What every instrument already does

Some things every SCPI instrument does the same way, and Crucible provides them ready-made. They are not declared in the profile.

Common commands

Compound messages

Several commands separated by ; run in order, and query answers come back on one line, also separated by ; (you saw it in chapter 7: 0.99999;1).

There is an SCPI rule worth being clear about. After a ;, the next header is relative to the previous one’s node, unless it starts with ::

$ printf 'VOLT 5;OUTP ON\n' | nc -q1 127.0.0.1 5025
$ printf 'OUTP?\n' | nc -q1 127.0.0.1 5025
1
$ printf 'SOUR:VOLT 3;OUTP OFF\n' | nc -q1 127.0.0.1 5025
$ printf 'SYST:ERR?\n' | nc -q1 127.0.0.1 5025
-113,"Undefined header;SOUR:OUTP"
$ printf 'SOUR:VOLT 3;:OUTP OFF;:OUTP?\n' | nc -q1 127.0.0.1 5025
0

• VOLT 5;OUTP ON works: VOLT hangs off the root, so OUTP does too.
• SOUR:VOLT 3;OUTP OFF does not: the current node is SOUR, and OUTP is read as SOUR:OUTP, which does not exist. The output stays on.
• With a leading : (;:OUTP OFF) you go back to the root, and it always works.

A real instrument does exactly the same. When in doubt, write ;:.

The state is shared

Each device has a single state, shared by every connection. It is not cloned per client or reset on disconnect: just as a real device does not reset when someone else picks it up.

The example multimeter has a command the real one does not, SIMulate:INPut, to set the voltage on its terminals from a script. Every command in this session is a separate connection:

$ printf 'SIM:INP 12\n' | nc -q1 127.0.0.1 5026
$ printf 'MEAS:VOLT?\n' | nc -q1 127.0.0.1 5026
+1.200014E+01
$ printf '*RST\n' | nc -q1 127.0.0.1 5026
$ printf 'MEAS:VOLT?\n' | nc -q1 127.0.0.1 5026
-6.313870E-05

The 12 V set by one connection is measured by the next. And after *RST it goes back to 0 V —the profile’s value—, not to the 3.3 V of the estado_inicial the bench started it with.

In practice:

• If your software configures something and exits, the next run finds it configured. Send *RST at the start, or restart Crucible, if you want to start from scratch.
• SIMulate:INPut is the way to chain instruments: devices do not see each other, so your script reads the supply and passes the value to the multimeter. You can add commands like it to any profile.

Errors do not drop the connection

An unknown command, a missing argument or a formula that cannot be evaluated leaves its error in the queue and the connection stays open. Your software sees it as on the real instrument: by asking SYST:ERR?.