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We are designing two interface (printed) boards and need to connect them by cables (0.5-1.5m). So we're thinking about which connectors to use. We'd like to use twisted-pairs like in a RJ45 cable.

The problem is, we are talking about 200 signals (-10V to +10V), which would require 50x RJ45 cables.

Alternatives I know of are RJ21 or DSUB-50 cables, with 50 pins, 2x25 wires each. Buying them is way too expensive (at least in Germany, it would be around 450€ or more for 8 cables) and soldering 800 contacts is considered too much work for now (+120€ material). For comparison, we could get 50x RJ45-cables out of the box for 70€.

So I wonder if there is an alternative? Are there any cable standards which are as reliable and cheap as RJ45, but suitable for a larger number of signals? (If there wouldn't be the twisted pair condition, flat ribbon cables would be a good choice).

For the maximum frequency of the signals one could assume something around 200-500 kHz, the impedance of the cable should not matter too much, as the entry impedance of the end device is 1 MOhm (but I'm far from an expert in this matter).

Further clarification: We have about 60 analog signals and about 100 digital ones, just some of them have a high frequency and stiff edges, a lot others are just rarely changing signals for controlling a state space machine. About 40 of the 200 "signals" are actually ground.

I'm not searching for an improvement of the basic setup as it cannot be changed. Behind the interface boards directly follow the end devices. Apart from some little amplification and filtering of the analog signals, the purpose of the interface boards is to collect the signals of all sensors and actuators and redistribute them, to enable better cabling.

The system basically looks like is: enter image description here

As the DSUB-50 connectors of the right system do not provide the possibility of twisted pair wiring in most cases, the idea was to directly connect the DSUB-50 with an interface board (without cables) to offer better suited connectors.

