1

I would like to isolate digital serial communication between 2 circuits. I don't know much about the protocol between two circuits. It is not a common protocol such as SPI or I2C. Shortest pulses are 30μs and communication is bidirectional over a single channel/wire. I have found some such as Si860x series. https://www.silabs.com/documents/public/data-sheets/Si860x.pdf

All of such isolators are labeled as I2C isolators. But my serial communication isn't I2C. I wonder if there are any isolators for general usage or if I can use it for my application?

Background: I am trying to power my DSLR camera without the battery (but with a constant power source). But the camera verifies the battery with some serial communication. I am trying to fool the camera's circuit in which I will let the battery communicate with the camera over an isolator, while the camera is powered with another power source. For this, I think, I need a bidirectional isolator.

Saren Tasciyan
  • 263
  • 1
  • 12
  • There are these magnetic isolators by Analog Devices. They also have bidirectional variants for SPI and I2C, or just plain old random digital IO. Some are _extremely_ low power too! Do check out their ADuM iCoupler isolators. :) The only downside is, they're not very cheap. – Richard the Spacecat Jul 12 '18 at 08:58
  • Actually, that probably wasn't too helpful. To isolate a bidirectional (tristate) thing, you often would also have to have some output enable-like pin on it, and figure out the direction. However, if the battery talks over some One Wire-style protocol where there's only one or a few lines constantly pulled up and only pulled down by the master/slave, a 'dumb' I2C isolator, like the one you've linked, may possibly work there. – Richard the Spacecat Jul 12 '18 at 09:13

2 Answers2

3

There is a very good possibility that the camera is managing the battery through the one-wire serial interface to a chip in the battery pack. These chips are commonly referred to as "battery monitors" or "gas gauge" chips. You would want to take a look at the data sheets of several of these chips to see if you can match up the protocol spec against what you see on your oscilloscope. That would be the shortest path to reverse engineering the protocol.

I am not so sure that you would really need to have an isolation. If you evaluate the camera subsystem there is a decent chance that you could share the GND between the power supply, the camera and your small control subsystem.

If it turns out that you really do need an isolator then you would have to take a close look at the one-wire physical layer interface to see what polarity it is. There are some interfaces that use a common pulldown resistance and the members on the wire use an active driver to pull the line high. Then there is the other polarity, like I2C where a common pullup resistor is used and the members on the wire use an active driver to pull the line low. The SiLabs part that you linked to is one that supports I2C and there is good chance that the part would work with a one-wire protocol that has the common pullup resistor type of polarity.

Michael Karas
  • 56,889
  • 3
  • 70
  • 138
  • It is proprietary. I don't have access to any official information. I analyzed the data in between with a logic analyzer (with and without battery) but it is different every time, which may mean a cryptographic communication. The reverse engineering is therefore difficult. I asked this before here: https://electronics.stackexchange.com/questions/382286/what-kind-of-serial-communication-is-this?noredirect=1#comment933119_382286 – Saren Tasciyan Jul 13 '18 at 21:44
  • Common ground might be a hazard too. Thanks for the input on I2C drivers. I think the members (battery and camera) pull down actively. – Saren Tasciyan Jul 13 '18 at 21:48
0

I don't think a bi-directional isolator is needed to run your camera, For that. it seems more like you need to forge the protocol, or just leave the battery connected to the camera and connect a constant voltage charger also at the same time.

Jasen Слава Україні
  • 31,874
  • 1
  • 31
  • 65
  • Yes, I thought about that but the protocol is proprietary and each time it looks slightly different, which is for me an indication of cryptographic method. I think connecting everything together is a hazard for the camera. That is why I was thinking of using a bidirectional isolator. – Saren Tasciyan Jul 13 '18 at 21:46
  • what does the term "bidirectional isolator" mean to you? – Jasen Слава Україні Jul 14 '18 at 04:19
  • It means to me a component, which can isolate a bidirectional (where both sides of the wire transmit information on the same wire) digital serial communication. Is that wrong? I am learning electronics as a hobby. – Saren Tasciyan Jul 14 '18 at 11:25
  • it is unclear to me what you mean by "isolate" – Jasen Слава Україні Jul 14 '18 at 11:27
  • That electrically there can't be flow of current between both sides of the isolator. – Saren Tasciyan Jul 14 '18 at 11:30
  • so just disconnect the wire? but if you want to power the camera how will that help? – Jasen Слава Україні Jul 14 '18 at 11:31
  • I want to power the camera with a power supply, without the battery. But the camera doesn't work without verifying the battery and rejects the power supply. Reverse engineering of the verification seems to be too difficult. Instead, I want to connect the serial wire of the camera with the battery, so that the camera verifies the battery. To prevent any current flow between camera and and battery (for protection), I want to isolate them with a bidirectional isolator. – Saren Tasciyan Jul 14 '18 at 11:43
  • my suggestion above is to connect a 2 wire charger to the battery and leave the battery fully connected to the camera. – Jasen Слава Україні Jul 14 '18 at 11:45
  • I asked this before here: https://electronics.stackexchange.com/questions/312779/hacking-dslr-battery-to-power-camera-from-power-supply One potential problem, which was mentioned is that charger's logic might be "confused". By which I understand that the circuit wasn't designed for that and it may cause unwanted issues. As a more secure alternative, I though a bidirectional isolator would be a more secure approach. – Saren Tasciyan Jul 14 '18 at 12:01
  • yeah it wouldn't necessarily be that charger, might have to be built from modules, and if that battery has multiple cells might need more than 2 wires... seems to be 5 pins on that battery. seems like a lot of work, and has the potential to cost much more than it saves you. – Jasen Слава Україні Jul 14 '18 at 12:10
  • Yes, there are 2 cells within the battery, which would potentially lead to unbalanced charge/discharge. Wouldn't you agree in this case best and simplest thing to do is to put a bidirectional isolator on the serial line? – Saren Tasciyan Jul 14 '18 at 12:15