RJ-45 for instrument interface

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

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ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
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https://hobbs-eo.com

On Mar 12, 2018, Steve Wilson wrote (in article):

I?d run RS-485/422 over Cat 6 with RJ-45 connectors.

Assume that accidental connection with anything using RJ-45 connectors will be common.

Power over this cable should follow PoE standards, for safety, and vailability of hardware. It took 802.3 a lot of effort to develop PoE in detail, and there is no percentage in trying to replicate that effort.

Joe Gwinn

Putting an FTDI USB-to-serial chip on your board is the easiest way to interface a small instrument. Your internal uP sees a serial interface, and the control computer sees a usb-serial chip. Most OS's are familiar with the FTDI chip.

It's equivalent to burying a USB-serial dongle inside your gadget. Use USB hubs and cables.

If you need to go long distances, go Ethernet but that's a bit more work.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

We used to use them in one of our main instruments. The two main reasons we stopped were:

1) Customers do amazingly stupid things. They WILL plug inappropriate things into those ports. 2) Too often we needed to feed 5V power at up to an amp through them, and the voltage drop was unacceptable.

We use FTDIs too. The problem in Windows is that you don't know how the ports are going to enumerate, so it's hard to use multiple instruments.

The Prologix GPIB-Ethernet can run a few GPIB instruments, but not a whole rack-full.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
https://hobbs-eo.com

Depends.... We inplemented a similar scheme but had full control over the PC controlling software and hardware and the same for the peripheral devices... PC had a 5 byte packet out and a 3 byte packet return. Each peripheral had its own ID which was part of the 5 bytes.. ID + command + parameter 1 + parameter 2 + crc. A responding peripheral sent back data1

• data2 + crc.All at 115K baud and a few hundred metres. Your issue will be POE..... CAT5 can't carry high currents. High voltages and low currents is the only way but beware of shorts from power to data !!!

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All of our gadgets ID themselves with model number and serial number, so an app can scan all the serial ports, looking for devices. That is a mild nuisance, but it works.

Do people still make instruments with GPIB? It's getting old.

We use Ethernet a lot too, both inside gadgets running Linux and bare metal, with "discrete" uPs and with SOCs. There are learning curves, but it becomes routine after the first one.

We recently persuaded the GbE interface of a ZYNQ to talk to a 100 Mbit PHY. That saved a lot of balls and long PCB traces.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

If you use an 802.3af, 802.3at, or 802.3bt interface for the power, it won't blow anything up. If you're pushing gigabit ethernet through the cable, it's about the only way to do PoE. Note that there are several different types of PoE, some of which are incompatible even though the RJ45 connector fits.

The handshake process is sufficiently complexicated that it would take a minor miracle or a very unlucky customer to fool the power injector into delivering full power. From my limited experience with various cheap PoE switches and phones, wrong injector type, weird midspan devices, insufficient injector power, rotten CAT5e crimp connections, creative pinouts, and AC power line glitches are the real problems, all of which will inhibit the injector from producing power. Methinks you're safe with PoE.

My guess(tm), based up not knowing your application or how much power you need to deliver, is that the price of a PoE system is likely to be the real problem.

Good luck.

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Jeff Liebermann     jeffl@cruzio.com 
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Well, you guys get a gold star then. ;) Not everybody does that, unfortunately.

Yup. Generally along with Ethernet and/or USB.

I've never actually implemented an Ethernet instrument--most of my stuff is still one-offs and proofs-of-concept. With DFH on full time and more licensing stuff in the works, that may change.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
https://hobbs-eo.com

We use the FTDI cables with a 3.5mm jack plug

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?

Yes, I need to go Ethernet.

Yeah, but... if you need the speed and flexibility of POE ethernet, USE IT, but don't pepper your devices with incompatible ports that mate with the same connectors.

Serial ports are a source of confusion, too, because of the variety of connectors and shortcuts... it has so much legacy, and so much cruft!

On the minimal side, Apple's old AppleTalk used a miniDIN-3 connector for a single shielded pair, would run a kilometer or so with multidrop connections. It autoterminated; two connectors on each device, and if one socket was empty, that switched in a 120 ohm termination resistor. With the right UART/SCC support chip, it would run transformer-isolated at 130 kbaud. There were protocol stacks for it, both ends needed some considerable smarts, but like gain, smarts is cheap.

More important, there's already USB dongles (Keyspan USA-28X) that support it. Well, they drive it adequately, you still need the transformers/connectors/ terminators. And if serial is required, that's available on the second port alongside the Appletalk.

Thanks for the informative reply. I do not need or want POE, I'm just concerned about someone plugging a cable in the wrong socket.

When you say "deliver full power", does that mean the POE can deliver partial power by itself or if it is misconfigured? I don't want to have to deal with 48V no matter what other faults may exist. 48V is not the problem, the difficulty is getting rid of the heat. I could add a shutdown circuit, but that adds complexity and another failure mode.

Thanks

Am 12.03.2018 um 20:58 schrieb Steve Wilson:

I did that with a BeagleBoneBlack.

Just ssh to it. vi / gcc / make / USB / Ethernet drivers / flash disk... everything already there. Just bang the port bits or use the i2c... of the chip set.

regards, Gerhard

Thanks. I'm thinking to use HTML as the interface to the host, so I probably want full Linux.

Using usbdeview one can find serial numbers or vendor or id numbers for many of the rs232 usb converters. And i found sime code on the web howto read that on a com port by c++. So it seems one can identify ports or chips. Klaus

you can run ubuntu on it, then you can also plug in a monitor and a keyboard if you like

GPIB: IT allows parallel poll and simultaneous sending commands to multiple listeners. So it is a good way to simultaneously trigger two lockin amplifiers for example. Set both as listeners then send trig to all devices that were set as listeners. Klaus