Scan and Identify Instruments

After connecting to a bridge, you need to discover what instruments are on the GPIB bus. mcgpib provides bus_scan for discovery and instrument_identify for detailed identification of individual devices.

Scan the bus

A bus scan sends the AR488 ++findlstn command, which polls every GPIB address from 1 to 30 and returns the addresses that acknowledge.

> Scan the GPIB bus on bench-a

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

Bus scan on bench-a: 3 listener(s)
  Address 1: HEWLETT-PACKARD 34401A (S/N: 3146A02377)
  Address 5: KEITHLEY INSTRUMENTS INC. 2000 (S/N: 1234567)
  Address 22: Agilent Technologies E3631A (S/N: MY12345678)

Scan without identification

If you only need to know which addresses have active listeners, pass identify=False:

> Scan bench-a without sending *IDN?

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

Bus scan on bench-a: 3 listener(s)
  Address 1: listener found
  Address 5: listener found
  Address 22: 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 address:

> Identify the instrument at address 5 on bench-a

The LLM calls instrument_identify("bench-a", 5):

Address 5 on bench-a:
  Manufacturer: KEITHLEY INSTRUMENTS INC.
  Model: 2000
  Serial: 1234567
  Firmware: A04 /A02
  Raw: KEITHLEY INSTRUMENTS INC.,2000,1234567,A04 /A02

The *IDN? response follows the IEEE 488.2 format: four comma-separated fields for manufacturer, model, serial number, and firmware version. mcgpib 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. mcgpib will show these as “(no identification)” in scan results, but you can still send commands to them if you know their programming interface.

List instruments across bridges

After scanning, list_instruments shows everything discovered across all connected bridges:

> List all discovered instruments

The LLM calls list_instruments():

Discovered instruments:
  [bench-a] Address 1: HEWLETT-PACKARD 34401A
  [bench-a] Address 5: KEITHLEY INSTRUMENTS INC. 2000
  [bench-a] Address 22: Agilent Technologies E3631A
  [bench-b] Address 3: Stanford_Research_Systems SR830

To filter to a single bridge, pass the bridge name:

> List instruments on bench-a only

The LLM calls list_instruments(bridge_name="bench-a").

Tip

list_instruments reads from cached scan results. It does not talk to the bus. If you have power-cycled instruments since the last scan, run bus_scan again to refresh the data.

Handle instruments that do not support *IDN?

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

1. Scan without identification to confirm the address is active:

   > Scan 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 address 22 on bench-a and read the response

   The LLM calls raw_scpi("bench-a", 22, "F1RA", read_after=True).

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 bus scan:

   > Scan bench-a again

   This replaces the previous scan 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 listeners are found at all, the scan will return an error. Before assuming a bus problem, try interface_clear("bench-a") to reset the bus state, then scan again.

Automatic scanning on connect

If auto_scan = true and auto_identify = true in the bridge config, instrument discovery happens automatically when you call connect_bridge. The scan results are immediately available through list_instruments without a separate bus_scan call.

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