SX1276_Receive_LBJ

A LBJ Message Receiver Based on TTGO LoRa 32 v1.6.1 (ESP32 + SX1276 868MHz)

The SX1276_Receive_LBJ project is modified from RadioLib's Pager_Receive.ino and LilyGo-LoRa-Series's templates, based on the TTGO LoRa 32 v1.6.1 dev board. This project aims to provide an alternative solution to the expensive programmable pagers commonly used in receiving the Chinese Railway Train Proximity Alarm (aka LBJ, on 821.2375MHz following POCSAG protocol) messages often transmitted by on board LBJ systems through CIR.

Notice

This is an experimental project to learn embedded device programming and sub-GHz signal receives, thus have bad coding quality and may have unknown issues. Use it at your own risk.

Hardware

Based on TTGO LoRa 32 v1.6.1 dev board, utilizes SX1276's FSK modem to receive LBJ messages. The schematic of this board can be found here.

Additional Functions

Note: This is optional and requires additional hardware to be connected to the board.

Additional functions can be enabled by connecting specific peripheral to the specified pin as following:

1. DS3231 RTC

Share IIC Bus with on board SSD1306 OLED.

#define I2C_SDA                     21
#define I2C_SCL                     22

Keeps time while power off. If not used, the time will be acquired from NTP server every time after reboot. If connected uncomment the following line in utilities.h while building.

#define HAS_RTC // soldered an external DS3231 module.

2. AD Buttons (WIP)

Due to lack of GPIO a four key analog button is used.

#define BUTTON_PIN                  34

Provides input to interactive functions including logs inspection and device settings on OLED(WIP). If connected use the button_equipped branch instead of master while building.

More information about pin definitions can be found in utilities.h

Features

• Shows formatted LBJ messages on OLED display.
• Save Received messages to TF card in plain text and CSV format.
• Host Telnet server and send formatted messages to client.
• BCH error correction migrated from MMDVM_HS_Hat's BCH3121.cpp

Known Issues

• Takes a very long time to startup if a large number of files are in TF card.
• TF card hot-plug is NOT supported. Unplug while power on will cause crash after next message receive.
• Unstable WiFi connection.
• No multi-client support for Telnet service.
• No buzzers for alert.
• Partially decoded or corrupted message will show on display.
• Documentation and license are preliminary, needs to clean up and complete.

Details

1. About LBJ long message

It does not appear on TB/T 3504-2018 standard. I received this kind of messages using SDR while listening to 821.2375MHz. Some train transmits them on POCSAG address 1234002. The formal name and structure of these messages is unknown, the contents are currently identified by guessing. Current identified information in one typical long LBJ message is listed in the following table.

Nibbles(4bit)	Encode	Meaning
0-3	ASCII	Type
4-11	Decimal	8 digit locomotive registry number
12-13	Unknown	Unknown, usually 30
14-29	GB2312	Route
30-38	Decimal	Longitude (XXX°XX.XXXX′ E)
39-46	Decimal	Latitude (XX°XX.XXXX′ N)
47-49	Unknown	Unknown, usually 000

In total of 50 nibbles / 200 bits, transmitted in 10 POCSAG frames.

2. SX1276 Configuration

• Freq = 821.2375MHz + 6 ppm (default)
• Mode = FSK, RxDirect (DIO2)
• Gain = 001 + LnaBoostHf (AGC off)
• RxBW = 12.5 kHz

Due to lack of TCXO on the SX1276 module used by the dev board, an automatic frequency adjustment mechanism is implemented by measuring the frequency error of the carrier and preamble received using SX1276's frequency error indicator (FEI). It tries to lock on the signal after receiving a carrier or preamble, thus compensate for the frequency error caused by crystal or the transmitter. This mechanism can be disabled via serial command afc off.

3. Telnet/Serial Commands

Serial

• ping Serial state test command, returns pong.
• time Returns system time.
• sd end Unmount TF card.
• sd begin Mount TF card.
• mem Returns memory left. (by esp_get_free_heap_size())
• rst Returns reset reason.
• ppm read Returns current ppm.
• afc off Disable automatic frequency correction.
• afc on Enable automatic frequency correction.

Telnet

• ping Telnet state test command, returns pong.
• read Returns 1000 bytes of data from log on TF card.
• log read [int] Returns [int] bytes of data from log on TF card.
• log status Returns if TF log is enabled.
• afc off Disable automatic frequency correction.
• afc on Enable automatic frequency correction.
• ppm [float] Set ppm to [float].
• bat Returns battery voltage.
• rssi Returns current RSSI from SX1276 module.
• gain Returns current gain of SX1276 module.
• time Returns system time.
• bye Disconnect from Telnet.

4. Reception Failure

Sometimes the received messages may be corrupt, partially decoded or wrongly corrected, thus may display unreliable results. If <NUL>, NA, ******** or part of these characters shows up, it means this part of the message is corrupted.

Libraries and codes used/referenced

• RadioLib (Modified)
• U8G2
• ESP32-Arduino
• PlatformIO
• ESP32AnalogRead (for battery voltage checking)
• BCH3121.cpp/.h from POCSAG_HS
• LilyGo-LoRa-Series
• ESPTelnet
• RTClib
• Project inspired by GoRail_Pager.
• More if I can remember, I apologize for that.

Huge thanks to all authors and contributors.