[TriEmbed] I2C range extension (Chip McClelland)
Fred Ebeling
FEbeling at ECPDesigns.com
Wed Sep 30 13:18:13 CDT 2015
This has been an interesting discussion, but perhaps a bit of history would
help. The original I2C protocal was developed by Philips to interconnect
HIFI equipment together and still be able to use a 3 connector plug. It was
designed for short distances, perhaps 10 feet max.
To go further the normal “standard” is to use RS 485 (or RS 422) standard. This
uses three wires, a ground, and two signal wires that are opposite of each other.
So when the TX+ sends a “1”, the TX- sends a “0”. The wires are also terminated
usually with a 120 ohm resistor on both ends. Thou some ignore this rule.
RS-485 can go thousands of feet. I have set up systems with the length of over
4000’ between the controller and the node.
Speed is based on distance, the major limitation is the “capacitance of the wire”. This
will cause the leading and falling edge to degrade and limit the speed to around 10KHz
for long distances. Shorter distances (100’) can get up to 1 MHz.
The problem thou is this is one way communications at a time. Think of it like the
old CB radios. You send a packet to the “good old boy” at the end node and say “Over”
so the end node can respond back.
RS 422 eliminates this by having separate xmit and rec pairs.
In either case, the RS485 IC are reasonable priced and there are even standards for
the connectors and connections.
Fred Ebeling
ECP Designs
From: Charles McClelland via TriEmbed
Sent: Wednesday, September 30, 2015 1:54 PM
To: triembed at triembed.org
Subject: [TriEmbed] I2C range extension (Chip McClelland)
Charlie,
I agree with John’s recommendations but wanted to pass on my experience. I can reliably go 8’ with i2c under the following conditions:
- Cat5 cable and paying attention to which pairs are used for SDA / SCL
- Speed set to 100kHz
- 4.7k pull-ups to 3.3V
I need to get to 50’ so I have ordered a few P82B715 chips. These chips advertise to get to 50’ but only if you are using 5V logic so we will see if they can go 50’ at 3.3V. I will share my results at the next meeting.
Hope this helps,
Chip
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Today's Topics:
1. Re: Bulk LED Order (Adam Haile)
2. I2C range extension (Charles West)
3. Re: I2C range extension (John Vaughters)
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Message: 1
Date: Tue, 29 Sep 2015 13:58:17 -0400
From: Adam Haile <email at adamhaile.net>
To: "triembed at triembed.org" <triembed at triembed.org>
Subject: Re: [TriEmbed] Bulk LED Order
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Final call! Putting in quote request tomorrow. Let me know if you want
anything.
On Fri, Sep 11, 2015 at 11:14 AM, Adam Haile <email at adamhaile.net> wrote:
I recently had to tear down my old porch lighting install when we had our
house repainted, so I'm going to be ordering a bunch of new lights from my
supplier in China.
Like last time, if anyone wants to get in on the order, the pricing is a
huge discount from normal suppliers like Adafruit, Sparkfun, Amazon, eBay,
etc.
I don't take a profit, the prices in the pricing lists (GDrive link below)
are direct from the manufacturer. The major point here is to save on
shipping for everyone. Which I'll divide between all buyers based on the
percentage of the overall purchase price which is yours.
Pricing lists here:
https://drive.google.com/folderview?id=0B7a_WsLpan5-SEl5TjAtTjctZm8&usp=sharing
So, if you would like anything, please let me know in the next couple of
weeks and give me the exact model number and quantity. Most things come in
5m rolls, so you can tell me rolls or meters.
Once I get everyone's details I'll will request a final quote (some of the
pricing can vary by quantity) and then send everyone their final cost.
*PLEASE NOTE*: Since I have to pay the manufacturer via PayPal, you *MUST* pay
me via Paypal. That way no money has to go through my own personal bank
accounts first. So, sorry, no cash or checks. And payment must go through
prior to submitting the final order to the manufacturer. Sorry if this is
an inconvenience, but it's the easiest way for me to do it.
Let me know.
Adam
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Message: 2
Date: Tue, 29 Sep 2015 22:03:01 -0400
From: Charles West <crwest at ncsu.edu>
To: TriEmbed <triembed at triembed.org>
Subject: [TriEmbed] I2C range extension
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Hello,
Thanks again for all the good discussion on electrical connectors.
We're still debating some of the specifics for the CANInstall and/or
I2CPotential protocols. One of the big things up in the air is how
reliable I2C is and how long the range can be.
My impression is that having a I2C bus of more than a meter or two is
considered pushing your luck. However, there seem to be range
extender/repeater chips available that promise rather drastically improved
range (such as the P82B715).
If I may ask, what is the longest I2C bus you have built/seen?
How has your experience been in terms of reliability?
What sort of bus speeds do you tend to use?
The bus speed is driven by the master, so theoretically you could have a
sub-hertz baud rate? Does this mean that you could get a really long range
with a super slow baud rate or are the edges not sharp enough to be
detected?
Thanks,
Charlie
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Message: 3
Date: Wed, 30 Sep 2015 12:54:03 +0000 (UTC)
From: John Vaughters <jvaughters04 at yahoo.com>
To: Charles West <crwest at ncsu.edu>, TriEmbed <triembed at triembed.org>
Subject: Re: [TriEmbed] I2C range extension
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Charlie,
The biggest problem I would be concerned about on i2c over a long distance is noise susceptibility that could affect reliability. So the importance of reliable communication is at question. I guess the first question would be, do you have an environment that is noisey? Then, Do you care if you have interruptions? If your environment is fairly clean of noise and you can live with communication interruptions, then long i2c may be a good low cost solution for long runs.
Here is what you may want to consider.
1. Take control of the pull up resistors by disabling them on the chip.
2. Tune the pull up resistors by starting at a safe low level and then increase them until you stop getting communication at the length you are running. Then back down. Keep in mind that you will gain more reliability as you reduce the pull ups.
3. Twisted pair cables would be your first attack on noise.
4. If you still encounter noise, consider ferrite beads to supress. This is best done if you have an idea of the noise frequency.
The alternative is if you truly desire reliable communications, then 485/CAN is the way to go. CAN being the most expensive, but gives you the similar capability to i2c with the ad hoc interruption communication capability. Or better stated an adhoc like network with a priority assignment.
I personally never ran i2c that long and cannot answer those questions. I generally have used 485 the most. The TI chips are usually the lowest cost for communication. For some reason people love the half duplex, but I prefer the full duplex.
I would be curious to know how far you make i2c run if you go that route.
John Vaughters
On Tuesday, September 29, 2015 10:03 PM, Charles West via TriEmbed <triembed at triembed.org> wrote:
Hello,
Thanks again for all the good discussion on electrical connectors.
We're still debating some of the specifics for the CANInstall and/or I2CPotential protocols. One of the big things up in the air is how reliable I2C is and how long the range can be.
My impression is that having a I2C bus of more than a meter or two is considered pushing your luck. However, there seem to be range extender/repeater chips available that promise rather drastically improved range (such as the P82B715).
If I may ask, what is the longest I2C bus you have built/seen?
How has your experience been in terms of reliability?
What sort of bus speeds do you tend to use?
The bus speed is driven by the master, so theoretically you could have a sub-hertz baud rate? Does this mean that you could get a really long range with a super slow baud rate or are the edges not sharp enough to be detected?
Thanks,
Charlie
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