Hameg HO88, Manual

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Änderungen vorbehalten / Subject to change without notice
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the data to the bus and uses the DAV line to indicate that the signals on the data lines
are stable (valid) and can be safely accepted (DAV at logic 1). It then checks the NDAC
line to determine whether the data have been accepted by all devices (logic 0), resets
DAV to a logic 0 state (= not valid), and outputs a new message byte to the bus.
NRFD (not ready for data)
This- line is driven by all devices when receiving commands and by Listeners when
receiving data messages. It indicates when a device is ready or not ready to receive a
message byte. A logic 0 denotes readiness to receive messages. As soon as valid
data are signalized by the DAV line, the receiving device puts the NDAC into its logic 0
state and strobes in the data. NDAC goes back to logic 1 when the device sending
data indicates „data not valid“ on the DAV line, and so on. Even if only one device is
sending a logic 1 signal on the NRFD line, the DAV line cannot be pulled low (logic 1)to
prompt acceptance of new data.
MDAC (not data accepted)
This line is driven by all devices when receiving commands and by Listeners when
receiving data messages. It is used to communicate to devices with data or commands
to send that the data on the signal lines have been accepted (NDAC logic 0). The
device that is sending the message byte cannot put the DAV back into its logic 0 state
(= data not valid) until this is done. After the DAV line carries the signal denoting that
the data states are no longer valid, the NDAC goes into a logic 1 state again, and the
device is ready for the next message byte. Even if just one device is transmitting a
logic 1 on the NDAC line, the data on the signal lines cannot be taken off the bus.
The interface management lines ATN, IFC, REN, SRQ and EOI
The interface management lines are used to manage the flow of information across
the IEEE-488 bus. They can be influenced by the user. ATN, IFC and REN can only be
driven by the System Controller. SRQ can only be driven by a peripheral device. EOI
can be driven by either the CIC or a peripheral device acting as Talker.
ATN (attention)
ATN can only be driven by the Controller-in-Charge. The state of this line indicates to
the devices connected to the bus whether the states on the data lines should be
interpreted as commands (logic 1) or device-dependent messages (logic 0). Acceptance
of the data is controlled by the handshake lines DAV, NRFD and NDAC.
IFC (interface clear)
IFC can likewise only be driven by the System Controller. It is used to initialize the bus
interfaces of all devices connected to the IEEE-488 bus (returning them to the same
state as when they a re powered up p). IFC should be the first instruction issued when
control of a device via the bus is begun;
REM (remote enable)
The System Controller drives the REN line. If REN is pulled to a-logic 1, this places all
devices enables as Listeners in remote control mode. A logic 0 puts all devices back
into local (manual) mode. The REN line can also be set globally to logic 1 by connecting
it with the IEEE-488 bus ground, or within devices on the bus by fixed wiring. As long
as REN stays at logic 0, none of the Listeners on the bus - unless they are internally
wired for a logic 1 - will accept remote commands!
SRQ (service request)
Any device connected to the IEEE-488 bus can drive the SRQ line to request service
from the System Controller, for instance if an abnormal condition exists or in order to
transfer measurement data to the System Controller. The System Controller can only
influence the SRQ line by indirect means, namely by performing a serial poll until it
finds the device which is the perpetrator of the SRO interrupt. That device is then
automatically cleared of SRQ data, thus resetting the SRQ line to logic 0.
EOI (end or identify)
The EOI line serves two purposes. A Talker (which can be the CIC) uses it to mark the
end of a message string. When a Listener detects EOI. it terminates the conversation