Connecting a scale to a PC, PLC or ERP: RS-232, USB, Ethernet, WiFi and I/O explained

A connection on a scale is only useful if the data received can actually be understood by your PC, PLC, printer or ERP system. That is why, when integrating, you need to look at two things: the physical interface and the communication protocol.

The most important rule: a connection is not the same as communication

A scale with an RS-232 port cannot automatically talk to every RS-232 device. The same applies to Ethernet and USB. You need to know:

• which interface is present;
• which settings apply, such as baud rate and parity for serial communication;
• which message format or protocol is used;
• whether the scale sends data by itself or first expects a command;
• which software or PLC driver the receiving side uses.

RS-232: simple and still widely used

RS-232 is very common on professional scales and indicators. It is suitable for point-to-point communication over relatively short distances and is widely used for printers, PCs and simple machine connections.

What to look out for

• baud rate, data bits, stop bits and parity;
• connector and pin assignment;
• continuous, stable or on-demand transmission;
• ASCII format and any status fields;
• zero, tare or print commands supported remotely.

RS-485 and RS-422: for longer distances and industrial environments

RS-485 uses differential signalling and is therefore suitable for longer cables and environments with more interference than classic RS-232. It can also be used in networks with several devices. The actual behaviour, however, is determined by the protocol, for example a manufacturer's protocol or Modbus RTU if the device supports it.

RS-422 is electrically similar to RS-485 but has a different topology. So check exactly what the indicator and PLC support.

USB: virtual COM port, HID or something else?

With USB there are several possibilities:

• USB virtual COM: the PC sees the scale as a serial port.
• USB HID keyboard wedge: the weight is 'typed' as keyboard input into the active field.
• USB device with driver/software: communication via the manufacturer's software.
• USB host: for example for saving to a USB stick; this is not the same as PC communication.

Ethernet and WiFi

Ethernet and WiFi give a scale access to a network, but say nothing yet about the application protocol. A device may, for example, use TCP sockets, a web interface, an API, Modbus TCP or manufacturer software. So always check the protocol and the available documentation.

Ethernet is generally preferred when fixed, reliable network communication is important. WiFi is useful when cabling is difficult or the scale is moved around.

Bluetooth

Bluetooth is mainly practical for local wireless communication with a tablet, smartphone or PC. Range and reliability depend heavily on the environment and implementation. Only use it for process-critical control when the solution has been explicitly designed for that.

Digital I/O, relays and analogue signals

Not every connection needs to be a digital weight telegram. For simple machine control, an indicator can have outputs for, for example:

• under / OK / over;
• setpoint reached;
• coarse or fine dosing (see filling and dosing by weight);
• stable weight;
• alarm.

Inputs can be used for external start, zero, tare or print. Some transmitters also offer an analogue output such as 0–10 V or 4–20 mA. The weight is then passed on as an analogue process value; resolution, scaling and fault detection must then be designed accordingly.

Connecting a scale to a PLC

For PLC integration, speed, determinism, error handling and documentation are more important than an attractive user interface. Determine in advance:

• which weight value is needed: gross, net, stable, peak or filtered;
• how often the PLC needs a new value;
• which commands must be possible in return: zero, tare, start, reset;
• what happens in the event of a communication error;
• whether legally relevant metrological data must be protected or recorded in an alibi memory;
• which protocol the existing PLC already supports.

For process integration, weighing indicators and transmitters are often more suitable than a stand-alone bench scale.

Connecting a scale to a PC, printer, database or ERP

For administrative applications it is usually about correct recording: weight, date/time, operator, item, order number and possibly batch or barcode. Think in advance about whether you only want to 'enter' a weight or record a complete transaction flow.

An HID connection can be perfectly fine for a single field. For traceability, software that stores the weight together with its context is far more robust.

Which connection should you choose?

Frequently asked questions about scale connections

Can I connect any scale to a computer?

No. The scale must have a suitable interface and a usable data protocol. A connector or cable alone does not mean the software can read the data.

What is the difference between RS-232 and USB?

RS-232 is a serial industrial connection with clearly adjustable communication parameters. Depending on the device, USB can behave as a virtual serial port, an HID keyboard or a manufacturer-specific interface.

When do I choose RS-485 instead of RS-232?

RS-485 is better suited to longer distances, environments with a lot of interference and networks with several devices, provided the protocol used supports this.

Can a scale send data directly to Excel?

Some systems can act as a keyboard HID or send data to Excel via software. Other models require a driver, terminal program, middleware or custom software.

What is more important: the connection or the protocol?

The protocol. Two devices can both have Ethernet or RS-232 and still not communicate directly if the message format, commands or protocols do not match.