# Swapping out RJ-45 connector on Differential I2C circuit

I'm trying to use Sparkfun's Differential I2C Breakout circuit into my own design, and I'm looking to replace the RJ-45 connector. It's too big (having a footprint size of 15.24 mm x 15.24 mm), and I have some requirements that I need to meet. The original board uses an Ethernet cable to transmit the differential I2C signals across long distances. I'm trying to do the same with another connector.

Due to the environment that my project will be in, I have to use a shielded, twisted-pair cable, with the length of the slave sensor being 20 meters (max) away from the I2C master. With these constraints, I was thinking about using HDMI, due to it being readily available and meeting all of these requirements. However, based on some footprints I researched, the HDMI footprint is too big. Thus, I'm thinking about using a micro-HDMI to HDMI cable instead. HDMI has more than enough pins for my needs, but I was wondering if there were any other cable alternatives that will fit my needs and has a small PCB footprint?

EDIT: So, after some discussion and thinking, mini/micro HDMI might be okay in the long run for 3 meters, but in the long run, when I need 20 meters, HDMI is a no-go. It won't be enough to transmit power, so I'm going to need a higher gauge wire than AWG-30. I can't think of any pre-assembled cable assemblies that fit the length and other requirements for shielding, so is my only choice to make my own cables for this endeavor?

EDIT: To give more information about my board, it's more or less an I2C multiplexer board. I'm working off of a Raspberry Pi Zero and an Arduino Nano, and the size of the board is 2 inches by 3 inches. This configuration is meant to read from 8 I2C sensors, so I'm trying to fit eight differential I2C circuits onto one board. The Pi Zero and the Arduino have to be mounted on the board (though for the Pi, I'm not including the 40-pin header as it's not needed); therefore, most of the space is being taken up by the mounting holes of the Pi Zero and the Arduino's footprint, if that gives any indication of the amount of space I'm left with. So, with the 8 I2C sensors, imagine having 8 of the Differential I2C Breakout circuits on one board. I'm trying to find a connector that can deliver the two differential signal pairs as well as power/ground just like the RJ-45 used in Sparkfun's board. In terms of maximum size, given the size of the RJ-45, my max dimensions for the connector is approximately 10 mm by 10 mm. This is mainly just to ensure that there is enough space for adequate routing of the 8 differential circuits.

• Is there any reason why you don't simply relocate the RJ45 connector rather than add a whole new connector on board? Jan 28, 2020 at 23:47
• There are so many board to wire plugs/sockets out there in relatively compact size and mechanically reliable. however they need expensive crimping tools. - A more affordable solution would be purchasing cable assemblies. - Or you can pick your desired connector pair (socket/plug), then search for people who own the crimping tool and are ready to make the cable for you for a reasonable fee - Also Manufacturers also provide this cable assembly services. Here is a good one from Harwin (an leading player in this business) harwin.com/connectors-hardware/cable-assemblies Jan 29, 2020 at 15:18
• Molex, Samtec, JST and many other manufacturers are also have huge range of low to high power, high speed, fine pitch and shielded connectors and cable assemblies many of them designed for differential transmission Jan 29, 2020 at 15:18
• You can't get a good answer if you don't specify the maximum size of the connector. All you have given us is that a couple of connectors you looked at are "too big". Jan 29, 2020 at 15:27
• @NatsuKage Thank you for your reply. I apologize, but I left out information that would have been very helpful, and I apologize for that. I'm working with 8 I2C sensors, each having their own differential circuit. With 8 sensor circuits, the RJ-45 jack is too large, so this is my issue in trying to replace it with something smaller. Jan 29, 2020 at 18:11

You can use RJ14/RJ25 connectors with 4P4C plug and any 2x2 UTP cable. These are smaller than RJ45.

Note, that the original board has too much empty space and too many connectors. If all you need is one I2C line adapter, you can make PCB roughly the size of 6p6c jack footprint with all SMD parts (including I2C jack) on the other side of the board.

You can use micro HDMI cable, but those get quite expensive over 6ft. Besides, cables longer than 10m are usually heavy 24AWG with standard HDMI. Most (if not all) of long micro cables are active, i.e. they have built-in signal booster chips that require power. IMHO they are not worth it, considering relatively small footprint size reduction.

UPDATE:

Just to clarify - the reason I suggested registered jack is because it is historically used for many purposes, so one cannot really guess the function by appearance. This is not so for HDMI, USB and similar connectors, where people expect them to function a certain way. If you use them for your project expect someone eventually trying to plug expensive smartphone or camera into it, and then suing you for damages.

Have you tried a simple search for "rectangular connector"? Some of them have 0.8mm pitch and footprint smaller than HDMI/USB, for example MQ172 and DF52 series from Hirose, or CLICK-Mate from Molex and many others.

Or is it that when you say "find the right cable to go with it" you mean pre-terminated cable of required length? Then you definitely out of luck, because at 20m you won't find pre-made cable for non-common connector.

