Readout System (Diagram 9) - Tektronix 2246 1Y Service Manual

Portable oscilloscope

Table of Contents

Address Latches

MULTIPLEXED AD BUS ADDRESS LATCH. Since the

ADO through AD7 bits are multiplexed between

address and data information, the addressing infor

mation needs to be latched to hold it for stable

addressing

(demultiplexed). The

Latch Enable) signal from the Measurement Pro

cessor (pin 61) goes high when the address bits are

stable, and the bits are latched into U2513. The

device is permanently enabled by the grounded

enable pin.

NONMULTIPLEXED

ADDRESS

LATCH

(U2512).

Some

address bits are also latched to maintain them

between ALE strobes. The latching also prevents

address line problems on the Main board from

locking

up the

Measurement

U2512,

latched

addresses

(A12-A15) are routed to the Display Controller

(U600) for addressing the internal registers in that

device. Those address lines are also applied to U501

(Diagram 4) for additional decoding to load the

Analog Control Shift Registers with the serial data

supplied from the MB DATA signal line. Two address

lines (A16-A17) are latched in U2512 for use by the

System ROM U2519.

Measurement Processor ROM

The operating code for the Measurement Processor

is stored in the System ROM (U2519). Immediately

after the Power On Reset ends, the Measurement

Processor fetches the first command from the reset

vector and begins running the program.

Measurement Processor RAM

The Measurement Processor RAM (U2521) provides

storage space for interm ediate-step calculation

results, the front-panel settings, and the system

calibration constants. The Processor RAM is battery

backed up so that data stored during operation

remains intact during periods of power off. When the

instrument is turned on again, the stored front panel

settings return the oscilloscope to the same oper

ating state that was present at power off. The stored

calibration constants preserve the accuracy of the

measurement system (assuming the instrument is

warmed up and was warmed up when the SELF CAL

routine was last done). If the backup battery is

dead, or if the stored calibration constants are lost

for some other reason, the instrument will do a SELF

CAL at power on. This restores accuracy to the

instrument (unless the problem is a RAM fault, in

which case the instrument cannot SELF CAL), but

Theory of Operation—2246 1Y and 2246 Mod A Service

ALE

(Address

BUS

ADDRESS

the

nonmultiplexed

Processor.

From

ADDR0-ADDR3

the battery circuitry should be checked and the bat

tery replaced if necessary. Also, the SELF CAL

routine should be run again after the instrument is

warmed

generate

constants at the operating temperature.

pow er-off front panel settings are lost for any

reason, the power-on conditions that are set up are

only restored in valid states (but not any predefined

setup).

Address Decoder

The Address Decoders (U2517 and U2518) allow the

Measurement Processor to enable any device on the

busses

for

communication.

BUSO SEL and DAC SEL from the processor select

the Address Decoder (eitherJJ2517 or U2518) that is

actively decoding when the WR signal is low.

Backup Battery

To keep the data stored

Processor RAM (U2521) during power off, a back-up

battery system, BT2501 (and BT2502 in 2246 Opt.

1Y), CR2502, and R2506, is used. The battery sup

plies the energy to maintain the memory states of

the static RAM. The battery is not rechargeable and

has an operating life of one or five years, depending

battery

type

(see

MEMORY" in Table 1-1 for more information). When

the instrument is on, CR2502 becomes reverse

biased to prevent any reverse current; when off,

CR2501 is reverse biased to isolate the back-up bat

tery from the +5 V supply. If the battery requires

replacement, observe the proper safety precautions

in the handling and disposition of the replaced

battery (see the WARNING under " Battery" in the

Specification).