After some thunderstorms came through, my OBi200 VoIP adapter stopped working. The network end worked fine, but there was no longer any dial tone, and the device status page for the PHONE port no longer showed any information.
I only had the device for 11 months, so I promptly contacted support. Since it was probably broken due to a surge/overstress event on the phone line, I decided to open it up and take a look. I wanted to see if there was anything obviously broken that I could just replace and bring it back online, as well as I was curious what parts they had used in the design (and if there actually was any protection on the ports).
As expected, there really isn’t much inside. There are three primary ICs:
- Marvell MCU which provides the Ethernet interface, system control, config pages, etc.
- RAM for the Marvell MCU
- A Silicon Labs Si32260-FM1 ProSLIC telephone interface IC
The rest of the board is power supplies and a few components required by the primary ICs. For what it’s worth there does appear to be an ESD protection IC on the USB port, but that’s good general practice for USB anyways.
I couldn’t find a full datasheet online for the Si32260-FM1. The best I found was a couple-page datashort with a block diagram, pinout, and a summary of what the device does. It essentially is everything necessary to provide a VoIP interface, including phone line voltage generation, DSP to encode and decode analog and FAX data, and a simple SPI interface for digital data transport (in this case to/from the MCU for transfer over Ethernet).
Note that the block diagram shows two channels, but the OBi200 only provides one phone channel. It appears that the second channel is connected and populated, so getting a second phone port is probably just a firmware change.
Unfortunately the datashort available didn’t have a reference circuit vendors usually provide, which more than likely is what Obihai used in this design. From what I can tell (without a bunch of probing to determine actually connections) there appears to be a simple analog filter on the frontend made of 0805- and 0603-sized components. The resistors appear to be either thick- or thin-film and the caps all MLCC. A quick check with my DMM didn’t find anything that was obviously open, short, or different than a neighboring part with a matched circuit shape.
I did not see anything in the way of TVS diodes, spark gaps, or any other component that would provide significant protection from a high-voltage transient event, which is somewhat unfortunate. Part of this is probably due to the small size, and the other due to there not being an actual ground lug anywhere on the product (the GND of the power port through a wall wart isn’t a true GND).
If I were to redesign this, knowing it’s probably going to connect to a set of phone lines that might be connected to a network of phone cable where lightning could possibly couple in, I would have probably added at least a couple TVS diodes and a GND lug. Most customers probably wouldn’t connect the GND lug, but it’s better than nothing.
Fortunately, there are surge suppressors for phone lines available, but of course it’s a separate product that needs to be purchased. I decided to go with a Tripp-Lite DTEL2 suppressor, which connects between the OBi200 and the phone network in my home.
In the end, Obihai honored their 12-month warranty, and I sent the broken device back before doing any additional debugging. I can only hope that adding an external suppressor will avoid another failure in the future.