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Theory of Operation- 4g4Al4g4ApService,Vol. 1

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lithium battery 8T2040, or in option 3g, silver batteries (Eveready392 or equivalent). See the Maintenance sectionfor replacementinformation.

Each ol the non-volatilememorylcs require less than 2 rl. They wiil hold.data as tong as the battery voltageis above 2.SV. In the batte{ circuit, RIO3O and R2037protectthe batteryagainst;ccidentalshort circuits. Thejumperon pins i anOe of p1040provides an easy way to rernovepower, thus clearingali data in the RAM.

The microprocessoruses ftip-flopU4O3Oto power- up and enabtethe battery-backed-up

RAMs. tnitiatty, U4.030.isreset by the RESETtinegoiogtow. This dis_ ablesthe non-volatit€RAMs,U1020and-U1030As. part

of thesequence,the microprocessorwrites _initialization

to instrumentbus address 73 to set U40gO,soutput high. (U4040decodes bus address73). This ailows q?qq to^c-hargeto +5 V. turningon e20i5. e2035, and Q2CX!7Q2037.connectsthe +5 V supptyto th€ RAMS, power-supply

inputs,back-biasingCRf03Oand discon- llcling the battery.e202S turns on e2OgO,puuingthe CE inputslow, allowingthe micropro"e"soito use the RAMs.

On power-downthe microprocessorsets the output of U4030 low. After C2090discharges, e2O2S,O20bO, 02035, and Q2037 switch off. R2Og2puils the CE1 lines high, to the RAM suppty vottage,disabtingthe memory. As the supptyvottagsfails, CR2030swiiches

the battery begins supptying power to the 9{, -and_

RAMs. R2036and R3034insurethat the-CE2tinesare grounded, not floating, in the standby condition, as is requiredfor lowestcurrentdrain.

Options. The OptionsSwitch settingstell the micro- cornputerwhich Instrumentoptionsare instaledso that the appropriatefirmwareis used. lt also enablesdiag- nostic checks and allows reportingonly settingsovir the GPIB in Tatk Only mode. Figure 7€B shows the switch settings. Seethe Maintenancesectionfor more informationaboutusingthe switches.

Octat switch S1050, bufer U2050, and decoder U3045form the OptionsSwitch. The decoderenables the bufferat addressof 7800. An open switchis read by the microcomputeras a 1, and a closed switch is read as a 0. When addressed,the buffer placesthe switchdata on the microcomputerdata bus.

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Flgure7.33. Optionsswitchbankon tha Memoryboard.

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