George G6HIG,
Looks like a useful instrument,?
I found?several manuals searching "agilent 8648b manual."
Also, try looking at the uP and the memory circuits supported by the backup battery.
An extension?card would be useful, some of the old HP Instruments?had one stored in the card cage.
Are the keyboard and display being scanned by the uP?
The best place to check is the keyboard,
here is a clip from the manual, page 5a-5 (203):
"The front panel contains two RPGs (rotary pulse generators), the keyboard, and the
display.
The two RPGs, one for frequency and one for amplitude, are connected directly to the
controller on the A3 board. Each RPG receives power and ground from the controller. Each
RPG returns two out-of-phase pulsed lines when the knob is turned.
The keyboard is a matrix of keys as shown in Table 5-1. on page 5-6. The keyboard is
scanned by the controller. Scanning pulses are sent alternately to the keyboard rows and
are read back on the columns when a key is pressed. The controller determines which key
was pressed based on the row that was pulsed and the column that the signal was
returned on. The column lines are pulled-up through resistors and are pulsed low when a
key is pressed. The row output latches are open-collector, therefore, pulses can not be seen
until the circuit is completed by pressing a key. The keyboard connects directly to the
controller at A3J3.
The display is driven by the controller through data latches on the A3 assembly. The
display control lines are eight bi-directional data lines, an enable clock line, a read/write
line, and a data/instruction line. The other lines going to the display are the display +5 V
and ground. The enable clock line is high during every data interchange. The read/write
line is high for a read operation and low for a write operation. The data/instruction line is
high for a data operation and low for an instruction operation. The data/instruction line is
used only during write operations. Data refers to the character data while instruction
refers to commands, such as return or space. When interchanging data, the controller
polls the display for acknowledgment.?
This means that if the display is disconnected the?controller will cease to attempt operations."
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