R&S FSL Instrument Model and Command Processing
1300.2519.12 5.11 E-11
Instrument Model and Command Processing
T
he block diagram in Fig. 5–2 shows how SCPI commands are serviced in the instrument. The
individual components work independently and simultaneously. They communicate with each other by
means of so–called "messages".
IEC/IEEE bus
Ethernet
Input unit with
input buffer
Command
recognition
Instrument settings
data base
Instrument
hardware
Output unit with
output buffer
Status reporting
system
IEC/IEEE bus
Ethernet
Fig. 5–2 Instrument model in the case of remote control
Input unit
The input unit receives commands character by character from the controller and collects them in the
input buffer. The input unit sends a message to the command recognition as soon as the input buffer is
full or as soon as it receives a delimiter, <PROGRAM MESSAGE TERMINATOR>, as defined in IEEE
488.2, or the interface message DCL.
If the input buffer is full, the traffic is stopped and the data received up to then are processed.
Subsequently the traffic is continued. If, however, the buffer is not yet full when receiving the delimiter,
the input unit can already receive the next command during command recognition and execution. The
receipt of DCL clears the input buffer and immediately resets the command recognition.
Command recognition
The command recognition analyses the data received from the input unit. It proceeds in the order in
which it receives the data. Only DCL is serviced with priority, for example GET (Group Execute Trigger)
is only executed after the commands received before. Each recognized command is immediately
transferred to the internal instrument settings data base but not executed immediately.
The command recognition detects syntax errors in the commands and transfers them to the status
reporting system. The rest of a command line after a syntax error is analyzed further if possible and
serviced. After the syntax test, the value range of the parameter is checked, if required.
If the command recognition detects a delimiter, it passes the command to an execution unit that
performs the instrument settings. In the meantime, the command recognition is ready to process new
commands (overlapping execution). A DCL command is processed in the same way.
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