Operation & Service Manual
Section III
Model 1805B RF Control Unit
Theory & Operation
Assembly (A6) provides a +20 ±2V, +5 ±0.5V, and ±15V ±2V outputs to all the Model 1805
Assemblies connected to the Motherboard section of this assembly. The +30V for the DC
Substitution PC Board Assembly (A2) is generated by the +30 V Power Supply Assembly
(A7). The +30 V Power Supply Assembly receives its operational power from the T1
Transformer located on the Power Supply\Motherboard Assembly (A6). A front panel
POWER ON (DS1) and STANDBY (DS5) indicator is provided to allow the user to verify that
an external power source is applied to the Model 1805B.
MICROPROCESSOR SECTION
The microprocessor board, functionally depicted in Figure 3-5 reads control information form
either the front panel (local control) or the rear panel IEEE-488 bus interface (remote control).
The board decodes this control information and performs the tasks needed to implement the
control function. The 1805B's functions include switching of the substitution levels to eleven pos
sible settings and the control of the RF ON/OFF switching.
The heart of the microprocessor board is U1, an INTEL eight-byte 8085 parallel microprocessor
with a 6-MHz clock. U1 interprets instructions stored in EPROM ( erasable electrically program-
mable ROM ) U10. Programmable logic array, U9, interfaces the microprocessor to the program
memory, U10 as well as providing I/O port capability. Serial EPROM U1 1 contains the instru
ment program for U9 which is loaded at instrument power up. These instructions in program
memory U10 guide the microprocessor in its operation through two main modes: local and
remote operation. In local mode the user programs the instrument by switching front panel con
trols that switch status receivers that are part of 1/0 chip U7. The program instructions
stored in EPROM U2 continuously update these status receivers, which are scanned by the
microprocessor to derive local mode instructions. If the front panel status changes, the micro
processor outputs the proper bits to the board drivers through the output latches ofU7 or U9.
The board drivers (U8 and U6) energize the relays associated with the 1805B circuits responsi
ble for this function. Due to the continual scanning of the microprocessor, detection of
any front panel function change occurs unless the 1805B is set to remote mode in which case
the microprocessor switches to a new set of instructions.
Figure 3-4 Simplified Microprocessor Block Diagram
3-9