Forums

RJ-45 for instrument interface

Started by Steve Wilson March 12, 2018
On 03/12/2018 03:56 PM, Steve Wilson wrote:
> Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> What do you actually want to accomplish? I's be very unlikely to use an >> RJ-45 for anything but Ethernet, myself. Most newer instruments have >> Ethernet connectivity out of the box, and older ones can be controlled >> via e.g. a Prologix GPIB-Ethernet box. > >> Cheers > >> Phil Hobbs > > I have the Prologix interfaces. I do not want to use GPIB, RS-232, USB, HDMI, > or any of the other interfaces that have been discussed. > > Say I need to monitor a hypothetical process and measure temperature, > pressure, flow, etc. I need to make the necessary sensors and connect them to > a controller to gather the data and send it back to a host computer. The > controller is connected via the LAN. > > Say I need a bidirectional interface to control the sensors. To connect the > individual sensors to the controller, I propose using the same RJ-45 > interface used on the LAN, but using my own protocol instead of ethernet. > > Why would you not use RJ-45? >
For one reason, everybody assumes that an RJ45 is an Ethernet connector, so care is needed so that nothing gets blown up. For another reason, they aren't the most reliable connectors in captivity, especially when daisy-chained. For a third, there are a lot of different styles of Ethernet patch cords out there--especially crossover cords--and some squaddy is bound to use the wrong one. You'll get blamed for the resulting failure, as sure as God made little green apples. I've often repurposed video cables (Displayport and HDMI) but not RJ45, thanks. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On 2018-03-14, Steve Wilson <no@spam.com> wrote:
> Jeroen Belleman <jeroen@nospam.please> wrote: > >> Nothing prevents the use of coupling transformers at the end(s) of >> a length of coax. > > No, that doesn't work. It allows return currents to flow on the outside of > the shield. The makes the cable radiate RFI.
How's that going to happen? As I see it with the coax isolated the only current flowing in the shield is the return of the current flowing in the core and that current will be drawn to the inside of the shield by the magnetic field of the core.
> To prevent it, the coax must be > grounded to the case. This can provoke ground loops.
that's pretty much guaranteed to cause ground loops. -- This email has not been checked by half-arsed antivirus software
On Mon, 12 Mar 2018 12:56:00 GMT, Steve Wilson <no@spam.com> wrote:

> >What are your thoughts?
Unrelated to this request, but I ran across this from HP on using RJ45 connectors for analog instrumentation applications. They ended up using it for strain gauges, which, as most of you know, are rather low level devices. http://www.vtiinstruments.com/Catalog/Technotes/RJ-45_Excels_For_Stria_Gage_Connection.pdf The combination of good electrical performance and cheap/reliable termination of shielded or unshielded twisted pairs is useful. --sp
Jeff Liebermann wrote:
> On Wed, 14 Mar 2018 18:20:45 -0500, Les Cargill > <lcargill99@comcast.com> wrote: > >> There's also something to be said for M12 on the outside of boxes. > > Yep: > <http://www.iebmedia.com/index.php?id=5873&parentid=63&themeid=255&showdetail=true> > >> They're harder to kill and make positive contact. You can have a >> throughhole pigtail like you'd have using the above object. > > They're also substanitally more expensive than RJ45. >
Very much so! -- Les Cargill
Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote:

> On Mon, 12 Mar 2018 12:56:00 GMT, Steve Wilson <no@spam.com> wrote:
>>What are your thoughts?
> Unrelated to this request, but I ran across this from HP on using RJ45 > connectors for analog instrumentation applications. They ended up > using it for strain gauges, which, as most of you know, are rather low > level devices.
> http://www.vtiinstruments.com/Catalog/Technotes/RJ-45_Excels_For_Stria_Ga > ge_Connection.pdf
> The combination of good electrical performance and cheap/reliable > termination of shielded or unshielded twisted pairs is useful.
> --sp
Very interesting information. The connector is much more reliable than I thought. Excellent stability, 750 mating cycles, inexpensive and available everywhere. Pretty hard to beat. Directly applicable to my request. Thanks, Speff
On Mar 15, 2018, Jasen Betts wrote
(in article <p8d2ah$mp0$2@gonzo.alcatraz>):

> On 2018-03-14, Steve Wilson<no@spam.com> wrote: > > Jeroen Belleman<jeroen@nospam.please> wrote: > > > > > Nothing prevents the use of coupling transformers at the end(s) of > > > a length of coax. > > > > No, that doesn't work. It allows return currents to flow on the outside of > > the shield. The makes the cable radiate RFI. > > How's that going to happen? > > As I see it with the coax isolated the only current flowing in the shield > is the return of the current flowing in the core and that current will be > drawn to the inside of the shield by the magnetic field of the core.
Yes. We use RF transformers to break ground loops in coax all the time. If it&rsquo;s really critical, one uses shielded transformers, but if it&rsquo;s low power, a transformer the size of a peppercorn is too small to radiate much.
> > To prevent it, the coax must be > > grounded to the case. This can provoke ground loops. > > that's pretty much guaranteed to cause ground loops.
Yep. For RF, what also works is to make the inputs to power-frequency ground loops. See MIL-STD-461 method CS-109. Joe Gwinn