TM 5-6675-318-14
x . M e m o r y t i m i n g . The memory timing generates the necessary waveforms from the
m i c r o p r o c e s s o r control signals to insure proper timing for the memory and I/O data
t r a n s f e r s . A typical READ cycle begins when the microprocessor places an address on
the bus. One clock period later, STM goes true indicating that the address is valid
and that the address latches should store the information. One clock period after
At this time, the microprocessor
r e c e i p t of STM, the memory timing sends back UMC.
One-half clock period later, XSMC goes high which
r e m o v e s the address from the bus.
allows the memory output drivers to be enabled. At the trailing edge of SMC, data
must be stable in order for the microprocessor to store the data in its accumulator
r e g i s t e r . At the same time, the output drivers are disabled.
y. Main PCA A2 internal I/O circuitry. The internal I/O circuitry provides
buffered interconnection between the front panel PCA A3 and the processor circuits.
A front panel READ will put the status of the front panel switches on the plotter
TTL bus. A front panel WRITE causes the selected front panel LEDs to light up .
R E A D registers 23, 26, and 27 are used to put front panel PCA data on the TTL bus,
while WRITE registers 21, 23,, and 27 are used to enable the LEDs on the front panel
These registers are stored in memory locations 2 0 8 t h r o u g h 2 78 a n d a r e
assembly.
d e c o d e d in the microprocessor circuitry by U33 and U38 using READ, RAL, and addres-
ses A0-A2.
z.
Front panel PCA A3.
The front panel PCA A3 provides a means for the operator
to set CHART HOLD or CHART LOAD, and to manually select a pen, control pen up or
down, and to enter certain position data to the plotter. The operator may also
receive status information concerning the plotter operation from the LEDs on the
f r o n t p a n e l . The front panel PCA circuits interface with the main PCA A2 through
t h e i n t e r n a l I / O c i r c u i t r y . The front panel PCA houses switches which indicate to
the plotter which pen stalls are loaded with pens, or if a pen is presently in the
pen arm.
The PCA also provides an interconnection for the X- and Y-limit switches.
aa. Main PCA A2 memory circuit.
The memory capacity of the plotter is 9k
s i x t e e n - b i t w o r d s . The memory includes the following:
(1) Read only memory (ROM)
8192 words of ROM. (8k x 16)
(2) Read/Write random access memory (RAM)
1024 words of RAM. (1k x 16)
Read only memory. The ROM consists of two 8-bit wide mask programmable ROMs
ab.
ROMs U55 and U56 are 8k devices, and they
configured as 8k of 16-bit wide memory.
These ROMs contain the firmware
r e p r e s e n t memory locations 0000 0 8 t h r o u g h 1 7 7 7 7 8.
instructions to drive the plotter and communicate with the HP-IB controller through
t h e I / O . P r o g r a m e x e c u t i o n b e g i n s a t m e m o r y l o c a t i o n 4 08 after the micro-
processor receives the power-on pulse.
The RAM consists of four 1k by 4 Read/Write Memories
a c . Random access memory.
The RAM is used as temporary
U 6 4 , U65, U66, and U67 configured as 1k by 16.
storage for various plotter operations and as a buffer for incoming data. The RAM
o c c u p i e s memory locations 76000 8 t h r o u g h 7 7 7 7 78.
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