4

SCDCT1775 Rev B

GENERAL

As shown in the block diagram of Figure 1, the CT1775

provides all functions required to implement a Dumb Remote

Terminal Unit (RTU). It is designed for the greatest flexibility

and ease of use. CT1775 can be operated with either an

internal or external clock. Simple control lines are provided to

interface with either single channel or dual redundant

configurations.

Control lines are available to implement either on line or off

line wraparound built-in test. CT1775 can be configured to

perform a parity check on its hard-wired terminal address. It

provides numerous output flags to simplify the use interface.

These flags indicate various decoded messages, as well as the

results of error checks. Sync selection, along with the flexible

controls, allows the CT1775 to operate as a Bus Controller as

well as a Remote Terminal.

INTERNAL OR EXTERNAL CLOCK

CT1775 may be operated with either its internal clock or an

external clock. Internal clock operation requires that a 12 MHz

parallel-resonant

fundamental-mode

crystal,

such

as

MIL-C-3098/42 TYPE CR64/U, be connected between pin 18

(XTAL) and ground. In addition, +5 volt power must be

connected to pin 2 (OSC/CLOCK POWER), and CLOCK

OUT (pin 19) must be connected to CLOCK in (pin 24).

For external clock operation, no connection is made to pin 2

(OSC/CLOCK POWER, and the external clock is applied to

pin 24 (CLOCK IN). Pin 19 (CLOCK OUT) is not connected.

The external clock must be capable of driving a load of 20

picofarads to within 0.5 volts of the + 5 volt power supply and

to within 0.5 volts of ground. Standard TTL voltage levels will

not work properly. It must have a rise time and fall time of less

than 10 nanoseconds. For compliance with MILSTD-1553, the

external clock frequency must be 12 MHz.

8 BIT OR 16 BIT INTERFACE

The CT1775 may be configured to interface with either 8 bit or

16 bit parallel data highways. For 16 bit operation, the 16 data

lines (D 15 through DO) are used directly. LATCH DATA 1

and LATCH DATA 2 are tied together, as are DATA SELECT 1

and DATA SELECT 2. This allows data transfer in 16 bit bytes.

For 8 bit parallel data highways, the 16 data lines must be tied

together in eight pairs (1315 to D7, D8 to 130, etc.) The two

LATCH DATA and DATA SELECT are used independently.

This allows transfer in two 8 bit bytes.

ADDRESS WITH PARITY

The CT1775 provides five lines for hard-wired terminal

address. Internal pull-up resistors are provided on these lines,

so logic "1" lines may be left open-circuited. Logic "0" lines

must be grounded. The CT1775 maybe configured to check the

parity of these five address lines. This function can be selected

by using the ENA PAR CHECK line. The address parity line

(TMADDP) is hard-wired for odd address parity if the function

is sued. the ODD PARITY output flag indicates a valid check

for odd parity of the six address lines.

DUAL REDUNDANT OPERATION

The CT1775 may be used in a dual redundant configuration

with a minimum of additional circuitry. A CHAN SELECT

signal is provided which simultaneously disables the LATCH

DATA, DATA SELECT, and ENC ENABLE lines of the

CT1775. Therefore, CHAN SELECT can be used to multiplex

a single set of LATCH DATA, DATA SELECT and ENC

ENABLE control signals between two CT1775 units, which

have these signals tied together in parallel.

WRAPAROUND BUILT-IN TEST

The CT1775 may be configured to implement either on line or

off line wrap around built-in test. By enabling the receiver with

RX STROBE during a normal transmission, the encoded word

will be fed back into the decoder by the receiver. In this on line

wrap around mode of operation, the CT1775 compares each

decoded word that is fed back with the original word that was

encoded. The BIT STATUS output flag indicates when the two

words are not the same.

Care must be taken when using this on line wrap around test

technique because an outgoing status word will be interpreted

by the decoder as a new command word. Since the status word

has the correct address and the same sync as a command word,

the CT1775 will set RT ENABLE and VAL CMD WD. For on

line wrap around operation, it is therefore necessary to reset

RT ENABLE after transmission of a status word. This can be

accomplished by inverting SEND DATA and applying it to

DEC RST during status word transmission. If it is required that

the status word be fed back, RT ENABLE should be reset

immediately after it goes HIGH by applying a LOW to DEC

RST for 1 microsecond (minimum). The status word will be

available at the receive register.

The CT1775 can be placed in an off line wrap around test

mode by use of the S/T SELECT signal. In this mode, the

transceiver is disabled and the encoder output is fed directly to

the decoder input. All other functions remain the same, and the

CT1775 compares each word that is decoded with the original

word that was encoded. The BIT STATUS line also indicates

the result of this comparison for the off line wrap around test.

FAIL SAFE TIME-OUT

The CT1775 contains a timer which continuously monitors the

length of each transmitted message. This timer detects a

transmitted message which exceeds 768 microseconds and

causes the transmission to terminate. At the same time, The

FAIL-SAFE flag is set to indicate a Terminal Fail-Safe

Time-out. Further transmissions are inhibited until the

FAIL-SAFE flag is reset by MRST or a valid command word

with the correct address is received.