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I2C cable

Started by Winfield Hill May 10, 2019
We need a 1-meter length of 100kHz (or slower) 
I2C onnection.  It'd be nice to use a pre-made
cable: micro-USB, 4-wire RJ11, or RJ45 ethernet.
Some considerations: SCL and SDA crosstalk(?),
high ground capacitance.  Also need 3.3V, Gnd.

Micro-USB: SCL and SDA twisted together, bad?  
RJ11: flat, SCL and SDA on outside lines?
RJ45: SCL and SDA shielded from each other.


-- 
 Thanks,
    - Win
Winfield Hill <hill@rowland.harvard.edu> Wrote in message:
> We need a 1-meter length of 100kHz (or slower) I2C onnection. It'd be nice to use a pre-madecable: micro-USB, 4-wire RJ11, or RJ45 ethernet.Some considerations: SCL and SDA crosstalk(?),high ground capacitance. Also need 3.3V, Gnd.Micro-USB: SCL and SDA twisted together, bad? RJ11: flat, SCL and SDA on outside lines?RJ45: SCL and SDA shielded from each other.-- Thanks, - Win
- 400pf. Max @100khz . Cat5 should work. There are bus extenders too. Cheers ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Martin Rid <martin_riddle@verison.net> Wrote in message:
> Winfield Hill <hill@rowland.harvard.edu> Wrote in message:> We need a 1-meter length of 100kHz (or slower) I2C onnection. It'd be nice to use a pre-madecable: micro-USB, 4-wire RJ11, or RJ45 ethernet.Some considerations: SCL and SDA crosstalk(?),high ground capacitance. Also need 3.3V, Gnd.Micro-USB: SCL and SDA twisted together, bad? RJ11: flat, SCL and SDA on outside lines?RJ45: SCL and SDA shielded from each other.-- Thanks, - Win- 400pf. Max @100khz . Cat5 should work. There are bus extenders too.Cheers----Android NewsGroup Reader----http://usenet.sinaapp.com/
https://www.i2c-bus.org/specification/ -- ----Android NewsGroup Reader---- http://usenet.sinaapp.com/
Just for lab use?  Or is this part of the flex or beehive question from 
earlier?

I'd be loathe to suggest anything not shielded at least in some rudimentary 
way.  There are lots of styles of flat or round multiconductor cable that 
would do, and then a regular header will do on the end, or modular 
connectors if you like those.

Incidentally, any idea if PVC (outgassing, plasticizers, chewing?) 
compatible with bees?  How about electric fields?

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"Winfield Hill" <hill@rowland.harvard.edu> wrote in message 
news:qb40m902842@drn.newsguy.com...
> We need a 1-meter length of 100kHz (or slower) > I2C onnection. It'd be nice to use a pre-made > cable: micro-USB, 4-wire RJ11, or RJ45 ethernet. > Some considerations: SCL and SDA crosstalk(?), > high ground capacitance. Also need 3.3V, Gnd. > > Micro-USB: SCL and SDA twisted together, bad? > RJ11: flat, SCL and SDA on outside lines? > RJ45: SCL and SDA shielded from each other. > > > -- > Thanks, > - Win
Tim Williams wrote...
> > Just for lab use? Or is this part of the flex > or beehive question from earlier?
This goes from the little PCB located on the outside of the beehive, mating to the flex.
> I'd be loathe to suggest anything not shielded > at least in some rudimentary way. There are > lots of styles of flat or round multiconductor > cable that would do, and then a regular header > will do on the end, or modular connectors if > you like those.
Well, a nice thing about a CAT5 connector is, readily available in many lengths. I'm adding a Harting RJ45 jack to the PCB now.
> Incidentally, any idea if PVC (outgassing, > plasticizers, chewing?) compatible with bees? > How about electric fields?
Bees are robust, maybe, but hive operators stick to natural materials = wood. Near the end of the season, the environment in the hive can get pretty nasty. That's why I'm adding gas sensors. But dunno what we'll find. Maybe nothing. https://www.dropbox.com/sh/w1xbcwovma602he/AADzRhQgieifJ9z1FoSscDrHa?dl=0 -- Thanks, - Win
On Friday, 10 May 2019 17:03:04 UTC+1, Winfield Hill  wrote:

