Question: Why does the 6514 in remote DDC mode operation return only 5 ½ digit readings?
Answer: The emulation mode on the Model 6514 enables it to accept device-dependent commands (DDC) for the Models 6512, 617 and 617 HIQ electrometers. However, utilizing the emulation mode does not permit the user to access many of the features that are unique to the 6514.
The Models 6512, 617 and 617 HIQ electrometers have 4½ digit resolution. The readings that are returned from these instruments in remote operation are always 5 ½ digits. The format for the return data string with the appropriate prefix is “NDCA +/- 1.23456 E +00”. The range on which the reading is made determines the value of the exponent. In DDC mode, the return string of the 6514 conforms to this format and therefore loses one digit of resolution. This digit, however, may be displayed on the front panel of the instrument. The loss of a digit is especially noticeable in the Coulombs function on the 6514, which is specified at 6 ½ digits. For example, a 5.0nC reading may appear as “5.00000 nC” on the front panel display (with the maximum number of digits displayed). When in DDC emulation mode, however, the return data string of the 6514 will appear as follows:
“NDCC +0.50000 E –08”
The readings over the bus appear to indicate that the sensitivity of the instrument is only 100fC, when it is actually 10fC. If the 6514 is instead configured to accept SCPI commands, as is default, then the return data string will appear as follows:
The user can then discard the digits that are not important to him or her.
Using the DDC emulation mode on the 6514 may be a programming convenience, but may produce unexpected results due to the differences between the 6514 and the electrometer that it is replacing.
©Copyright 2003, Keithley Instruments, Inc.