LAN Instrument Control: VXI-11 vs HiSLIP

LAN vs USB vs GPIB

Modern test instruments typically offer three connectivity options: USB, LAN (Ethernet), and GPIB. LAN has become the preferred choice for most new installations because it:

• Supports long cable runs (up to 100 m with Cat5e/Cat6)
• Allows remote access from anywhere on the network
• Enables multiple simultaneous connections from different computers
• Requires no special hardware beyond a standard network switch
• Transfers data faster than GPIB (typically 10–100× faster with HiSLIP)

The challenge is that LAN instruments communicate using one of two application-layer protocols — VXI-11 or HiSLIP — and choosing the wrong one can result in slow transfers or connection failures.

VXI-11: The Classic Protocol

VXI-11 was standardized in 1995 and is built on Sun RPC (Remote Procedure Call) over TCP/IP. It was designed to emulate GPIB behavior over Ethernet, making it easy to migrate existing GPIB-based code to LAN.

How it works:

• The instrument runs an RPC port mapper on port 111
• The VISA library queries the port mapper to find the VXI-11 server port (usually dynamically assigned)
• Two separate TCP connections are established: one for commands (core channel) and one for asynchronous interrupts (abort channel)
• Each write/read transaction involves multiple round trips to set up the RPC call

This round-trip overhead is VXI-11's main weakness. Each command incurs several network round trips beyond the actual data transfer, making it noticeably slower for high-throughput applications.

VISA resource string format:

TCPIP0::192.168.1.100::inst0::INSTR

The inst0 field is the logical name defined by VXI-11. Most instruments use inst0; some Rohde & Schwarz instruments use hislip0 or inst0 depending on firmware version.

HiSLIP: The Modern Standard

HiSLIP (High-Speed LAN Instrument Protocol) was introduced in 2010 by the IVI Foundation as a direct replacement for VXI-11. It uses plain TCP (no RPC layer) on a fixed port (4880) and was designed specifically for high-speed instrument control.

Key improvements over VXI-11:

• Fewer round trips — a single TCP write delivers the command directly, no RPC setup needed
• Overlapped mode — the host can send the next command while still reading the previous response, enabling pipelining
• Encrypted connections — HiSLIP 2.0 adds TLS support for secure communication
• IPv6 support — VXI-11 is IPv4 only; HiSLIP works with both
• Fixed port 4880 — no port mapper required, firewall rules are simpler

VISA resource string format:

TCPIP0::192.168.1.100::hislip0::INSTR

The hislip0 identifier tells the VISA library to use the HiSLIP protocol. Some instruments number additional HiSLIP interfaces as hislip1, hislip2, etc.

Side-by-Side Comparison

Which Protocol to Use

Find Your Instrument's IP Address

There are three ways to find a LAN instrument's IP address:

1. Front panel menu

On most instruments: System → Connectivity → LAN or Utility → I/O → LAN. The IP address, subnet mask, and gateway are displayed. Note the address and whether DHCP or static IP is used.

2. Instrument web interface

Most LXI-compliant instruments host a web page at their IP address. If you can see the IP on the front panel, open http://<ip-address> in a browser. The page will show full network details and often allows you to send SCPI commands directly.

3. LXI Discovery (mDNS/Bonjour)

LXI instruments broadcast their presence on the local network using mDNS. Use the following Python snippet to discover them without knowing the IP in advance:

import pyvisa

rm = pyvisa.ResourceManager()
# list_resources with a TCPIP filter finds LAN instruments
resources = rm.list_resources('TCPIP?*::INSTR')
print(resources)

Connect with PyVISA

Once you have the IP address, connecting is straightforward. The only difference between VXI-11 and HiSLIP is the resource string:

import pyvisa

rm = pyvisa.ResourceManager()

