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Service
Model 8901B
one assembly, the A20 LO Control Assembly, decodes the Instrument Bus for the entire section.) Up
to 16 data codes for each of the 16 select codes are possible for each active enable line. The select code
typically selects a functional category on an assembly, and the data code selects the specific function or
configuration. On a given assembly, the select codes are decoded only while the corresponding enable
line is active. The data codes are, in turn, decoded and latched only when triggered by the decoded
select line. The latched data drive the digital-to-analog devices which control the instrument hardware.
On the schematic diagrams the lines leaving the 1/0 ports of the CPU are labeled with a mnemonic
such as s2(L) for 1/0 line 02. The "s" indicates a select code, "2" indicates that it is the third, least-
significant bit of the undecoded select code, and "(L)" indicates that the line is true
(1)
when the logic
level is low. All bit-position numbering begins with
0.
The select codes
go
out on the Instrument Bus
through Select Buffers, which are simple inverters. Thus s2(L) goes out on the bus as s2(H). Decoded
codes are labeled as e=l(L) for example. The "e" indicates an enable code,
u=n
indicates decoding,
"1"
indicates a decoded hexadecimal 1 (binary OOOl), and "(L)" indicates the logic level corresponding to
a true. The mnemonic "e=l" corresponds to e3e2ele0 = 0001. Data codes are also buffered. However,
unbuffered data lines are also connected to the Instrument Bus for reading back data to the
1 / 0
ports.
Figure 8D-7 illustrates how the 50 Hz High-Pass Filter
is
selected. The filter (not shown) is activated
when the output line of the High-Pass Filter Control (A3U16) labeled 50 Hz HPF(L) goes low. Register
U16 is simply a latch; it does not decode the data. To activate the 50 Hz High-Pass Filter, the CPU
sends out the binary enable code 0001 (hexadecimal
l),
select code 0100 (hexadecimal 4), and data
code 0010 (hexadecimal 2). The Enable Decoder activates the line e=l(L). The decoder was enabled
because e3(H) was low. Since s3(H) is low, and since e=l(L) is also low, the Select Decoder (A3U20)
is enabled. The three least-significant bits of the select code are decoded and activate the s=4(L) line
out of the decoder. This line clocks the data into the High-Pass Filter Control latch. Since the dl(H)
line is high, the 50
Hz
HPF(L) line goes low. This selects the 50 Hz High-Pass Filter.
There is a direct relationship between the codes output on the Instrument Bus and the Direct Control
Special finctions discussed in paragraph 8-7. The enable, select, and data codes, combined into
a
hexadecimal number "esd", create the Direct Control suffix. In the example used in the previous
paragraph, the suffix is 142 (corresponding to Direct Control code 0.142). Instrument control can be
visualized as a series of Direct Control Special finctions issued under program control.
The example above decoded only three of the four select code bits and used the data bits directly
(or
inverted them). Notice that if the code
esd
= 147
were issued, the through path (NO HPF), 50 Hz
High-Pass Filter, 300 Hz High-Pass Filter, and De-emphasis Pre-Display On switches would all be
activated. On some assemblies the data codes may be decoded and select codes above 7 may be used.
On other assemblies certain select codes are used to enable readback devices which read back status or
measurement data onto the unbuffered data lines. This is discussed in more detail in connection with
Direct Control Special Functions in Service Aids.
Keyboard and Display Assembly ( A l )
The Keyboard and Display Assembly
is
both an input peripheral and an output peripheral to the
Controller. The pressing of a key is sensed by the Keystroke Detector. The detector interrupts the
Microprocessor which then enters an interrupt service routine. The routine causes the key rows and
columns to be scanned sequentially via the Key Row and Column Scanner to ascertain which key is
down. This scan is accomplished by driving the rows in sequence with the select decoder and reading
the state of the columns with the data-readback lines. If no key closure is found (due, perhaps, to
key bounce), the scan repeats.
If no key closure is found after 50 ms, the Microprocessor leaves the
key-service routine and returns to the measurement being made prior to the key interrupt (starting at
the beginning of the measurement cycle).
Lighting of the key and annunciator lights, display digits, and decimal points is by a straight forward
decoding of the Instrument Bus. Note that the lights in the keys do not light as a direct result of a key
closure. Instead the Microprocessor, having recognized a key closure, sends the command out on the
Instrument Bus to light the key light.
8D-50
Service Sheet BD5
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