with the options, I decided to search again for a better edge connector. To my surprise, I stumbled across the EDAC 346-036-520-201, which is a 2x18-position, 0.125” spacing edge connector. It doesn’t have mounting ears, which is perfect for this application. EDAC is a Canadian company and the only place I could find stock was at Sayal Electronics in Toronto and other Canadian cities. They have a $100-minimum US order policy but I needed at least 14-connectors to build the seven top connectors, anyway. To cover future needs, I ordered quantity-20, at which they quoted $6 each. Ground shipping was $27 and they arrived just a few business days later. As of this writing, they show more than 300 units in stock.
Making PCBs for the Top Connector
Since there are 36-lines per top connector and I needed at least seven units, I decided that connecting with a small PCB really made sense. It would look a lot nicer, too. The DEC H851 measures 2.4 x 1.0 inches but the EDAC connector is longer. My unit (which I will call the H851SL) was designed at 2.55 x 1.0 inches. Actually, the connector measures 2.535 and I later ended up trimming the PCBs down to match that.
 An Oddity of the DEC Edge Connectors
The reason that the EDAC connector (and therefore the PCB) is wider can be seen by comparing a DEC edge connector tab (upper) with an S100 tab (lower) at right. They have the same finger spacing. Ignoring the fact that DEC fingers are longer and wider, we can see that DEC fingers extend to the left edge of the tab, whereas the S100 board below has some blank PCB margin at the left side of the row of fingers. All edge connectors that I have seen are designed with space on the sides to accommodate the extra margin. Since the EDAC connector has that, its body is about 0.2” wider than the DEC H851.
As a result, the DEC PCB tabs can be inserted into the EDAC connectors off-center, whereas the H851 fits snugly. It’s not a real problem though, because it turns out that you can easily visually align the connector contacts with the tab fingers of the DEC PCB, as you slide the H851SL on. I thought about filling-in the sides of the EDAC connectors with epoxy paste. I haven’t tried that though, because it’s easy to align them and now that the system is being completed, they won’t be removed much.
The only place where the wider H851SL has been an issue is where a card has two of them in the center of the board, as shown in the top picture on this page. The PCB tabs are closer together there. However, with the H851SL boards trimmed down to the length of the EDAC connectors and the H851SL units touching, they just align with the fingers and make reliable contact.
Choosing a PCB Vendor
This was a weird situation from the standpoint of getting PCBs made. Requirements on the process were minimal—it didn’t need to be double sided, nor did it need a silkscreen or fine lines. Since debugging the PDP-8/m would be waiting, turnaround time was important (but so was cost). The boards are miniscule and I only needed 11 or so. I knew that the vendor’s minimum policies would dictate. Here is the rundown on the vendors I looked at (prices are without shipping):
- BatchPCB was charging $2.50/sqin (rounded up) plus a $10 setup fee. That would have resulted in $8.41 per board. However, I noticed that they seemed to have fallen down in customer service. There were lots of issues raised on the forum, without answers from management. Also turnaround was 3-4-weeks. I decided to look elsewhere. NOTE: I see that now that BatchPCB has closed and OSH Park has taken over operations, with a different pricing structure.
- Checking the PCB cost comparator on Ladyada.com, Olimex DSS came in cheapest at $6.91 per board. However, that was shipping from Europe and the prices were five years old!
- I had used Advanced Circuits before, with their special of $132 for four boards up to 60sqin each. That would be $12 per board, assuming that the cutting didn’t add extra charges (not likely).
- Advanced Circuits was also offering a “Barebones PCB” service: This sounded like a good match for this case. The deal is for no soldermask, no silkscreen and no solder dip. It does include double sided, plated through holes, tin plating and 1-3-day turnaround. This sounded great to me. (I used metal to write a few words but made sure that it passed ground rule checks. ) The quote came back at $8.24 per board. The only thing was, they had a minimum board size of 1.25 x 1.25”. Would my 2.55 x 1.0” board work? I asked sales and they said they thought so. More about this below.
The way I looked at the PCB cost was that I had $12 in the connectors and generally, I would be willing to pay $30 or more for an H851. With $19 shipping for the eleven pieces, the Advanced Circuits Barebones deal would result in at about $22 total, per completed H851SL. I was good to go!
PCB Layout
I used Eagle to create the PCB, as seen at right. Traces were made 20-mils to match DEC’s product. They of course, tie each pin of the left connector to the corresponding pin on the right connector. All traces are on the solder side of the PCB. You can download the Eagle schematic file here and the board file here. While retaining the copyright, I am granting a free license for anyone to use and modify all files of this project for noncommercial purposes, so long as due credit is given.
Production Files Package
You can download the zip file containing Gerber files and everything needed for fabrication, here. You will need to edit the Readme file, changing “My-Board-number” to your board designation, “My-PO-number” to yours (if any) and filling-in contact info, where appropriate. Note that oddly enough, the only common way that I have heard of for specifying the PCB outline in a layout-program fabrication files package is to include it in the silkscreen layer. Unfortunately, the Barebones process does not use that layer. For the purpose of specifying the board outline, I included it anyway, noting that it otherwise isn’t used, in the readme file.