UPDATE 2:

I think by fixating on minimizing PCB footprint you are missing the whole point of miniaturization, which is to minimize the space occupied by final product including all the connections required for normal operation. Let me illustrate the point: on top is "IX Industrial" connector with PCB footprint 12.1 x 9.8 mm. Note, that routing traces to those SMD pins might take up more space yet. On the bottom is CAT5 certified 4P4C jack and RJ11 plug, with PCB footprint 12.0 x 12.55 mm.

The difference for the PCB is negligible, however the plug in the first case protrudes 18 mm more from the device. If you consider minimal bending radius of 10-conductor shielded cable comparing to 4-conductor UTP you will be adding easily 40mm to your device dimensions, while saving 2.7mm on PCB

Now, I could not find ready made IX cable longer than 5m, and that one was $50. On the other hand 20m UTP + 2 plugs can be made for about$4.50, if buying cable in bulk and crimping plugs yourself.

If all this does not sound important to you, at least go with something like MQ172 series I've mentioned above. They also have huge plugs, but at least you can terminate those yourself to the cable of any length. And the jack footprint is even smaller at 10 x 8 mm.

• Thank you for your reply. I was thinking of at first using micro-HDMI along with micro-HDMI to HDMI adapters, but I'll have a look at RJ-25. Are 6p6c cables twisted pair or single ended? Even if an RJ-25 jack will fit on my PCB, it's kind of moot if I can't find the right cable to go with it. Jan 24, 2020 at 20:56
• Sorry, I did a little update playing around in my PCB layout, using RJ-12 footprint which is roughly the same size as RJ-25. Certainly, it's smaller than RJ-45, but it still seems too large for my board. Jan 24, 2020 at 21:02
• 6p6c is a type of Registered Jack, not the cable. There are many UTP options available, including with 2 twisted pairs. You can even use basic CAT5, which is 4 twisted pairs, leaving other 2 unused. Jan 24, 2020 at 21:02
• You have a problem, then. 20m cable must have at least 24AWG (or better 22AWG) to keep attenuation at bay. There are no miniature connectors supporting such a heavy cable. Yes, you can use micro-HDMI to HDMI adapters but those will be larger than your board. Doesn't it defeat the purpose? Jan 24, 2020 at 21:12
• Thanks for your reply and revision. The main point of replacing the RJ45 jack was to find something with a smaller footprint for the distance we need, and the application is not commercial. With regards to 'finding the right cable', I meant finding a pre-terminated cable of required length that fit the requirements mentioned above. HDMI seemed to fit and was easily available so that's what I went with at first. Thank you for your suggestions. The rectangular connectors certainly seems more manageable than HDMI so they might work. Jan 24, 2020 at 21:43

the LAN cable is provide enough characteristic of your criteria, but if you mean the specification are twisted, shield grounded, the length of cable will be 20m or so, you can use specific LAN/ethernet cable type 6 / 6a / 7. which they are all twisted, shield ground and capable delivering your power over 20meter.

You can choose CAT type 7 over 6 because it will gives you more shield from noise / voltage spike from EMP/surge or RF interference.

but in my experience your power supply current must enough to deliver the power it needs for 20meters long. Tips: Use dedicated adjustable power supply for your Vcc/power input. if you have voltage drop, you can adjust it to higher enough to meets your requirement on the end of cable.

forget about HDMI cable, is useless for over 5meters, and your voltage across will drop enough to not powering up.

if your concerns is the plug, you can replace it with DIP connector pin. either you bought female and male adapter or you soldering it on your pcb.

https://www.mouser.co.id/ProductDetail/Harwin/M50-3100445?qs=sGAEpiMZZMs%252BGHln7q6pmzlZUuX%2F53qjn%252BBn1132V0U%3D

the smallest gap DIP 1.27mm x 5 = <10mm even with 2rows is still smaller than your criteria 10mm*10mm

the LAN/Ethernet cable is only 8 inside of it. give it +1 ground is 9. so 10pin is adequate for your projects. if the next concern is height problem, you can choose horizontal dip / connector. the layout must be sufficient enough to plug it.

• Thank you for your reply. To clarify, my cable needs to fit the criteria of being twisted pair and shielded (let's say length ~3.5m for simplicity). My main concern regarding the footprint is the space it takes on the board, not the connector's height, so yes, my concern is with the plug. In your suggestion with the DIP connector, I'd have to create my own cables to fit the plug, correct? Jan 29, 2020 at 21:39
• it seems you have misunderstanding the problem which cause by my suggestion with female header on board. you can choose male header with 10p 2rows 1.27mm pitch, if you never open the IDE cable type. the female header it's like ide header (but smaller of course with 1.27mm pitch.) the housing header is sold the way you can attach the nee cable just plug it. mouser.co.id/ProductDetail/TE-Connectivity/… Jan 30, 2020 at 2:07