RS232, Ethernet, USB, Wi-Fi or Bluetooth LE: Choosing a Scale Interface

Scale interfaces compared at a glance

For a scale on a vessel, Ethernet is the default choice for permanent integration with networks and software, RS232 for simple point-to-point links to printers, displays and older equipment, USB for configuration and short connections to a nearby device, and Wi-Fi or Bluetooth LE for portable use with tablets and phones where cables are impractical. The right choice depends on distance, how many devices share the link, and how wet, electrically noisy and metal-enclosed the location is.

Distances are typical limits from the standards and interface literature cited below; practical limits on a vessel are often shorter because of noise, connectors and steel structures.

RS232: simple, short and still useful

RS232 is a single-ended, point-to-point serial interface that remains useful for connecting a scale to one nearby device, such as a label printer or a remote display, with minimal configuration.

According to a Texas Instruments application note, older versions of the standard required 19.2 kbps at a cable length of 50 feet (about 15 m), while modern revisions specify a minimum of 19.2 kbps into a capacitive load of 2,500 pF, so the maximum length depends on the capacitance per metre of the cable. Transmitter logic levels are between ±5 V and ±15 V, and receivers must interpret ±3 V correctly.

• On board: keep runs short, use shielded cable, route it away from power cables and motor drives, and consider galvanically isolated converters where equipment is powered from different circuits.
• Data format: RS232 defines electrical signals, not content. Baud rate, data bits, parity, stop bits and the text format of the weight string must match on both ends.
• Longer distances: RS485 uses differential signalling and supports multi-drop buses; the TI RS-485 design guide gives a maximum cable length of 4,000 feet (about 1,200 m) at 100 kbps and up to 32 unit loads.

Ethernet: the backbone for integration

Ethernet connects scales, graders and computers into a vessel network, supports many devices at once and carries modern protocols such as HTTP APIs and MQTT, which makes it the standard choice for permanent installations.

Twisted-pair Ethernet as specified in IEEE 802.3 supports segments of up to 100 m between a device and a switch; longer distances use additional switches or fibre. Speed is rarely a constraint for weighing data. The practical issues on a vessel are physical:

• Standard RJ45 plugs are not sealed. In wet areas use connectors and cable glands with a suitable IP rating, or terminate cables inside a sealed enclosure.
• Use shielded cable where runs pass near high-current equipment such as winches, hydraulics and frequency drives.
• Plan the network: fixed IP addresses or reserved DHCP for scales, a separate segment for operational equipment, and a switch location that stays dry.

Once a scale is on Ethernet, software can use its API; see Integrating Scales with Your Software via an API.

USB: configuration and short local links

USB is best used for short, local connections: configuring a scale from a laptop, uploading label designs to a printer, or transferring data to a nearby computer.

The USB 2.0 specification (USB Implementers Forum) supports high-speed data at 480 Mbit/s over cables of up to 5 m; longer runs need hubs or active extension cables. On a vessel, the weak points are consumer connectors, which are neither waterproof nor locking, and the host-device model, in which a PC or tablet must control the connection. Sealed or panel-mounted USB connectors help where a port must be accessible in a wet area.

Wireless: Wi-Fi and Bluetooth LE on a steel vessel

Wireless links remove cables that can be damaged or corroded, but steel bulkheads, decks and water attenuate radio signals strongly, so wireless works best within one open space and should be tested at the installation location.

Wi-Fi

Wi-Fi (the IEEE 802.11 family) lets tablets, laptops and phones connect to a scale over a local network, often directly through an access point built into the scale or its indicator. It carries enough data for browser-based interfaces and APIs. Plan for the fact that a steel deck or bulkhead between the scale and the user can block the signal entirely, and that the 2.4 GHz band is shared with Bluetooth and other devices.

Bluetooth LE

Bluetooth Low Energy is designed for low power and short-range connections to phones and tablets. The Bluetooth SIG describes effective range as anything from less than a metre to more than a kilometre, depending on the radio, transmit power, receiver sensitivity, antenna and path loss, and notes that obstacles including metal and humidity reduce it (Bluetooth SIG: Understanding Bluetooth range). On a fishing deck, with steel around and people and wet product in the path, design for short distances and verify the link in place.

Choosing by application

Choose the interface from the job the connection has to do, not from the list of ports on the scale.

For the link from the vessel to shore, see Ship-to-Shore Data: Connectivity Options for Fishing Vessels.

Installation checklist for wet, noisy environments

1. Select connectors and cable glands with an ingress protection rating suited to wash-down, as defined in IEC 60529, and keep unused ports capped.
2. Route data cables separately from power cables and away from frequency drives, winch motors and hydraulic pumps.
3. Use shielded cable and ground the shield at one defined point unless the equipment manufacturer specifies otherwise.
4. Use galvanic isolation between equipment on different power circuits to avoid ground loops.
5. Place switches, converters and printers in dry, ventilated enclosures.
6. Label both ends of every cable with the device and port.
7. Test wireless coverage at the working position with the vessel's steel structures in place.
8. Document settings: IP addresses, baud rates, data formats and pairing codes.

Material and enclosure choices are discussed further on the hygiene and materials hub.

How WPL approaches this

WPL scales offer several interface options so the connection can match the application. The M2 Series Marine Scale connects through USB, RS232 and Ethernet to printers, labelling equipment, networks and software, with Bluetooth LE and the WeightControl IOT module as options. The M5 and M6 platform scales offer RS232, Ethernet, Wi-Fi, Bluetooth LE and USB, as well as remote displays, relay boards and digital I/O. The M3 offers Bluetooth LE and USB. WeightControl can be reached from a laptop or tablet through the scale's built-in access point. For the full picture, see the data integration hub.