Robert Seifert
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  • @Transistor I added some details, though I'm not entirely sure. But as a starting point it should be alright. The currents flowing are neglible. – Robert Seifert Oct 19 '17 at 17:10
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    What about the cost of the connectors though? Cables are one thing, but you are not allowed to forget the thing you plug them into, nor the boardspace for those connectors. – Joren Vaes Oct 19 '17 at 17:18
  • Are you sure you need twisted pair? Can you add filters at one or the other end of the cable? The wavelength of 500 kHz is 600 m, so you shouldn't expect much in-band pick-up from a 1.5 m cable, even with flat ribbon cables. – The Photon Oct 19 '17 at 17:31
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    Have you considered Amphenol RJ21 25 pair telephone connectors (2 of them)? They are physically pretty big but cables and connectors are relatively cheap and easy to source. Ultra-wide SCSI cables are another possibility. – Dean Franks Oct 19 '17 at 18:38
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    If the first board is signal conditioning and the second is ADC, why not put everything on one board and not even have a cable at all? – bobflux Oct 19 '17 at 19:31
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    RJ45 is a connector not a cable. – R.. GitHub STOP HELPING ICE Oct 20 '17 at 01:36
  • @ThePhoton - maybe not. We don't really know, but if there would be a feasible option with twisted pairs, we would be on the safe side. – Robert Seifert Oct 20 '17 at 10:18
  • @DeanFranks Yes I considered them, seems one of the better alternatives. But it appears that its not a common standard in Germany/Europe and therefore not so cheap or there are just lengths >2m available. – Robert Seifert Oct 20 '17 at 10:27
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    @peufeu the second one is actually a whole Hardware-in-the-loop system with 4 DSUB-50 female connectors. So the idea was to mount a an intermediate board to get more possibilites for better cabling. – Robert Seifert Oct 20 '17 at 10:28
  • You can get IDC DSUB connectors for ribbon cables at a good price, then use a twisted ribbon as suggested below... IDC is good, no soldering! – bobflux Oct 20 '17 at 10:39
  • Even 1 or 2 pairs in RJ11s would save quite a bit of money if you needed the cables to go in different directions.These are available off the shelf (but be sure to get twisted pair not UTP) – Chris H Oct 20 '17 at 12:49
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    I see a lot of suggestions here for alternate cabling arrangements, but have you considered muxing the signals over a high-speed serial connection like USB? – Robert Harvey Oct 20 '17 at 19:33
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    @RobertHarvey As a great part of the signals are analog, that's not an option. – Robert Seifert Oct 20 '17 at 19:58
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    Analog signals can also be multiplexed, though the technique is certainly different. Have you thought about using an optical cable? – Robert Harvey Oct 20 '17 at 20:03
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    IMO, it's a https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem. 200 wires with 20V p2p seems like an absurd setting - for 1.5m and 500kHz, you're actually creating a nice EM noise generator rather than an interface... So I'd rather rethink the design here, than search for a device matching this particular one. Just either a) separate ADCs from the rest of the B, put them in A, transmit muxed digitals over a fibre or any other hi-speed medium - that's the reason why SATA>PATA, b) if that's not an option for practical purposes, at least mux analogs (/agree with Harvey here). –  Oct 20 '17 at 21:45
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    Why not co-locate the analog side of things and talk to that via digital? Not sure if the power electronics you mention are for power supply with maybe a few rails (then just remove them to make space for other things) or power outputs, but splitting up your system another way might render this moot. – Nick T Oct 21 '17 at 00:22
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    @vaxquis I was asked in the comments to state the maximum frequency and amplitude of the signals, I did. That does not mean I have 200 oscillating signals with 500 kHz and 20V peak to peak. It means I have some analog measurement signals, which interest me up to 500 kHz and I have some rectangular control signals (PWM) oscillating with 20 kHz, with stiff edges. The rest are basically constant control signals of a state machine. Changing system A and B is just not possible and not point of the question. We have a similar system operating for years and just want to improve some details. – Robert Seifert Oct 21 '17 at 11:22
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    @NickT the power electronics are two pulse-width modulated 6H-Bridge AC/DC converters for controlling electrical motors and magnetic bearings. Changing system A and B is not an option and not necessary. The question was intended to improve some details of a similar system we're operating for years without great problems. – Robert Seifert Oct 21 '17 at 11:24
  • @RobertHarvey I'm still having trouble imagining how it actually is possible to multiplex dozens of measurement signals with a bandwidth of >200kHz without losing too much information, but I will read up on this. – Robert Seifert Oct 21 '17 at 11:33
  • @thewaywewalk `It means I have some analog measurement signals, which interest me up to 500 kHz and I have some rectangular control signals (PWM) oscillating with 20 kHz, with stiff edges.`- then you don't have *a single problem to solve*; you actually have *two problems*. XY problem, once again. You *can* `just (...) improve some details`; you can also *fix the engineering problem(s)*. You asked your question in a wrong way, because you've actually asked *two distinct things* (cont.) –  Oct 24 '17 at 22:08
  • 1. *Is there a low-cost alternative to RJ45 cables for large number of signals?* 2. We are designing two interface (printed) boards and need to connect them by 0.5-1.5m cables. The problem is, we are talking about 200 signals (-10V to +10V), which would require e.g. 50x RJ45 cables. *Can this be done more efficiently than by just using them*? ... some people answered the first (title) question, and I don't blame them for that - their ideas are pristine. Still, you also asked the second question... and some people (me, Nick T, Robert Harvey included) tried to answer it *as well*. –  Oct 24 '17 at 22:11
  • @vaxquis I'm sorry but I never asked the second question, you invented it or interpreted it. The basic setup was never in question and its improvement never asked for. Because it cannot be changed. It's not an XY-problem, it looks kind of clear too me. Its not the best setup I can think of, and you would never use in a real application. But this is a research project and we need some more signals than usual. I didn't stated that, because it didn't matter imho. – Robert Seifert Oct 25 '17 at 05:51
  • @thewaywewalk Your question(-s) on EE serve mostly for other people - that's the very idea of SE network. You *asked* the second question, implied - `So I wonder if there is an alternative?` (see the 1st version of your question) - *only* after your `I'm not searching for an improvement of the basic setup as it cannot be changed.` (nb proving the question should probably been closed as "recommendations for specific products") the question nr 2 got out of consideration. BTW saying "it cannot be changed" it false - you *don't want* to change it, and I understand that. Still, it *can* be changed. –  Oct 25 '17 at 13:36

6 Answers6

45

There are ribbon cables with twisted wires:

enter image description hereenter image description here

Every few cm, there is a flat section to allow attaching an IDC connector.

