HP 3457A Service Manual page 189

8-50. Reading instructions from ROM U602.
To read instructions from ROM U602, the microprocessor:
© Sets the read/write (R/W) output (U601, pin 32) high.
© Sets the address code of the memory location to be read onto address lines AO through AIS.
© For the ROM to be enabled, one or more of A13-15 address lines must be high. U602 is en-
abled by the output of the NOR gate U606a. Being enabled only when one of the address
lines, A13-AI5, is high, effectively maps microprocessor memory locations 2000-FFFF (hex)
into ROM U602.
The positive portion of timing signal €+a (U624a, pin 3) and the high r/W signal sets the output of gate
U623a (RD) low to activate the output of ROM U602. The microprocessor then reads the information
from data bus lines DO through D7.
8-51. Reading information from RAM U603.
To read information from RAM U603, the microprocessor:
© Sets the read/write (R/#) output (U601, pin 32) high.
© Sets the address code of the memory location to be read onto address lines AO through A12
© Sets at least one of the following address lines, A6-A12. high in order to place the output of
NOR gate U606c low (Ram).
© Sets address lines A13, Al4 and AIS low to enable NOR gate U606a, The combination of
addresses A6-A15 required to enable the RAM effectively maps processor memory locations
40-1 FFF into U603.
The positive portion of timing signal €+9 (U624a pin 3) and the high R/W signal pulses the output of gate
U623a (RD) low to enable the output of RAM U603. The microprocessor then reads the information
from data bus lines DO through D7.
8-52. Writing to the unprotected section of RAM U603.
To write to the unprotected section of RAM U603, the microprocessor:
© Sets the read/write (R/W) output (U601, pin 32) low.
© Sets the address code, of the memory location where the information is to be stored, onto
address lines AO through A12. (When addressing the unprotected section of U603, at least one of
the address lines A9-A 12 will be set high forcing the output of U608a high).
© Sets address lines A13-A15 low to force the RAM output of decoder U606a high.
Since at least one of the address lines A9-A12 is high the output of U608a is forced high (CALRAM), This
output is inverted by inverter U634c forcing the input to AND gate U622b low. The low output of
gate U622b is input to NOR gate U642c which causes the output of U642c to track its other input, the
timing signal —+0. The low portion of the timing signal activates the input of RAM U603 through
control line EN3 (RAM 603 pin 27). The microprocessor then writes the information to U603 through
data lines DO through D7.
HP 3457A Multimeter
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