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9.4. Multi-drop Programming Using SCPI Commands
9.4.1. Selecting One Power Supply in a Multi-drop Chain
All the SCPI commands in section 7 may be sent to any one of the supplies in an RS-485
chain by first sending the INST:SEL address command. All commands and queries will
then apply only to the selected supply, until a new INST:SEL is sent
When a multi-drop chain is first powered-up, the LAN supply is automatically the one
selected until the first INST:SEL is sent
9.4.2. Global Commands in a Multi-drop Chain
The GLOBAL commands affect all of the chained supplies at the same time, including the
LAN supply
There are differences between a normal SCPI command (in section 7) and a global
command. The differences are:
− There is no SYSTEM:ERROR? response message (see section 7.8.1) if one or
more supplies cannot execute a global command
− It is required that the user’s application add a 200 millisecond delay after a global
command before sending any other messages. This is because there is no
feedback from the multi-drop chain to indicate the command is still being processed
− There is no query version of these commands. Global settings must be queried by
selecting one power supply and reading it’s setting
− The error and status registers operate differently. See section 9.4.10 for details
− The global commands are not SCPI compliant
The global commands do not affect which power supply has been selected using the
INST:SEL command. For example, if the following commands are sent:
INST:SEL 4
:VOLT 50
GLOB:VOLT 70
(wait 200 mSec)
:VOLT 90
then all the supplies on the multi-drop chain will be set to 70 volts except the supply at
RS-485 address 4 will be set to 90 volts (after the global command, another INST:SEL
was not required)
9.4.3. Selecting a Supply
This command will select one power supply in a multi-drop chain. Subsequent commands
and queries will operate only on that supply, until a new one is selected. At power-up, the
LAN supply is automatically selected
Syntax: INSTrument:SELect <nn>
Parameter: <nn> is the supply’s RS-485 address. It is a number from 0 to 30
see sections 9.2.1.F and 9.3.2.E for set-up instructions
Example: INST:SEL 6
Query: INST:SEL? will return 06 in the example
Errors: SYSTEM:ERROR? may return errors including:
-241,”Hardware missing;address nn”, if nn is an invalid address
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