FieldKit
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Modbus RTU over TCP on iPhone, through an RS-485 converter

An iPhone has no RS-485 port. A transparent Wi-Fi or Ethernet converter solves that, and FieldKit speaks the RTU framing it needs.

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$5.99 one-time No subscription No account iOS 18 or later

FieldKit showing a device that answered in RTU over TCP, with a button to switch framing

What you need

  • An RS-485 device, such as a meter, drive or sensor.
  • A Wi-Fi or Ethernet to RS-485 converter in transparent mode, for example an Elfin EW11, USR or Waveshare model.
  • The device’s Unit ID, or a few minutes to scan for it.

Set baud rate, parity and stop bits in the converter. They have to match the device.

How to connect

  1. Enter the converter’s IP address and TCP port, or tap Find devices, which checks 502, 1502, 8899 and 4196.
  2. Under Framing, choose RTU over TCP.
  3. Enter the Unit ID of the device on the line, from 1 to 247.
  4. Read a small range with Quick read.

The app sends [unit][PDU][CRC] with no MBAP header. It closes the connection after a read timeout, so a late answer can’t be mistaken for the next one.

If you don’t know the mode

After a read times out, the app offers Try RTU over TCP, or Try Modbus TCP if you are already in RTU. It runs one read on a new connection and writes nothing. A success shows the values and offers to switch and to save the choice in a profile. How to tell which mode your converter uses.

Limits to know

  • The server mode speaks Modbus TCP only.
  • RTU over TCP takes Unit IDs 1 to 247, without broadcast 0 or 248 to 255.
  • Solarman V5 loggers can listen on the same port, and FieldKit doesn’t support them.
  • Keep one master on the line. RTU responses carry no register address, so a second master can make the app accept someone else’s values.
  • On adapters that echo, an exception after a 05 or 06 write can go unseen. Read the value back.