Scan and Identify Instruments

After connecting to a backend, you need to discover what instruments are available. mcpyvisa provides discover_instruments for discovery and instrument_identify for detailed identification of individual devices.

Discover instruments

discover_instruments finds all reachable instruments on a backend. For AR488 backends, this sends the ++findlstn command to poll GPIB addresses 1 through 30. For pyvisa-py and system backends, it queries the VISA resource manager.

> Discover instruments on bench-a

The LLM calls discover_instruments("bench-a"). With the default identify=True, it also sends *IDN? to each instrument found:

Instruments on bench-a: 3 found
  GPIB0::1::INSTR  -> HEWLETT-PACKARD 34401A (S/N: 3146A02377)
  GPIB0::5::INSTR  -> KEITHLEY INSTRUMENTS INC. 2000 (S/N: 1234567)
  GPIB0::22::INSTR -> Agilent Technologies E3631A (S/N: MY12345678)

If you have configured aliases, they appear alongside the resource strings:

Instruments on bench-a: 3 found
  GPIB0::1::INSTR  (voltmeter) -> HEWLETT-PACKARD 34401A (S/N: 3146A02377)
  GPIB0::5::INSTR  (ammeter)   -> KEITHLEY INSTRUMENTS INC. 2000 (S/N: 1234567)
  GPIB0::22::INSTR (psu)       -> Agilent Technologies E3631A (S/N: MY12345678)

Discover without identification

If you only need to know which instruments are reachable, pass identify=False:

> Discover instruments on bench-a without sending *IDN?

The LLM calls discover_instruments("bench-a", identify=False):

Instruments on bench-a: 3 found
  GPIB0::1::INSTR  -> listener found
  GPIB0::5::INSTR  -> listener found
  GPIB0::22::INSTR -> listener found

This is faster and avoids potential issues with instruments that do not handle *IDN? gracefully.

Identify a specific instrument

To get detailed identification for a single instrument:

> Identify the ammeter

The LLM calls instrument_identify("ammeter"):

ammeter (GPIB0::5::INSTR on bench-a):
  Manufacturer: KEITHLEY INSTRUMENTS INC.
  Model: 2000
  Serial: 1234567
  Firmware: A04 /A02
  Raw: KEITHLEY INSTRUMENTS INC.,2000,1234567,A04 /A02

You can also use VISA resource strings directly:

instrument_identify("GPIB0::5::INSTR")

The *IDN? response follows the IEEE 488.2 format: four comma-separated fields for manufacturer, model, serial number, and firmware version. mcpyvisa parses these into structured fields.

Interpret *IDN? responses

Not all instruments return clean *IDN? strings. Here is what to expect:

Instrument era	*IDN? behavior
Modern IEEE 488.2 (Keysight, R&S, Keithley)	Clean four-field response
Older HP/Agilent (34401A, E3631A)	Standard response but may vary in field formatting
Pre-488.2 instruments (HP 3478A, Fluke 8842A)	No response — times out
Mixed-protocol instruments	May return partial or non-standard strings

Note

Instruments manufactured before the IEEE 488.2 standard (1987) generally do not implement *IDN?. They respond to GPIB bus commands but use proprietary or HP-GL command sets. mcpyvisa will show these as “(no identification)” in discovery results, but you can still send commands to them if you know their programming interface.

Check server status

To see an overview of all configured backends and discovered instruments:

> Show server status

The LLM calls server_status():

mcpyvisa server status:
  Backends: 2 configured, 1 connected
    bench-a (ar488/serial): connected, 3 instruments
    lan-bench (pyvisa-py): disconnected

  Known instruments:
    dmm       -> GPIB0::22::INSTR (bench-a) -> HEWLETT-PACKARD 34401A
    ammeter   -> GPIB0::5::INSTR (bench-a)  -> KEITHLEY INSTRUMENTS INC. 2000
    psu       -> GPIB0::1::INSTR (bench-a)  -> Agilent Technologies E3631A
    scope     -> TCPIP::192.168.1.100::INSTR (lan-bench) -> not connected

Tip

server_status reads from cached state. It does not communicate with instruments. If you have power-cycled instruments since the last discovery, run discover_instruments again to refresh the data.

Handle instruments that do not support *IDN?

For pre-488.2 instruments, you need a different approach:

1. Discover without identification to confirm the instrument is reachable:

   > Discover instruments on bench-a without identifying

2. Try a known command for the instrument you expect at that address. For example, the HP 3478A uses F1RA to configure DC voltage autorange:

   > Send "F1RA" to GPIB0::22::INSTR and read the response

   The LLM calls instrument_query("GPIB0::22::INSTR", "F1RA").

3. Check the instrument’s front panel for a response. Many older instruments display “REMOTE” or “RMT” when they receive a valid command over GPIB.

Re-scan after power cycling

When you power cycle instruments or reconnect GPIB cables:

1. Wait for instruments to boot. Many instruments take 5—30 seconds for self-test after power-on.

2. Run a new discovery:

   > Discover instruments on bench-a again

   This replaces the previous discovery results entirely. Any instruments that are no longer on the bus will disappear from the list.

3. If instruments are missing, check:

   • GPIB cable connections (the connectors are stackable — make sure they are seated firmly)
   • Power status on each instrument
   • GPIB address switches or menu settings have not changed

Caution

If no instruments are found at all, the discovery will return an error. Before assuming a bus problem, try interface_clear("bench-a") to reset the bus state, then run discovery again.

Automatic discovery on connect

If auto_discover = true and auto_identify = true in the backend config, instrument discovery happens automatically when you call connect_backend. The results are immediately available through server_status without a separate discover_instruments call.

To skip automatic discovery (for faster connections or to avoid disturbing instruments), set auto_discover = false in the config and discover manually when ready.