Looking at two product drawings it appears to be pretty typical to have 50 mm of flat cable out of every 500 mm.

The Photon
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    The big advantage of this as a solution over 50x separate RJ45 cables is the reduction in the possibility of connecting the wrong cables! 8x 25-pair cables reduces the possibility of mis-plugging considerably... only 8 connectors at each end! – matt Oct 20 '17 at 11:07
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    So probably 6 of these (2x32 wires each) would be enough, together with 12 connectors it would be around 120€. Seems like quite a good option. I didn't know that these cables exist. Thank you! – Robert Seifert Oct 20 '17 at 11:13
  • I was going to say the same. Flexi cable or flat cables. – Dimitri Oct 25 '17 at 07:02
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SCSI cables come to my mind.. they have lots of pins (68 in the ultrawide version), are well shielded, and can be gotten rather cheaply since nobody uses SCSI storage anymore. If you go for the LVDS type, the connectors are even acceptably small (although painful to route then).

SCSI LVDS and HD68 connectors

Community
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WooShell
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    "can be gotten rather cheaply since nobody uses SCSI storage anymore" meaning they will soon become pretty expensive because there's no market – pipe Oct 20 '17 at 12:15
  • More expensive than the twisted ribbon cables, but still an option. Thank you! – Robert Seifert Oct 20 '17 at 12:48
  • https://www.ebay.com/itm/131038638426 two of these lots (each lot is 2) are 36 USD shipped worldwide. Then you need four of https://www.ebay.com/itm/371862526654 these. One advantage here is the really small space the connectors would take especially compared to 50 RJ-45. – chx Oct 20 '17 at 21:26
  • Low Voltage Differential SCSI was my first thought too. 34 pairs per connector and off-the shelf cables and connectors. And the cables are designed for reasonably high frequencies too. – Tonny Oct 21 '17 at 09:52
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You can use some form of ribbon cables. A cheap option is to use IDE cables, like those used by older PATA hard drives:

IDE cable image

Each of those has 40 pins, so 5 of them gets you the 200 pins you want.

Another option would be to use direct pins to connect the boards. However, if the PC/104 standard is already a bit hard to connect (due to sum friction from all the 104 pins), then 200 will surely be harder to connect.

If you have more freedom when connecting the boards, you can also use a SODIMM (or even the older DIMM) slot and connect boards directly: SODIMM card image

Some versions have 200 pins. By the way, this is exactly the same interface that the Raspberry Pi Compute Module uses (also 200 pins): Raspberry Pi Compute Module image

On the other hand, if you can design it in, it might be better to just serialize your data before getting it out of the board, unless your whole circuit is analog.

Ronan Paixão
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    OP already considered and rejected ribbon cable. And the board-to-board options don't reach 1.5 m as required. – The Photon Oct 20 '17 at 14:57
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RJ21. And look for used, because they are falling out of favor rapidly in favor of VoIP. They're the kind of thing that's such a glut on the market that people don't even bother putting them on Craigslist.

