KEPCO MST 488 Programmer - Manual (Page 32)

3-6
MST488-27 013004
3.3.4 PROGRAMMING TECHNIQUES TO OPTIMIZE POWER SUPPLY PERFORMANCE
Kepco's auto-crossover digital supplies can operate in either voltage mode with current limit, or
current mode with voltage limit. The operating mode is determined by the voltage and current
commands received, as well as the load. Each time voltage and current commands are
received, the unit must evaluate the commands and the load conditions to determine the proper
operating mode. Reducing the number of times this evaluation must be made is desirable
because Kepco's digital auto-crossover supplies employ two separate feedback loops. Each
time there is a potential mode change, there is always an uncontrolled period of a few millisec-
onds while the two feedback loops compete for control of the output. By changing only the
active parameter (e.g., voltage for voltage mode), there is no doubt as to what the operating
mode will be, so the unit is never uncontrolled, response is quick and no transients are possible.
Recommended programming techniques are:
1. Minimize programmed mode (voltage or current) changes. Unless absolutely required by the
test parameters, allow the power supply to automatically switch modes as determined by the
load. This will improve response time and reduce undesirable transients. For those power
supplies that employ relays (Kepco's MBT with "R" option, MAT and MST) this will also
increase the life of the relay.
2. Once the mode (voltage or current) is programmed, program the active parameter to zero
and the complementary limit parameter to the maximum anticipated for application. Then
program only the active parameter. The active parameter is the parameter that controls the
output, e.g., voltage controls the output in voltage mode.
3. Never program both the active and complementary limit parameter to zero. This can result in
long response times. Set the active parameter to zero and the complementary limit parame-
ter to a minimum, e.g., 10% of maximum, to ensure that the active mode is defined.
3.4 SCPI PROGRAMMING
SCPI (Standard Commands for Programmable Instruments) is a programming language con-
forming to the protocols and standards established by IEEE 488.2 (reference document ANSI/
IEEE Std 488.2, IEEE Standard Codes, Formats, Protocols, and Common Commands). SCPI com-
mands are sent to the MST 488-27 controller as output strings within the selected programming
language (PASCAL, BASIC, etc.) in accordance with the manufacturer’s requirements for the
particular GPIB interface card used.
Different programming languages (e.g., BASIC, C, PASCAL, etc.) have different ways of repre-
senting data that is to be put on the IEEE 488 bus. It is up to the programmer to determine how
to output the character sequence required for the programming language used. Address infor-
mation (GPIB address) must be included before the command sequence. (See PAR. 2.2.1 to
establish the MST 488-27 controller GPIB address.)
NOTE: Although some basic information is provided, the procedures in this manual assume
that programming of the unit using SCPI commands will by done by personnel who are
experienced programmers and understand the protocols required by SCPI and IEEE
488.2.