Hameg HO88, Руководство пользователя

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and dumps any data bytes that follows. The EOI line is also used by the System
Controller to perform a parallel poll of up to 8 different instruments on the bus. This
involves setting both EOI and ATN to their logic 1 states.
IEEE-488 bus communications
Both uniline and multiline messages can be sent over the bus. The interface
management lines ATN, IFC, REN. SRO and EOI are used uniline messages, and the
data lines D101 through D108 areused in conjunction with the ATN line and the EOI
line for multiline messages. The ATN line is used to distinguish between device-
dependent messages and interface messages.
Uniline messages
These messages have the highest:priority, and are detected by devices on the bus
regardless of the states on the data lines. They are sent using the following signal
lines:
Line Function
DAV Handshake: data valid
NRFD Handshake: not ready for data
NDAC Handshake: data not yet accepted (notdataaccepted)
IFC
Initialize interface (interface clear)
REN Remote/manual selection (remote enable)
SRQ Interrupt service request (service request)
ATN Device-dependent message/interface message (attention)
EOI End of message (end or identify)
EOl with Parallel poll (status byte query) ATN
Multiline messages
This category comprises both device-dependent messages and interface messages.
Device-dependent messages. The ATN line is at logic 0 during transfer of this type of
message. The data are transferred with aid of the handshake lines DAV. NRFD and
NDAC. The end of a transmission is identified by a line feed and/or asserting EOI.
Device-dependent messages contain device-specific information such as programming
instructions, measurement results, machine status, and data files. Coded in the for-
mat stipulated by the instrument maker.
Interface messages
The ATN line is at logic 1 during transfer of this type of message. These messages
contain commands issued by the System Controller to maintain order and initiate
proceedings within the bus system, and are sent with the aid of the handshake lines
DAV, NRFD and NDAC. If ATN is asserted, this tells the receiving devices that interface
message units are being sent over the bus.
Interface messages are used for:
• Enabling the Talker function of devices (TAG) by sending their primary talker
addresses, and disabling them.
• Enabling the Listener function of devices (LAG) by sending their primary
listener addresses, and disabling them.
• Transfer of the addressed commands GTL, SDC, PPC, GET and TCT.
• Transfer of the unaddressed (universal) commands LLO, DCL, PPU, SPE and SPD.
• Transfer of secondary addresses (SCG: Secondary Command Group).
The ASCII character table (Figure 3) also contains the interface messages that can be
sent while ATN is asserted.
These are summarized in the following:
Function Description
Talk and Listen addresses
These are used to address instruments as Talkers or Listeners.
TAG For addressing an Talker Address Group instrument as aTalker
LAG For addressing an Listener Address Group instrument as a Listener