It is the connector used with old office phones, when the standard wiring was 25 pair. It's a fairly bulky connector, but offices had them by the millions and it's still widely supported.

enter image description here

Harper - Reinstate Monica
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  • OP mentions RJ21 in the question, and that it's too expensive. – derobert Oct 20 '17 at 17:59
  • @derobert I just cleared out an office full of them, so they're dirt cheap to me, basically rubbish. OP might look for them used. – Harper - Reinstate Monica Oct 20 '17 at 18:12
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    @Harper I believe they were not that common on this side of the ocean, I actually have never seem them outside of our laboratories. However buying them from the US via ebay seems quite cheap. – Robert Seifert Oct 20 '17 at 19:57
  • @thewaywewalk Not a world standard then? Surely Europeans must have a plug connector they used for much the same purpose in the 1960s-1980s. – Harper - Reinstate Monica Oct 20 '17 at 21:21
  • @Harper I was born on the east-side of the wall, maybe they were hiding these connectors from us? ;) – Robert Seifert Oct 20 '17 at 21:41
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    @Harper I believe in the 60s we simply wired them in - you couldn’t unplug them. There were other plugs which looked like large headphone jacks, but this was rare - and expensive. Later than that (81) we used the British BS6312 431A plug - similar to the RJ11 but not compatible. This is still in use today in Britain, and other (mostly commonwealth) countries. – Tim Oct 21 '17 at 10:20
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Cables are expensive, what could you do? RJ45 is probably the cheapest option. If you even take 100 of them, it will still be cheaper than 500euro. But I have to say that for such things you should either not save money and get a good solution (because changing it later will anyway cost you a ton) or think, what is your system mistake. Maybe taking all those signals into one serdes channel over a fiber optic cable would be easier, cheaper and more reliable?

  • Something like a fiber optic cables and a serialized transmission is no option. But yes thinking about the systems flaws is part of the discussion. – Robert Seifert Oct 20 '17 at 10:51
  • What kind of signals are you transferring? I know systems where such amount of data goes through a slipring, so 100 pairs of wires really look too much –  Oct 20 '17 at 10:56
  • We have 48 analog signals (measurement signals from sensors) and about 120 digital control signals (where some are just inverted signals of others, some others are basically "reference"/ground - so there is room for optimisation here - we don't need all of them) – Robert Seifert Oct 20 '17 at 11:18
  • In that case you need a multiplexed ADC (plenty of them, but start from Analog Devices, they have actually 48 or 64 channels) and FPGA for digital ones. Communication may be anything actually. On the other side i believe you have a DSP or something, so you will not need to turn analogs back to analog, just use the communication. –  Oct 20 '17 at 11:21
  • The system on the first side of the cable is actually a HIL-System with FPGA and 48 ADC/DACs, on the other side are the power electronics and sensorics. The two interface boards I'm talking about are necessary to actually be able to connect both parts. It's just wiring and redistributing as well as some filtering and amplification going on on the interface boards. The complicated stuff happens behind each interface board. Basically I have 20 different connectors (which can change in some cases) on one side and 4 DSUB-50 plugs on the other. – Robert Seifert Oct 20 '17 at 15:35
  • In that case i wonder why 500euro is a problem –  Oct 20 '17 at 15:59
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    Have you ever worked in academics? The left part of the system is 10 years old or built by ourselves with few money, the right part is just borrowed from the german government and for everything in the middle we need to turn every cent twice ;) – Robert Seifert Oct 20 '17 at 16:02
  • Oh. Sell the one from the government to Iran. –  Oct 20 '17 at 16:03
  • @GregoryKornblum borrowed, not given. They can’t sell it. – Tim Oct 21 '17 at 10:23
  • I was kidding, don't take it seriously –  Oct 21 '17 at 10:24
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Going along the SCSI path that @WooShell started, these come in long ribbon cables as well as the molded plastic version picture in @WooShell's post.

http://www.mouser.com/Wire-Cable/Flat-Cables/_/N-5ggm

That link has options for new cables even longer than 1.5m. Specifically the 1.5m option has 50 pins. In fact, there's 10 in stock that they can ship immediately for $18.85 USD. They ship internationally, too.

Digikey.com has many options for electronics, including cables. I can't get to the site right now, but they should have other options for you.

Going with a SCSI cable gives you the dual row 2.54mm spacing pin header already built onto the cable, instead of having to add it yourself. If for some reason you're looking to daisychain the boards together on the same cable, some SCSI cables come with connectors mid-cable, too.

computercarguy
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