A Minor Problem with the PCBs
As noted above in the last bullet item, Advanced Circuits’ Barebones process specifies a minimum board size of 1.25 x 1.25” but sales “thought” that my 2.55 x 1.0” board would be okay. When the boards arrived, the only thing that I found wrong was that instead of being cut to 1.0” wide, they had an additional 1/4” of width. Apparently, the 1.25” minimum spec is a necessary condition. It meant that I had to find a clean way to cut all of these boards. Unfortunately, cutting PCBs is one of my Achilles heels. Here are some options I considered to do the trim:
- On rare occasions, I would press my heavy-duty paper cutter into service cutting a board but it really seemed to be at the limit of that tool’s abilities. Also, the cut comes out pretty rough.
- I keep dreaming of buying a sheet metal shear which would also be able to cut PCBs but can never seem to quite justify it.
- I looked for PCB cutter tools but didn’t find anything affordable and effective enough.
- Trying to use a scroll saw would result in too rough a cut.
- Thought about setting up a Dremel tool horizontally on its drill press stand and use it like a table saw (perhaps with the blade overhead). Would use a cutoff wheel for a blade. Couldn’t quite see doing this effectively.
- Could use a cutoff wheel on a table saw. Thought that this might require a jig to hold the board. Was a bit scared of working that close to the table saw blade with such a small board.
- The Dremel router table or the big router table could do it. Either way, I had some concern about bits being rapidly dulled by the fiberglass material. I might go through too many Dremel router bits or might risk dulling an expensive full size router bit. Plus, a router would generate a lot of fiberglass dust.
The solution I finally found and  went with (at right), was to buy a QEP 3/5 HP 7-inch tile saw from Home Depot. it has good reviews and costs only $49 at this time, including free shipping and a diamond cutting wheel. Since it promised to solve my PCB cutting needs once and for all, I felt that helped relieve this project of having to justify the full cost.
The saw worked well for this job. I highly recommend that you do employ wet cutting by filling the included tray with water. That helps trap fiberglass dust, though wearing a respirator is still mandatory. Of course the downside of wet cutting is that it does splash drops of water on the saw, table and floor. It wasn’t excessive; just needed to be wiped up with shop rags (below left).  This assumes that you DO push the opaque blade guard  all the way down. Without that, it really will sling lots of water at you.
At right, you can see a safe way of guiding the PCB through the saw. First, notice that the yellow blade guard is all the way down. The fence has been setup to the right of that, for the proper cut. A block of wood (at bottom) serves to push the PCB forward. A scribe tool (or ballpoint pen) can be inserted in a PCB hole to hold it down and against the fence.
Trimming the Length of the PCB
As explained above, we need to minimize the length of the top connector. As drawn, the PCB is slightly longer than the connector, so I sanded the boards down a little. Less than 1/32” was removed from each end.
 Assembling and Using the Top Connectors
Soldering the two connectors onto each PCB is easy. It doesn’t matter which way the connectors are inserted. Be careful to make sure that the connector is flat against the PCB, though. Begin by soldering two opposite corners. The completed unit is shown at right, alongside the original.
The new H851SL is installed into the system just like the original, except that you must visually align the socket and card-finger contacts (as discussed above). I’ve found that it isn’t critical but must be done with reasonable care. I hold the connector against the rear of the two PCB tabs, tilted up, so I can see both the socket and finger contacts. Align and then rotate the connector down onto the PCB tabs. Works every time. By the way, it doesn’t seem to be adequate to try to align the overall connector with the PCB tab. For some reason, when properly aligned, the overall position actually seems slightly off-center to me. Just align using the socket contacts and card fingers and you shouldn’t have any problem.
Making an Extension Top Connector
 Normally, the top connectors constrain the order of DEC PCB sets. For example, in the 8K memory three-board set at left, the middle board cannot be in front, because the top connectors would not reach from it to the board in back. This makes it difficult to troubleshoot the middle board. Similarly, the three RK8E disk controller boards have top connectors which constrain their order.
 To facilitate debugging, I decided to hand wire a couple of top connectors with sufficient wire to let me me put any board in front. To do that properly, I taped-down two connectors, with 1-inch between their center lines, then wired them together with lengths of fine stranded wire as seen at right. I left a somewhat generous loop between them. While I’m not certain of the gauge of the wire, it strips well at a 28-gauge setting on the high quality stripper I use for Kynar wire.  Fine gauge wire helps keep the assembly flexible and doesn’t push the boards out of position. The resulting connectors allow the middle board of the 8K memory to be moved to the front for debugging, as seen at left.
Summary of Resources
So there you have it: You can indeed make your own H851SL top connectors. Just to bring it all together, here are all the resources you need:
Copyright © 2013 by Stephen H. Lafferty. All rights reserved.
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