What's the current practice to make circuit boards configurable? For example, a signal coming into the board might be returned to system ground or to +24vdc, depending on the customer, and I need a way to strap some inputs appropriately. For a prototype system I could solder in a header block and use jumpers, but for a run that will be configured consistently, I want something hard-wired so that customers can't misconfigure the board. I understand that "zero ohm resistors" are available that can be soldered into place. What other solutions have you encountered?
For systems that already have plenty of digital smarts, 8 pin serial EEPROMs are quite common. For just setting a few configuration "jumpers," you can buy "solid state DIP switches" which are a handful of analog switches controlled by a tiny bit of non-volatile memory (all built into the IC); software running on a PC and a small program header are used to "flip" the switches' positions. I was going to suggest Maxim as a source, although I'm only finding digital pots there... and an application note on using it as a DIP switch. Still, I'm sure I've seen such things.
None of these solutions are directly applicable (without additional circuitry) at 24V. If you don't mind the extra manufacturing hassle, the zero ohm resistor approach works quite well. A step up from there is, of course, a mechanical DIP switch. While I understand you don't want customers misconfiguring the board, if the board is enclosed or otherwise difficult to physically access (i.e., takes enough effort that a customer can't realistically claim they accidentally bumped it and it changed!), there is great value to field service personnel as well as your own stock room to only requiring having one physical version of the board kicking around.
If you have a number of similar cards that you're thinking of jumpering, sometimes you can instead make all the cards the same and isntead build the back panel that they plug into differently so they know who they.
Sometimes it's worth it to put whatever electronics in there so that it works either way, or so it can be configured from a front panel. Keeps fingers and screwdrivers out of the works, which means higher reliability.
For solder configuration, zero-ohm resistors or cut pads that can be bridged by solder can be okay. Zero-ohm resistors have the advantage of being almost as cheap as the pads, small and can be machine-assembled. Sometimes you can have two positions for a part and just install one of them. Sometimes those two positions are deliberately made to physically overlap so that only one can be assembled. Mechanical switches are another solution. Lots of ways to skin the cat.
Best regards, Spehro Pefhany
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"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
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Here's yet another solution that is cheap in high volume- a flat cable (perhaps 0.1" or 2mm pitch) stripped and soldered into one position or another (.. or another..). (formed into a U). This can configure lines of several amperes and relatively high voltage (or microvolt level signals) for under a penny a pin.
Best regards, Spehro Pefhany
--
"it\'s the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Spehro mentioned 'keyed cables'. I have often seen another simpler variety of keying: A multi-contact plug that has shorts and opens that differ between versions. In the old days a dummy tube plug, in this day and age often automotive type connectors. Many tube radios had their mains voltage configuration done is a similar fashion. If tampering is a concern these can be potted in or sealed so that tampering will be evident should something happen.
This has the advantage that you can produce boards regardless of where they will be shipped and they all have the same P/N. Only after a customizing connector has been installed and sealed in place do they become individual assemblies with individual assembly numbers.
Our current policy is no switches, pots, or jumpers if at all possible. Everything is programmable, and all cal factors and stuff are in eeprom. Analog switches, programmable-gain amps, diffamps, mdacs, all sorts of stuff is cheap nowadays.
Sometimes we use one pcb layout for two or more products, but that's an assembly-time thing, not something the customer is ever aware of.
Don't know if that is current, but you can place jumper pads on your PCB. Basically two or three field of copper with a tiny gap and no solder mask. The reflow won't close the gap, but with a soldering iron it can be done quickly. Few clients will scratch those fields free agian.
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board.
For *one time* configuration, I've once used small breakable tabs one one side of the PCB (with the short on the tab).
For on site configuration, pliers are easier to use than an soldering iron. You break, it's open. And making this breakable seems to inhibate the user's temptation to 'adjust' the configuration. Ooops, in case of mistakes you still can get get the iron out and solder one of the spare 0R provided on the board onto the void footprints you've provided with each breakable tab.
Do they still make solderable DIP plugs? You can get a lot of options with a 16-pin DIP jumper - they used to call them "personality modules" in EPROM programmers. :-)
Well, seeing as how I hate wild goose chases... ... ... Ah! Found one!
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"DIP Header", right squack in the middle of the page. :-)
Sure but in production the crimping procedure of an automotive plug and the fact that it's keyed to avoid reversal is usually preferred. Also, you can't easily bend the pins on these.
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