Tektronix 494AP service manual Oe o, i o

Theory of Operatlon- 4g4A/494ApService,Vot. 1

+ G E N R U N N I N

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441S120

Flgure 7-21. Character timing dlagram.

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lnstrument Bus Interface

The microcomputercontrolsthe crt readoutand fre- quencymarkerdot over the instrumentbus throughthe followingports.

DecoderU3051asserts5F when it seesa valueof 5 on the upper four bits of the instrumentbus address lines,and 2F when it sees a value of 2. The decoder mustbe enabledby DATAVALIDhigh on the instrument bus. The false transitionof DATA VALID causesthe

addressedport to latchthe data on the instrumentbus.

ConlrolPort Controladdressport U3034turnsthe readouton or off, steersdata sent to the address/data port, controlsthe mode of the frequencymarker dot, and containstwo bits of the RAM address. The bits are definedin Table7-9. Bit numberingon the instrument bus starts al zero. However,the D and Q pins of U3034(and some other lCs) are numberedstartingat one,followingth€ manufacturersdata.

Bit 0 turns the crt readoutdisplayon (1) or off (0). When set, this bit releasesCLEARfrom the GENRUN- NING flip-flopand allows the off timer, U1055,to set

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