# VXI-11 connection
inst_vxi = rm.open_resource('TCPIP0::192.168.1.100::inst0::INSTR')

# HiSLIP connection
inst_hi  = rm.open_resource('TCPIP0::192.168.1.100::hislip0::INSTR')

# Both support the same PyVISA interface
print(inst_hi.query('*IDN?'))

Set a timeout appropriate for your application. LAN latency on a local network is typically under 1 ms, but slow instruments (e.g., during a long sweep) may need several seconds to respond:

inst.timeout = 10000  # 10 seconds — adjust for slow measurements

Throughput benchmark example

To see the speed difference in practice, use a spectrum analyzer or oscilloscope that returns large binary trace data:

import pyvisa
import time

rm = pyvisa.ResourceManager()

for proto, addr in [('VXI-11',  'TCPIP0::192.168.1.100::inst0::INSTR'),
                    ('HiSLIP',  'TCPIP0::192.168.1.100::hislip0::INSTR')]:
    inst = rm.open_resource(addr)
    inst.timeout = 10000

    t0 = time.perf_counter()
    for _ in range(20):
        inst.query('*IDN?')
    elapsed = time.perf_counter() - t0

    print(f"{proto}: 20 queries in {elapsed:.3f}s ({elapsed/20*1000:.1f} ms each)")
    inst.close()

Typical results on a 1 Gbps LAN: VXI-11 ~15–25 ms per query, HiSLIP ~2–5 ms per query.

Raw Socket Connection

Some instruments expose a plain TCP socket (no VXI-11 or HiSLIP layer) on a fixed port, typically 5025 or 5555. This is the simplest possible interface — you send ASCII SCPI text and read back the response.

Check your instrument's manual for the socket port number. The VISA resource string uses the SOCKET suffix:

import pyvisa

rm = pyvisa.ResourceManager()

# Raw socket — port 5025 is common for Keysight and Rigol instruments
inst = rm.open_resource('TCPIP0::192.168.1.100::5025::SOCKET')

# IMPORTANT: raw sockets do not auto-append a terminator
inst.write_termination  = '\n'
inst.read_termination   = '\n'

print(inst.query('*IDN?'))

Common raw socket ports by manufacturer:

• Keysight: port 5025
• Rigol: port 5555
• Rohde & Schwarz: port 5025
• Tektronix: port 4000 (varies by model)

Troubleshooting

Connection refused (VisaIOError: VI_ERROR_CONN_LOST)

The instrument is not accepting connections on the requested port. Verify:

• LAN is enabled on the instrument (check front panel LAN settings)
• The correct protocol is used — try inst0 (VXI-11) if hislip0 fails
• Port 4880 (HiSLIP) or port 111 (VXI-11 portmapper) is not blocked by a firewall

# Quick connectivity test from terminal
ping 192.168.1.100

# Check if HiSLIP port is open (Linux/macOS)
nc -zv 192.168.1.100 4880

# Windows equivalent
Test-NetConnection -ComputerName 192.168.1.100 -Port 4880

Timeout on first query after connection

Some instruments take 1–2 seconds to fully initialize a new LAN session. Increase the timeout for the first command, then restore it:

inst = rm.open_resource('TCPIP0::192.168.1.100::hislip0::INSTR')
inst.timeout = 5000          # generous timeout for initial handshake
idn = inst.query('*IDN?')
inst.timeout = 2000          # restore normal timeout
print(idn)

Instrument found by ping but not by list_resources()

pyvisa-py's LAN discovery relies on mDNS, which may not work across VLANs or subnets. Skip discovery and open the resource directly using the known IP address:

inst = rm.open_resource('TCPIP0::192.168.1.100::inst0::INSTR')

Slow transfers with VXI-11 on high-latency networks

VXI-11 performance degrades significantly over WAN links or VPNs due to RPC round trips. Switch to HiSLIP if possible, or use a raw socket connection which has less protocol overhead.

Multiple simultaneous connections fail

VXI-11 typically limits connections to one client at a time. HiSLIP supports multiple simultaneous connections. If you need concurrent access from multiple scripts or computers, use HiSLIP.

Next Steps

With LAN connectivity working, explore multi-instrument automation: