HP 8560E, Service Manual

Page: 400 / 665

Chapter 7 399
ADC/Interface Section
A16 Assembly Fast ADC Control Circuits (8560E with Option 007)
The post-trigger counter begins counting upward in "write" mode on the
first rising edge of PCLK after the LCOUNT signal from the trigger
circuit goes low. The frequency of PCLK is the programmed sample
rate. When the post-trigger counter reaches its terminal count, the
LSTOP signal goes low and disables the static RAM address counter
from further counting. LSTOP also forces LCOUNT high in NAND gate
U11D, which disables the post-trigger counter.
15-Bit (32 K) Circular Address Counter
Refer to function block G of the A16 fast ADC assembly schematic
diagram in the HP 8560 E-Series Component Level Information.
This 15-bit programmable circular counter provides the address lines of
the static RAM (U32). The counter consists of U23, U24, U25, and U26.
It counts upward from 0 to 32767 and then back to 0 in a circular
fashion. When a trigger occurs, latches U27 and U28 latch the current
static RAM address so that the CPU on the A2 controller assembly can
later read the latches and determine the static RAM address of the
sample that was taken when the trigger occurred.
The CPU loads the address counter during "read" mode by first setting
LLOADADDR (bit 6 of the fast ADC control word) and LREADCLK (bit
9 of the fast ADC control word) to their low state. The CPU then writes
the 16-bit load value to the CPU secondary address. The rising edge of
ACLK then latches the 16-bit data into the address counter.
After the address counter is loaded by the CPU during "read" mode, the
static RAM is read by the CPU. The RAM is read by first setting
LREADMEM (bit 10 of the fast ADC control word) and LREADCLK (bit
9 of the fast ADC control word) to their low state. Since the
LREADCLK control bit is low, a negative-going pulse on the ACLK line
will occur on every static RAM "read" by the CPU. This causes the
address counter to increment at the end of each static RAM "read" so
that the address counter automatically post-increments to the next
address of RAM U32. In order for this address post-increment to occur,
the LSTOP count enable signal from the post-trigger counter must be
high. LSTOP goes low when the post-trigger counter reaches its
terminal count in the "write" mode to stop the address counter from
counting. When the fast ADC assembly is changed from "write" mode to
"read" mode, LSTOP will be low. So the CPU on the A2 controller board
must always first program the post-trigger counter to a value other
than the terminal count (65535) to force LSTOP high.
Video Trigger Comparator
Refer to function block M of the A16 fast ADC assembly schematic
diagram in the HP 8560 EC-Series Component Level Information.
This 8-bit digital magnitude comparator, U34, compares the digitized
samples from the flash ADC (latch U29 output) to the programmed