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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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Joren Vaes
    Commented Oct 19, 2017 at 17:18
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    \$\begingroup\$ 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. \$\endgroup\$
    – Dean Franks
    Commented Oct 19, 2017 at 18:38
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    \$\begingroup\$ 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? \$\endgroup\$
    – bobflux
    Commented Oct 19, 2017 at 19:31
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    \$\begingroup\$ RJ45 is a connector not a cable. \$\endgroup\$
    – R.. GitHub STOP HELPING ICE
    Commented Oct 20, 2017 at 1:36
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    \$\begingroup\$ 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? \$\endgroup\$
    – Robert Harvey
    Commented Oct 20, 2017 at 19:33

6 Answers 6

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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.

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    \$\begingroup\$ 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! \$\endgroup\$
    – matt
    Commented Oct 20, 2017 at 11:07
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    \$\begingroup\$ 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! \$\endgroup\$
    – Robert Seifert
    Commented Oct 20, 2017 at 11:13
  • \$\begingroup\$ I was going to say the same. Flexi cable or flat cables. \$\endgroup\$
    – Dimitri
    Commented Oct 25, 2017 at 7:02
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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

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    \$\begingroup\$ "can be gotten rather cheaply since nobody uses SCSI storage anymore" meaning they will soon become pretty expensive because there's no market \$\endgroup\$
    – pipe
    Commented Oct 20, 2017 at 12:15
  • \$\begingroup\$ More expensive than the twisted ribbon cables, but still an option. Thank you! \$\endgroup\$
    – Robert Seifert
    Commented Oct 20, 2017 at 12:48
  • \$\begingroup\$ ebay.com/itm/131038638426 two of these lots (each lot is 2) are 36 USD shipped worldwide. Then you need four of ebay.com/itm/371862526654 these. One advantage here is the really small space the connectors would take especially compared to 50 RJ-45. \$\endgroup\$
    – chx
    Commented Oct 20, 2017 at 21:26
  • \$\begingroup\$ 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. \$\endgroup\$
    – Tonny
    Commented Oct 21, 2017 at 9:52
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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.

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    \$\begingroup\$ OP already considered and rejected ribbon cable. And the board-to-board options don't reach 1.5 m as required. \$\endgroup\$
    – The Photon
    Commented Oct 20, 2017 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

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  • \$\begingroup\$ OP mentions RJ21 in the question, and that it's too expensive. \$\endgroup\$
    – derobert
    Commented Oct 20, 2017 at 17:59
  • \$\begingroup\$ @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. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Oct 20, 2017 at 18:12
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    \$\begingroup\$ @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. \$\endgroup\$
    – Robert Seifert
    Commented Oct 20, 2017 at 19:57
  • \$\begingroup\$ @thewaywewalk Not a world standard then? Surely Europeans must have a plug connector they used for much the same purpose in the 1960s-1980s. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Oct 20, 2017 at 21:21
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    \$\begingroup\$ @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. \$\endgroup\$
    – Tim
    Commented Oct 21, 2017 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?

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  • \$\begingroup\$ 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. \$\endgroup\$
    – Robert Seifert
    Commented Oct 20, 2017 at 10:51
  • \$\begingroup\$ 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 \$\endgroup\$
    – user76844
    Commented Oct 20, 2017 at 10:56
  • \$\begingroup\$ 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) \$\endgroup\$
    – Robert Seifert
    Commented Oct 20, 2017 at 11:18
  • \$\begingroup\$ 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. \$\endgroup\$
    – user76844
    Commented Oct 20, 2017 at 11:21
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    \$\begingroup\$ 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 ;) \$\endgroup\$
    – Robert Seifert
    Commented Oct 20, 2017 at 16:02
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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.

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