Tommy Yu

Posted on Oct 12

Reading a 905 nm Laser Rangefinder Module Over UART: Arduino, Raspberry Pi and ROS 2

#arduino #raspberrypi #ros #hardware

Disclosure: I work for the manufacturer of these modules (ERDILRF, Chengdu). Every frame, baud rate and unit below is transcribed from the manufacturer manuals and implemented in an MIT-licensed driver. I am not publishing bench measurements here, and the last section lists what has not been verified.

The two classic first-integration failures with a UART laser rangefinder are a silent serial port and readings that are off by a factor of ten. Both usually trace back to protocol details, not wiring. This walkthrough goes in the order that saves time: identify the protocol family, wire it, then run code.

Parts

One ERDI 905 nm module (this write-up uses the LRF3000A1 as the worked example)
An Arduino with a spare hardware UART (Mega, ESP32, etc.), a Raspberry Pi with a USB-TTL adapter, or a ROS 2 machine
Jumper wires and a supply that matches your exact model

Step 1: Identify the protocol family

ERDI's 905 nm catalogue uses two UART protocols.

Family A: 8-byte 55 AA command/response. LR1000E2/E4, LR1200E2/E4, LR1500E2/E4, LR2000E2/E4, LRF1200A1, LRF3000A1, SPD1200N0/N2/N4, SPD1200D03. Default 115200 bps.

Family B: free-running 5C frames. LRF50H/100H/200H/300H/600H/1500H and LRF22VB/LRF50VB. Default 460800 bps (adjustable), 8 data bits, 1 stop bit, no parity, little-endian.

Step 2: Know the frames

Family A request, single shot: 55 AA 88 FF FF FF FF 84.
Reply: 55 AA 88 STA FF DIS_H DIS_L SUM. STA = 1 means success. Distance is DIS_H<<8 | DIS_L, and the manual states results are output as the real value x 10, so one count is 0.1 m.

The classic trap: transmit and reply checksums cover different bytes.

Transmit: low 8 bits of Function + D1..D4
Reply: low 8 bits of 0x55 + 0xAA + Function + D1..D4

They differ by exactly 0xFF. Use the transmit formula on replies and you will be off by one on almost every frame. A hand-computed example (constructed, not captured from hardware): 55 AA 88 01 FF 04 D2 5D sums to 0x35D, low byte 0x5D, and the distance 0x04D2 = 1234 counts = 123.4 m.

Family B frame: 5C dist_lo dist_hi check, e.g. 5C 02 11 EC. The check byte is the bitwise inverse of the sum of the bytes between the header and the check: ~(0x02 + 0x11) = 0xEC.

Units are per model, not per family. The same four bytes 5C 02 11 EC mean 43.54 m on an H-series module (centimetres) and 4.354 m on a VB-series module (millimetres). LRF1500H is the only 5-byte frame, with a 3-byte distance field.

Step 3: Wire it

Supply voltage and UART signal level are separate specifications. Do not infer one from the other.

LRF3000A1, 6-pin connector (A1002WV-S-6P) per its manual:

Pin	Signal	Note
1	GND	Ground
2	Power	3.3-5 V DC
3	NC	Reserved
4	TTL_TXD	Module transmit, 3.3 V TTL
5	TTL_RXD	Module receive, 3.3 V TTL
6	Enable	Power ON at low level

Module TX goes to host RX, module RX to host TX, and grounds are common. Other models differ: LRF50H takes 9-36 V and LRF22VB takes 3.0-3.6 V, and some manuals do not state a signal level at all. If yours does not, get the level confirmed in writing before connecting. Never reuse another model's electrical numbers.

Step 4: Arduino

Install ErdiLRF from the Library Manager (or PlatformIO: erdi-tech/ErdiLRF).

#include <ErdiLRF.h>

ErdiLRF lrf(Serial1, ERDI_LRF3000A1);

void setup() {
  Serial.begin(115200);
  Serial1.begin(erdiDefaultBaud(ERDI_LRF3000A1));  // 115200 for family A, 460800 for family B
}

void loop() {
  ErdiMeasurement m;
  if (lrf.measure(&m) == ERDI_OK) Serial.println(m.metres, 3);
  delay(500);
}

Note: SoftwareSerial on an Uno cannot reach 460800 bps. Family B needs a hardware UART, or set the module to a lower documented rate first.

Step 5: Raspberry Pi (Python)

pip install https://github.com/yu911517778-a11y/erdi-lrf-drivers/releases/download/v0.1.0/erdi_lrf-0.1.0-py3-none-any.whl
erdi-lrf --port /dev/ttyUSB0 --model LRF3000A1 measure

from erdi_lrf import ErdiLRF

with ErdiLRF("/dev/ttyUSB0", model="LRF3000A1") as lrf:
    print(f"{lrf.measure().metres:.1f} m")

with ErdiLRF("/dev/ttyUSB0", model="LRF100H") as lrf:
    for m in lrf.stream():
        print(m.metres, "(out of range)" if m.out_of_range else "")

The frame codecs are pure functions with no serial dependency, so you can unit-test them offline.

Step 6: ROS 2

cp -r ros2/erdi_lrf_ros2 ~/ros2_ws/src/
cd ~/ros2_ws && colcon build --packages-select erdi_lrf_ros2
source install/setup.bash
ros2 run erdi_lrf_ros2 range_node --ros-args -p port:=/dev/ttyUSB0 -p model:=LRF3000A1 -p rate:=10.0 -p frame_id:=lrf_link

The node publishes sensor_msgs/Range; min_range/max_range come from the model manual, and field_of_view defaults to 0 because the protocol manuals carry no beam divergence.

Where the manuals disagree with themselves

SPD1200D03: the protocol section says 115200 bps, the technical table says 15200/9600, and the manual says to confirm the delivered configuration first. The driver demands an explicit baud and returns no metric distance.
LRF1200A1 / LRF3000A1: continuous ranging is requested with 0x89 but the recorded reply uses 0x88, and the manual declines to authorise a reply parser. Single-shot is unaffected.
LRF50VB: the printed serial-number request fails the manual's own checksum function.

The drivers surface these instead of guessing.

What is not verified

No measurement results or performance claims are made here. Read the datasheet for your model.
The ROS 2 node has been tested in software replay only, not against a live ROS 2 runtime with hardware.
The driver tests are built from the frames printed in the manuals.

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