> Well, a nice thing about a CAT5 connector is, > readily available in many lengths. I'm adding > a Harting RJ45 jack to the PCB now.
RJ11 connectors work nicely for i2c in conjunction with the following four-core flat cable: Farnell 130-2745 which has a pvc outer sheath and polypropylene insulated conductors. I have used this combination in production with cable lengths of 7m. For such lengths it is important to follow the more recent NXP recommendations: Power and ground in the middle, sda and sclk on the outside. In most systems the largest source of interference is from the adjacent signal transitions. Power and ground decoupled to each other at each end of the cable. Many devices can be daisy chained, but keep them in a continuous line without branches to form a constant impedance transmission line. Consider some gentle RC damping at the controller end to minimise reflections. Treating the bus as a transmission line really does have benefits. More info later... John
jrwalliker@gmail.com wrote...
> > On Friday, 10 May 2019, Winfield Hill wrote: > >> Well, a nice thing about a CAT5 connector is, >> readily available in many lengths. I'm adding >> a Harting RJ45 jack to the PCB now. > >RJ11 connectors work nicely for i2c in conjunction with the following >four-core flat cable: Farnell 130-2745 >which has a pvc outer sheath and polypropylene insulated conductors. >I have used this combination in production with cable lengths of 7m. > >For such lengths it is important to follow the more recent NXP >recommendations: >Power and ground in the middle, sda and sclk on the outside. In most >systems the largest source of interference is from the adjacent signal >transitions. >Power and ground decoupled to each other at each end of the cable. > >Many devices can be daisy chained, but keep them in a continuous >line without branches to form a constant impedance transmission line. >Consider some gentle RC damping at the controller end to minimise >reflections. Treating the bus as a transmission line really does >have benefits. >More info later... >John
OK, John, yes, crosstalk is the big concern. And, hey, CAT5 is UTP, unshielded twisted-pair, not shielded. -- Thanks, - Win
Winfield Hill <hill@rowland.harvard.edu> wrote:

> OK, John, yes, crosstalk is the big concern. And, hey, > CAT5 is UTP, unshielded twisted-pair, not shielded.
SHIELDED CABLE (STP) vs UNSHIELDED CABLE (UTP) Cat 5E cables are most commonly utilized with unshielded twisted pairs (UTP). However, environments with large motors, generators, and certain types of high-voltage lighting are major sources of Electromagnetic and Radio Frequency Interference (EMI/RFI). In these applications, shielded twisted pair (STP) cable will help protect data signals when connected properly https://www.warehousecables.com/landing/cat5-shielded-cable.php Many other examples on google
On 10 May 2019 10:50:57 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>jrwalliker@gmail.com wrote... >> >> On Friday, 10 May 2019, Winfield Hill wrote: >> >>> Well, a nice thing about a CAT5 connector is, >>> readily available in many lengths. I'm adding >>> a Harting RJ45 jack to the PCB now. >> >>RJ11 connectors work nicely for i2c in conjunction with the following >>four-core flat cable: Farnell 130-2745 >>which has a pvc outer sheath and polypropylene insulated conductors. >>I have used this combination in production with cable lengths of 7m. >> >>For such lengths it is important to follow the more recent NXP >>recommendations: >>Power and ground in the middle, sda and sclk on the outside. In most >>systems the largest source of interference is from the adjacent signal >>transitions. >>Power and ground decoupled to each other at each end of the cable. >> >>Many devices can be daisy chained, but keep them in a continuous >>line without branches to form a constant impedance transmission line. >>Consider some gentle RC damping at the controller end to minimise >>reflections. Treating the bus as a transmission line really does >>have benefits. >>More info later... >>John > > OK, John, yes, crosstalk is the big concern. And, hey, > CAT5 is UTP, unshielded twisted-pair, not shielded.
Why do you need shielding ? Put clock in one twisted pair, data in an other pair and Vdd/Gnd in the third pair. If only two pairs available, use the non-active members of each pair for Vdd resp. Gnd. Use heavy AC decoupling between these non-active pair members. After all, 100 kHz is a quite low frequency.
On Friday, 10 May 2019 21:21:07 UTC+1, upsid...@downunder.com  wrote:

> After all, 100 kHz is a quite low frequency.
But the falling edges can be fast enough for transmission line effects to be important when the cable is a few metres long. Double clocking due to reflections makes a mess of i2c. Using the lowest possible pullup resistors or using active pullup that approximates to a constant current clamped at Vcc can help with noise immunity in the high state. John