Scpi.NET

Welcome to Scpi.NET, a pure .NET library for controlling SCPI devices. If you ever tried to control an oscilloscope or digital multimeter from your computer, you probably came across the SCPI commands which allow to control such devices.

Although this standard looks like a simple text communication, the real use is not so easy because these text commands have to be wrapped into a lower-level communication protocol like TCP/IP or USBTMC which the device understands. The most of manufacturers provide their own libraries like Tektronix's TekVISA or National Instruments' NI-VISA. Unfortunately, in my opinion these libraries are too heavy, create unwanted software dependencies, make installation packages larger, usually focus on one manufacturer hardware only and in the end they only provide a "pipe" which is able to transfer the SCPI commands. The goal of this project is to create a simple, lightweight and manufacturer-independent library which can talk to any SCPI device over USB or Ethernet.

SCPI over TCP/IP

This is the easy part of the library and I have implemented it only to give the same control interface to both USB and Ethernet devices. The devices listen to the TCP port 4000 and the SCPI commands are sent directly in the payload without any headers.

SCPI over USB

This is the hard part of the library. The devices implement the USB Test & Measurement Class (TMC) which requires quite low-level USB communication. Both read and write requests have their own headers which create additional level of complexity. Although I read several documents about the TMC, I still had to reverse-engineer the communication of my oscilloscope to get it working. Another useful bits of code were found in the Linux kernel drivers. Although there are still some places where I am not sure if the implementation is correct, the library works well with several Tektronix oscilloscopes and digital multimeter from Keysight.

USB drivers

My USB TMC driver relies on Windows API calls to the kernel32.dll and SetupApi.dll libraries which are the integral part of the Windows operating system, so that no additional software dependencies are needed.

In order to make USB TMC device working, you need to install the correct USB driver. It's quite tricky to get the driver because it is usually part of the VISA libraries and cannot be downloaded separately. I cannot share the driver I use because of the license restrictions, but as a starting point, I can recommend the link below where exactly the same driver is used and the connectivity checklist is perfectly valid for all USB TMC devices:

USB Connectivity Checklist

Usage

Just install the NuGet package or clone the repository and add reference to that. Then you can start with the examples below.

using ScpiNet;
...

// List available USB devices. We will get back USB identifiers which can be used to open the device.
List<string> devices = UsbScpiConnection.GetUsbDeviceList();

// In order to get instrument name from the USB identifier, we need to open the device and ask it for
// the name. The connection implements IDisposable, so it's recommended to use it in the using block.
using (IScpiConnection connection = new UsbScpiConnection(devices[0]))
{
	// Create the connection:
	await connection.Open();

	// Get the instrument name:
	string id = await connection.GetId();

	// Send some SCPI command:
	await Connection.WriteString("My special SCPI command", true);

	// Read the response:
	string response = await Connection.ReadString();
	...
}

// Connecting the TCP/IP device is even easier because there is always one device listening
// on the port 4000:
using (IScpiConnection connection = TcpScpiConnection("192.168.1.100", 4000))
{
	// The rest is same as with the USB connection:
	await connection.Open();
	string id = await connection.GetId();
	...
}

Reading of instrument ID is fine, but you will probably want to send more SCPI commands to the device. In order to keep the application architecture clean, you should create a separate class for the instrument you are controlling. This can be done by inheriting from the ScpiDevice class which already provides some useful methods such as Query(). Please see the SampleApp directory for more details.

Instrument driver considerations

This library focuses only the the transmission of the SCPI commands and responses. It does not provide any higher-level functionality like instrument drivers. The reason is that implementing of such drivers is a very complex task:

• Every programmer needs to control different functions of the measurement instruments. If we wanted to address all possible requests, it would result in really complex and bloated API. Therefore I decided to create my private drivers which are tailored to the needs of my applications only.
• I don't know why, but the SCPI devices I encountered offered almost no means of reflection: You cannot ask the oscilloscope how many channels it has or what time bases are available. This means that you have to hard-code all these things into your driver. Therefore it is almost impossible to crete a driver without physically having the device in your hands.