Why don't the front panel controls of my instrument work properly when it is operating in DDC mode?
Most current Keithley instruments are controlled remotely using SCPI commands. SCPI is an acronym for Standard Commands for Programmable Instruments. Numerous instruments also have one or more DDC (device-dependent command) programming modes that allow them to emulate predecessor Keithley products, or instruments from other manufacturers. This allows you to reuse existing control software that was written using DDC syntax. For instruments with this feature, you use the front panel to set the GPIB language as either SCPI or DDC. In some cases you select a specific instrument model number rather than DDC.
In any case, the DDC mode of operation should only be used when the instrument is being programmed remotely via the GPIB interface. It is not intended for front panel operation. Pressing a key actually sends a series of SCPI commands to the language processor. This is always the case, independent of the GPIB language setting. Since the processor is expecting DDC commands, unpredictable results can occur when SCPI commands are received instead, thus causing erratic behavior. Therefore, if you intend to use the front panel, make sure that the GPIB language is set to SCPI. Note: Instrument setups saved while in the DDC mode are also not restored properly. If you want to store setups, then you must be in the SCPI language mode.