Lake Shore Model 642 Electromagnet Power Supply User’s Manual
Computer Interface Operation
5-3
5.1.3.1
Bus Control Commands
A Universal Command addresses all devices on the bus. Universal Commands include Uniline and Multiline Commands.
A Uniline Command (Message) asserts only a single signal line. The Model 642 recognizes two of these messages from
the BUS CONTROLLER:
Remote (REN)
and
Interface Clear (IFC)
. The Model 642 sends one Uniline Command:
Service Request (SRQ)
.
REN (Remote)
– Puts the Model 642 into remote mode.
IFC (Interface Clear)
– Stops current operation on the bus.
SRQ (Service Request)
– Tells the bus controller that the Model 642 needs interface service.
A Multiline Command asserts a group of signal lines. All devices equipped to implement such commands do so
simultaneously upon command transmission. These commands transmit with the Attention (ATN) line asserted low.
The Model 642 recognizes two Multiline commands:
LLO (Local Lockout
) – Prevents the use of instrument front panel controls.
DCL (Device Clear)
– Clears Model 642 interface activity and puts it into a bus idle state.
Finally, Addressed Bus Control Commands are Multiline commands that must include the Model 642 listen address
before the instrument responds. Only the addressed device responds to these commands. The Model 642 recognizes four
of the Addressed Bus Control Commands:
SDC (Selective Device Clear)
– The SDC command performs essentially the same function as the DCL command
except that only the addressed device responds.
GTL (Go To Local)
– The GTL command is used to remove instruments from the remote mode. With some
instruments, GTL also unlocks front panel controls if they were previously locked out with the LLO command.
SPE (Serial Poll Enable)
and
SPD (Serial Poll Disable)
– Serial polling accesses the Service Request Status Byte
Register. This status register contains important operational information from the unit requesting service. The SPD
command ends the polling sequence.
5.1.3.2 Common
Commands
Common Commands are addressed commands which create commonalty between instruments on the bus. All
instruments that comply with the IEEE-488 1987 standard share these commands and their format. Common commands
all begin with an asterisk. They generally relate to “bus” and “instrument” status and identification. Common query
commands end with a question mark (?). Model 642 common commands are detailed in Paragraph 5.3 and summarized
in Table 5-8.
5.1.3.3
Device Specific Commands
Device specific commands are addressed commands. The Model 642 supports a variety of device specific commands to
program instruments remotely from a digital computer and to transfer measurements to the computer. Most device
specific commands perform functions also performed from the front panel. Model 642 device specific commands are
detailed in Paragraph 5.3 and summarized in Table 5-8.
5.1.3.4 Message
Strings
A message string is a group of characters assembled to perform an interface function. There are three types of message
strings: commands, queries and responses. The computer issues command and query strings through user programs, the
instrument issues responses. Two or more command strings or queries can be chained together in one communication but
they must be separated by a semi-colon (;). The total communication string must not exceed 255 characters in length.
A command string is issued by the computer and instructs the instrument to either perform a function or change a
parameter setting. When a command is issued, the computer is acting as ‘talker’ and the instrument as ‘listener’. The
format is:
<command mnemonic><space><parameter data><terminators>.
Command mnemonics and parameter data necessary for each one is described in Paragraph 5.3. Terminators must be
sent with every message string.
