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UART hardware notes for laser rangefinder modules

Maintained by ERDI TECH LTD (ERDILRF). These notes cover the wiring problems we see most often. They are generic electrical guidance, not a wiring diagram for a specific model; use the model's own document for pin order.

Logic level is not supply voltage​

A module that runs from 5 V may still use 3.3 V UART levels, or the reverse. Read both rows of the electrical table before connecting a host. If the host is 3.3 V and the module output is 5 V, use a level shifter or a series/divider network on the module TX line; do not rely on the host pin being "5 V tolerant".

Background: UART/TTL integration guide.

Grounds and returns​

  • Connect signal ground and power ground at one point near the module.
  • Do not use the cable shield as the return conductor.
  • On a vehicle, a long ground loop between the module and the controller picks up ESC current. Twisted pair for TX/RX and a separate supply pair is usually enough; a ferrite at the module end helps with motor noise.

Supply​

  • Size the regulator for the peak current, not the average.
  • Put 10–100 µF bulk capacitance near the module and a 100 nF ceramic at the connector.
  • Check the module's brown-out behaviour: some units emit a valid-looking frame with a zero or maximum distance during start-up.
  • If the module shares a rail with a motor or a servo, power it from a separate regulator.

Cable length​

TTL UART is not a field bus. At 115200 baud, keep the run short and shielded; if the module must sit far from the controller, convert to RS-485 or move the reader next to the module and send the parsed result over a robust link. Do not "fix" long-cable errors by lowering the baud rate alone: the checksum will still fail under motor noise.

Connector discipline​

  • Record the mating connector part number in your build documentation.
  • Pin order is not standard across vendors; a mirrored connector can put the supply on the TX pin.
  • Strain-relieve the cable at the module; the connector is not a load-bearing part.

What the module does not include​

  • No galvanic isolation.
  • No reverse-polarity protection on every model; check the datasheet.
  • No surge protection for outdoor cable runs.
  • No guarantee that the UART is silent during power-up.

First power-on sequence​

  1. Current-limited bench supply at the nominal voltage.
  2. Measure the TX idle level before connecting the host.
  3. Connect ground first, then TX/RX, then power.
  4. Capture bytes with a logic analyser or a terminal at the documented baud rate.
  5. Confirm the frame header and checksum before writing a parser.

Frame layout for the two ERDI protocol families is in Protocol A and Protocol B. Model index and per-model documents: https://erdilrf.com/models/.