I'd like to use the i2c on the 16c74 over long distances i.e. 1000ft
and have thought of using rs485 drivers. However, I want to use
multimaster, is there any real problems i.e. when 2 masters clash?
i.e. current drain etc...
I'd like to use the i2c on the 16c74 over long distances i.e. 1000ft
and have thought of using rs485 drivers. However, I want to use
multimaster, is there any real problems i.e. when 2 masters clash?
i.e. current drain etc...
> I'd like to use the i2c on the 16c74 over long distances i.e. 1000ft
> and have thought of using rs485 drivers. However, I want to use
> multimaster, is there any real problems i.e. when 2 masters clash?
> i.e. current drain etc...
The idea of I2C is in a open-drain driver. You may use it over a
longer distance if you down-clock it (there is no limit - clock can
go down to DC), with heavier pull-up resistors and powerful
pull-down drivers.
The multi-master protocol features an 'arbitration procedure', that
is, if both masters start transmitting at the same time, the one to
pull the data line low while the other one has released it (sent 1)
shall win arbitration. The other master will wait until the bus is
free again before attempting to retransmit.
As a suggestion: there are other serial protocols which may work
better at the distances you need: Dallas 1-wire, CAN, etc... I2C is
really designed as an 'Inter IC' protocol.
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Extracted from The I2CFaq (c) Vincent Himpe.
Available from FTP//ftp.uni.paderborn.de/elrad
Check the INDEX.00 file there to know where it is
5.0) Q&A section
Q - What is the maximum distance of the bus ?
A - This depends on the load of the bus and the speed you run it at.
In typical applications a few meters (3 to 4). Better: The maximum
capacitive load has been specified (electrical Spec's in this FAQ).
If you run at a lower clock frequency then you could go further.
If you are careful in routing your PCB's and cabling then you can
take it further.I once had an application that had a total of about
100 meter cable in it. The entire system was clocked on something
like 500 Hz.
I used twisted pair cable and twisted SCL with GND and SDA with VCC.
No problem.The systems is now up and running for over 2 years.
Q - I want to extend it ''by the book''. Is there something like a Buffer
for I2C ?
A - Yes indeed this exists. Philips manufactures a special chip to buffer
the bidirectional lines of the I2C bus. Typically this is a current
amplifier. What it does is force current into the wiring
(a couple of mA). That way you can overcome the capacitance of long
wiring.
