# Should it be expected that some shielded wire is not good at data transmission?

I have recently purchased a reel of this specification screened wire from RS Components to use in I2C communications, expecting it to be very well suited to purpose. I am running at approximately 400 kHz clock on the wire and did not think for a second that the choice of cable might limit the speed I could run at. However on oscilloscope inspection, the bus signals are extremely dulled by the connection of a 3m length of this wire.

Is the problem with the I2C master not having a sufficient pullup resistor say, or is the wire genuinely at fault? Is there a significantly better wire specification I could use for this application?

Further Information:

It is true this wire is listed as 95pF/m. I have found a different spec of wire (for microphone use) that seems potentially impossible based on the physics discussed, but here it is, 55-105pF/km.

• At 100pF per meter, that 300pF and 1K ohm pullup implements 0.3uS time constant to 63%, 0.6uS to 90%. – analogsystemsrf Jun 22 '17 at 13:59
• I2C is not designed to be run over long lengths of wire like that. It is designed for connecting chips on a PCB (or at worst on interconnected PCBs with short jumper wires). – Majenko Jun 22 '17 at 14:10
• I2C isn't really designed to sent signals over a cable, it's designed for interconnection of ICs on a circuit board. It can be made to work over short cables, but 3 meters is going to be challenging. Certainly lower value pull-ups will help but I wouldn't run I2C over 3m of cable. – John D Jun 22 '17 at 14:11
• @Majenko Ha, you beat me to it :) – John D Jun 22 '17 at 14:12
• All I2C devices use an open-drain driver with a pull-up resistor. In combination with the cable (or board) capacitance this severely limits how fast you can drive I2C into a long cable. Try reducing your clock speed from 400kHz to 10kHz and see if it works. There are I2C Bus Extender ICs from Texas that can work at up to 50m using twisted pair cable. – Steve G Jun 22 '17 at 14:46

$\sqrt{\dfrac{L}{C}}$