i2C extender IC: how does shorts on the cable destroy my master i2c bus - Page 1
Forum section: electronics / projects

author: drakejest
author_rank: Regular Contributor
timestamp: December 30, 2020, 07:31:45 pm
content: So i have here aP82B96i2c extender IC which i can use to extend the i2c upto several meters away. Since i have a cable several meters away i decided to place a digital isolator to have galvanic isolation on my host controlle . On the datasheet onP82B96 page 19it mentions that a short circuit on the cable could damage the i2c buss of the host and given a sample protection circuit buut unfotunately i dont understand the configuration.another thing i do not understand is how a short circuit on the cable can damage the host. If its a Vcc short to ground then the bus would remain unharmed since power is going directly to ground.Here is thecurrent circuitDo i still need to place the shot circuit protection even though i have a digital isolator on the host?

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 30, 2020, 08:53:36 pm
content: If you short Vcc to SDA or SCL the will try to pull that low. Won't end well.

author: drakejest
author_rank: Regular Contributor
timestamp: December 31, 2020, 11:19:38 am
content: Quote from: Ice-Tea on December 30, 2020, 08:53:36 pmI see so when SDA/SCL will sink the short circuit current. But shouldn't it be that the P82B96 is the only thing that will be destroyed? not the master?Unfortunately i cant do the recomended circuit as is wwill make the SCL unidirectional and i need it to be bidirectional. So if thats the cause of the problem would adding a series resistance to the i2c buss on the cable side so that when a vcc shorts to either SDA SCL there is a current limiting resisto

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 31, 2020, 11:32:17 am
content: If you're running meters of cable, I2C is simply not the right solution - it's only designed for on-board distances.

author: drakejest
author_rank: Regular Contributor
timestamp: December 31, 2020, 02:20:46 pm
content: Well thats the reason for that IC P82B96 , it extends i2c to upto 25 meters

author: Renate
author_rank: Super Contributor
timestamp: December 31, 2020, 03:21:23 pm
content: Quote from: drakejest on December 31, 2020, 02:20:46 pmEh, I still say "ugh".I like RS-485 and Modbus.RS-422 or CANBUS is fine too.

author: drakejest
author_rank: Regular Contributor
timestamp: December 31, 2020, 04:40:02 pm
content: Quote from: Renate on December 31, 2020, 03:21:23 pmBut the slave device has only i2c, would complicate a bit to add another middle man micro- controller.Lets not fight over what protocol is better and please help me understand how the circuit protection circuit is protecting against short circuit

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 31, 2020, 04:56:23 pm
content: Quote from: mikeselectricstuff on December 31, 2020, 11:32:17 amHDMI does I2C over several meters just fine. Granted, it's shielded cable.

author: bson
author_rank: Unknown
timestamp: December 31, 2020, 08:40:53 pm
content: An I2C extender almost certainly has to include beefed-up drivers, with active pullup, to overcome capacitive cabling loads. It would be surprising if it died on a short though, you'd think a device like that would be rugged enough not to. I think 25m is just too long for I2C at any sensible clock and given the potential problems if a single clock pulse is lost (clocking until the device responds and then reconfiguring it to make sure it's in a desired state, which may not always work) - I'd say this is an ill-conceived idea to begin with.

author: drakejest
author_rank: Regular Contributor
timestamp: January 01, 2021, 12:24:43 am
content: Quote from: bson on December 31, 2020, 08:40:53 pmI doubt they wouldnt list it as 25meters at 390Khz speed that it wont perform, Texas Instruments is a reputable manufacturer after all.I dont mind the chip dying, a lot can go wrong on that 25meters, what im concerned is it dying taking the master controller along with it.