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Model 8901B
Service
Remote Interface Assembly (A14)
The Remote Interface Assembly interfaces the Controller with the Hewlett-Packard Interface Bus
(HP-IB). It performs necessary handshake operations, interprets the HP-IB control lines, and is both
an input and output peripheral to the Controller.
The Remote Interface Assembly consists of Handshake Logic, HP-IB Input/Output Transceivers,
Interface Control Logic, Address Decoder, part of the Microprocessor, and Instrument Bus interface
circuits.
As an input peripheral, the Remote Interface accepts a byte from the HP-IB data lines under control
of the bus handshake lines. It then interprets the data byte and the bus-control lines to see if the
byte is an address (talk or listen), a command, or a data byte. When a byte is processed, one of three
things happens: (1) the byte is ignored,
(2)
the byte is processed in hardware (for example, some
bus commands), or (3) the byte causes a Microprocessor interrupt (for example, codes received while
addressed to listen). The Microprocessor treats an HP-IB interrupt as it would an interrupt from the
Keyboard. However, the HP-IB interrupt service routine first checks whether the byte is a command
(for example, Device Clear), address, or data (for example, "Ml").
If
the byte is an address or command,
it is processed. If the byte is data, the routine first checks whether the instrument is in remote. If it
is, the incoming byte is processed as program code.
If
not, the byte is ignored. After processing a byte,
Microprocessor tells the Remote Interface what to do next (for example, input another byte, set a
status latch, or prepare to output a byte).
As an output peripheral, the Remote Interface takes a byte of status or measurement data from
the Microprocessor and processes it over the HP-IB. It does this only after determining that the
Modulation Analyzer has been addressed to talk. The require-service message (SRQ) is also output via
the Remote Interface.
The Handshake Logic controls the asynchronous transfer of bytes over the HP-IB. The bytes are
transferred without interrupting the Microprocessor in two cases: when the byte is data but the
Modulation Analyzer is not addressed to listen, or when the byte is not an interrupting bus command.
The Handshake Logic also provides the means for the Microprocessor to complete the handshake if
the byte is an interrupting type.
When the Modulation Analyzer is accepting bytes, the Handshake Logic
(1)
monitors the Micropro-
cessor and HP-IB and signals the HP-IB talker or bus controller when the Modulation Analyzer is
ready to receive, (2) tells the Microprocessor when valid data is on the HP-IB, and (3) tells the HP-IB
talker when the Microprocessor has accepted the data. When the Modualtion Analyzer is outputting
data or status bytes, the Handshake Logic (1) tells the Microprocessor when the HP-IB listener is
ready to receive, (2) provides the Microprocessor with logic
to
tell the listener when data is valid, and
(3)
tells the Microprocessor when the listener has accepted data.
The HP-IB Input/Output transceiver acts as an HP-IB buffer and sendheceive switch. It is controlled
by the Interface Control Logic.
The Interface Control Logic, together with the Address Decoder, determines the talk or listen status of
the interface and whether or not the Microprocessor should be interrupted. The ROM in the Interface
Control Logic
is
addressed by two of the HP-IB data lines, the Address Decoder, and one of the HP-IB
control lines (Attention, ATN). The ROM contains the control information for the Interface Control
Logic and the Microprocessor.
If the Modulation Analyzer's listen address is recognized by the Address Decoder, the Microprocessor
attempts to set the Remote Enable Flip-Flop.
If
the HP-IB Remote Enable
(REN)
control line is
true, the flip-flop
is
set (if not already set), and the Microprocessor sets a status bit in memory. Each
time the Microprocessor performs any remote-dependent operation, it checks both the status bit and
the flip-flop output (Remote Enable Latch, RNL). Both must be set for the instrument to remain in
remote. If REN goes false at any time, the Remote Enable Flip-Flop
is
cleared, and the instrument is
no longer in remote.
The Address Decoder compares the address set by the Address Switches with the five least-significant
1/0 bytes
to
determine
if
the instrument
is
being addressed. The Interface Control Logic looks at the
Service Sheet BD5
8D-51
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