Watkins-Johnson Company WJ-8718A/MFP Instruction Supplement page 123

MAINTENANCE
WJ-8718A/MFP
internal to the IC. If no external short circuit can be found, replace
the common IC.
4.1.4
4.1.4.1
8. When open outputs and short circuits have been eliminated, the
problem could be an open input on the IC being driven from the node,
an internal IC failure, or an open signal path. The current tracer, or
an ohmmeter, can be useful in finding an open signal path. For an
opened input or an internal failure, the IC must be replaced.
MICROPROCESSOR SYSTEM TROUBLESHOOTING
Modular Concept
The microprocessor system consists of a number of modules, parallel-connected to
the microprocessor data bus. A module can be described as a block of functionally related
components capable of transmitting or receiving data on the bus. The IF Interface (MFP-A3),
the Synthesizer Interface (MFP-A4), and the Front Panel Encode Board (MFP-AIAl) each
contain modules which are a part of the microprocessor system. If an optional remote interface
is)n use, the interface board is also a part of the modular structure. The practical approach to
troubleshooting the system is to isolate the trouble to a specific circuit board and then to a
specific module.
Each module typically consists of address decode and functional circuitry. In order
to troubleshoot within the modUle, the technician must be able to locate a module and identify
the address and function circui ts.
The microprocessor accesses each module in the system for a data transfer by a
unique hexadecimal address. The address is placed on the bus at the beginning of each data
transfer cycle. Typically, the address decode components in a module provide the following two
services:
1.
Because the eight lower-order bits of the data bus are multiplexed,
the address data cannot remain on the bus throughout the entire data
transfer. Therefore, a circuit is required which will capture and hold
the address data. The typical address capture circuit in the MFP
system is a D-Type flip-flop which latches the address information on
the negative-going transition of a special microprocessor signal
(ALE).
2.
The address information must be decoded by a circuit which can
provide clock or toggle type output signals. Generally, address
decode circuitry is shared by more than one module. The decoder
must utilize the address data inputs to provide one unique toggle
output for each address input. The address decode circuits in the
system are typically BCD-to-decimal integrated circuits with select
and enable inputs tied directly or through logic gates to the address
capture circuit,· to the microprocessor data bus, or to selected
processor signals.
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