DIGOO DG-HOSA – Part 1 (Teardown and Hardware)

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This project started with the idea of purchasing a cheap security system off one of the Chinese stores. After a little hunting, I found Digoo DG HOSA 433MHz 2G&GSM&WIFI Smart Home Security Alarm System Protective Shell Alert with APP which looked interesting so picked one up to tear apart. I was curious about how various communication methods were implemented.

This is the first part of this adventure the next part will be exploring the firmware of the device. With that let's take a look at the hardware.

Teardown Time

After the device showed up, I quickly got down to taking the device apart. In my haste, I didn't take many good photos of it intact. The front side of the board is straight forward; it contains the screen, button array for all user input, and a lot of useful test points. The front side is pictured below.

Board Front

The most significant information found on the front side of the board is the notation PG-103, which is also found in the firmware (spoiler). After some searching, I found this device is also branded as the PGST PG-103. This kind of rebranding of hardware is not unusual for a lot of Chinese devices.

Now switching to the back of the board, which is the business side of the board with the main chips and modules providing the various communication methods. When opening that device I encountered the intrusion detection button. This button causes the device to go into an alarm mode and require a reset of the device to come back online. For my testing, I bypassed this button bridging both sides of it.

Back of Board
Board Back

Component List

When inspecting the board, I found a few significant components and modules on the board. I was not surprised to see that most of the major communication parts are off the shelf modules. The components listed below are highlighted in the image above and the relevant data sheets where available are linked.

The main processor is a GigaDevice GD32 chip which is a series that is very similar to the of STMicroelectronics STM32 chips. The GD32F105 chip uses an ARM-based instruction set and has the same pinout as the STM32F105 component.

Block Diagram

The high-level block diagram for the device is pretty straight forward. The GD32F105 chip is the primary processing and control of the external communication modules. This allows for a modular architecture all of the peripherals.

 |  Cellular             +-----------+
 |  Quictel M26          |           |
 +-----------------------+           |
 +-----------------------+  +--------+-------+
 |  WIFI                 +--+   CPU          |
 |  HF-LPB120-1          |  |   GD32F105RCT6 |
 +-----------------------+  +--------+-+-----+
 +-----------------------+           | |
 |  433mhz receiver      |           | |
 |  SYN511R              +-----------+ |
 +-----------------------+             |
 +-----------------------+             |
 |  Keypad Controller    +-------------+
 |  Holtek BS83B16A-3    |

Pin Out

When exploring the board there are many test points on the board and tracing them out I was able to trace out most of the pins to where they connect on the controller.

  • SYN515R Pin 10 (DO) -> CPU PB9 (62)
  • Unknown -> CPU PA5
  • Unknown -> CPU PA6
  • Unknown -> CPU PA8
  • U7 SCL -> Unknown
  • U7 SDA -> Unknown
  • DAC_OUT -> CPU PA4 (20)
  • WIFI UART TX -> CPU PA2 (16)
  • WIFI UART RX -> CPU PA3 (17)
  • GSM UART TX -> CPU PA12 (45)
  • GSM UART RX -> CPU PA13 (46)
  • U1 (F117) Pin 6 -> CPU PB 8


After investigating the hardware I was able to extract the firmware and start the reversing process. I will cover what I have found in future posts. For now, if you are interested in more higher resolution photos of the board I have posted them on my Flickr account.

OpenSky Radio Trunking System

gqrx capture

OpenSky is a proprietary trunking radio that is designed to carry both voice and data traffic. the protocol is marketed as to be secure and private. Opensky operates on the 700, 800, and 900 MHz bands.

OpenSky was originally developed by M/A-Com as part of the Monarch wireless voice and data system for FedEx in the 90s. Later M/A Com was purchased by Tyco Electronics who was then purchased by Harris RF Communications. Harris has now merged with L3 Technologies to become L3Harris. This protocol has gone on a wild ride of Mergers and Acquisitions for this protocol hasn't it? The original OpenSky protocol was upgrades in 2010 and named OpenSky2.

The integrated data capabilities in OpenSky allow for more features in one single base station than voice-only trunking systems. This integration has allowed for dispatchers to have location data for radios in the field and, the ability to send data to terminals in for example police car, and for users log into the handsets pulling down their profile with the various talk groups and other preferences.

OpenSky and OpenSky2 are TDMA based protocols they have been designed to operate using 25W micro repeaters. Opensky2 introduced support for the 900mhz band and a more narrow bandwidth. The table below lists them out.

Number of Slots42
Raw bit rate (bps)19,200 9,600
Channel Width (khz)2512.5
Frequecy bands700 / 800700 / 800 / 900

Signal Harbor describes there are 3 major components to the OpenSky signaling protocols:

  • FMP (Federal Express Mobile Protocol) - Providing Digital Voice
  • OCP (OpenSky Communication Protocol)
  • OTP (OpenSky Trunking Protocol)

These protocols are based off a modified CDPD (IS-732) similar to a an IS-54 (D-AMPS) network. I could not find a lot of exact details on lower level protocol operations other then Each radio assigned an IP address.

Digital voice is encoded using the Advanced Multi-Band Excitation (AMBE) speech encoding standard. This a proprietary standard that was developed by Digital Voice Systems, Inc. Interestingly this standard has been using in the Iridium Network and XM Satellite radio. There are more details on this standard here and here.

This protocol came to my attention while visiting my parents in Oakland County Michigan that has adopted OpenSky as a Countywide standard in 2002 for radio communications. Oakland county like many other areas is now replacing their OpenSky systems with a P25 system. Many of the large deployments have run into issues, for example, the State of Pennsylvania has had many issues and is replacing the system with a P25 Phase II system. The State of New York has had an OpenSky deployment which has run into many issues which are detailed in the OpenSky Wikipedia article.

I found this protocol interesting because, like most technology, it's a product of it's time. In this case OpenSky comes from a time before the pervasive presence of 4G/LTE wireless. Today you can accomplish many of the same goals as a OpenSky system by utilizing the current carrier LTE networks.

Below is a list of resources I used when researching this protocol: