Theory of Operation— 492/492P Service Vol. 1 (SN B030000 & up)
Fig. 5-20. C haracter generator (U1028) block diagram.
Instrument Bus Interface
The microcomputer controls the crt readout and frequen
cy marker dot over the instrument bus through the following
ports:
Port
Hex Address
Control
5F
Address/data
2F
Decoder U2038 asserts CONTROL when it sees a value
of 5 on the upper four bits of the instrument bus address
lines and DATA when it sees a value of 2. The decoder must
be enabled by GEN ON low and DATA VALID high on the
instrument bus. The false transition of DATA VALID causes
the addressed port to latch the data on the instrument bus.
Control Port. The control port (U2034) turns the readout
on or off, steers data sent to the address/data port, and
controls the mode of the frequency marker dot. The lower
four bits are defined (see Table 5-9). Bits are numbered in
this discussion as on the instrument bus— starting at zero.
However, the D and Q pins of U2034 (and some other ICs)
are numbered as on their data sheets, starting at one.
REV AUG 1981
5-51
