HP 8112A Operating, Programming And Servicing Manual page 272

Read Only Memory
Random Access
Memory (RAM)
RAM Battery Supply
HP-IB General Purpose
Interface Adapter
10.7·2 Servicing the Microprocessor and Front-panel
or write data to, the addressed location. Note that some devices can
only be read from, others can only be written to, and others can both
be read from and written to.
The ROM is a permanent data-store which contains the
microprocessor program and other fixed data such as the standard
(ROM)
parameter set.
The RAM is a data-store which the microprocessor can write to and
read from using the read/write (R/W) control line to choose which
operation is required.
The RAM is used to store the current parameter set and temporary
data needed by the microprocessor. The HP 8 1 1 2A RAM has a
battery back-up power supply, described in the following section,
which means the data in the RAM is maintained while the
instrument is switched off. This allows the current parameter set to
be restored when the instrument is switched on again.
The normally negative "Power Down Detected" signal is used to
ensure that the RAM data is not corrupted when the instrument is
switched off.(Refer to Figure 1 0.7-5). Normally U27A output is low,
U27B output is therefore positive and the RAM SELECT output
from U 1 5C can enable the RAM by switching on U29C. When the
"Power Down Detect" signal goes high, U27A output goes low, U27B
output goes low and U15C cannot switch on U29C. Therefore the
RAM becomes, or remains, disabled.
When the instrument is operating, U27C output is high, QI is
switched on and the RAM UlO is powered from the +5 V supply.
(Refer to Figure 10.7-5). "Power Down Detected" signal goes high,
U27C output goes low and switches off Q l . The +5V supply to the
RAM is now maintained by the battery BTl.
The GPIA IC U30 interfaces between the microprocessor and the
HP-IB , as shown in Figure 10.7-4. The IC pin confi g uration is given
in Figure 10. 7-2.
Microprocessor Interface Signals
D0-07. Eight bi-directional, tri-state data lines allowing data transfer
between the microprocessor and the GPIA.
CS. A negative edge selects the GPIA enabling the microprocessor to
communicate with the GPIA.
R/W. The READ /WRITE input controls GPIA register access and
the direction of data transfer on the data pins. It is connected to the
microprocessor READ/WRITE output .