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CT1990

MIL-STD-1553B Remote Terminal, Bus Controller, or Passive Monitor Hybrid with Status Word Control

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Figure 6 shows the operation of the data handshaking signals during transmit command with three data words.

As with the receive command discussed previously, NBGT is pulsed low if the command is valid and for the RT.

TX/RX will be set high indicating a transmit data command. While the RT is transmitting its status word, it

requests the first data word from the subsystem by setting DTRQ low. The subsystem must then reply within

approximately 13.5 µs by setting DTAK low. By setting DTAK low, the subsystem is indicating that it has the data

word ready to pass to the RT. Once DTAK is set low by the subsystem, DTRQ should be used together with H/L

and TX/RX to enable first the high byte and then the low byte of the data word onto the internal highway

IH08-IH715. The RT will latch the data bytes during IUSTB, and will then return DTRQ high. Data for each byte

must remain stable until IUSTB has returned low. Signal timing for this handshaking is shown in Figure 11.

Additional Data Information Signals

At the same time as data transfers take place, a number of information signals are made available to the

subsystem. These are INCMD, the subaddress lines SA0-SA4, the word count lines WC0-WC4 and current

word count lines CWC0-CWC4. Use of these signals is optional.

INCMD will go active low while the RT is servicing a valid command for the RT. The subaddress,

transmit/receive bit, and word count from the command word are all made available to the subsystem as

SA0-SA4, TX/RX and WC0-WC4 respectively. They may be sampled when INCMD goes low and will remain

valid while INCMD is low.

The subaddress is intended to be used by the subsystem as an address pointer for the data block. Subaddress

0 and 31 are mode commands, and there can be no receive or transmit data blocks associated with these. (Any

data word associated with a mode command uses different handshaking operations. If the subsystem does not

use all the subaddresses available, then some of the subaddress lines may be ignored.

The TX/RX signal indicates the direction of data transfer across the RT - subsystem interface. Its use is

described in the previous section.

The word count tells the subsystem the number of words to expect to receive or transmit in a message, up to 32

words. A word count of all 0s indicates a count of 32 words.

The current word count is set to 0 at the beginning of a new message and is incremented following each data

word transfer across the RT - subsystem interface. (It is clocked on the falling edge of the second IUSTB pulse

in each word transfer). It should be noted that there is no need for the subsystem to compare the word count and

current word count to validate the number of words in a message. This is done by the RT.

Subsystem Use of Status Bits and Mode Commands

General Description

Use of the status bits and the mode commands is one of the most confusing aspects of MIL-STD-1553B. This is

because much of their use is optional, and also because some involve only the RT while others involve both the

RT and the subsystem.

The CT1990/1 allows full use to be made of all the Status Bits, and also implements all the Mode Commands.

External programming of the Terminal Flag and Subsystem Flag Bits plus setting of the Message Error Bit on

reception of an illegal command when externally decoded is available. The subsystem is given the opportunity

to make use of Status Bits, and is only involved in Mode Commands which have a direct impact on the

subsystem.

The mode commands in which the subsystem may be involved are Synchronize, Sychronize with data word,

Transmit Vector Word, Reset and Dynamic Bus Control Acceptance. The Status Bits to which the subsystem

has access, or control are Service Request, Busy, Dynamic Bus Control Acceptance, Terminal Flag, Subsystem

Flag, and Message Error Bit. Operation of each of these Mode Commands and of the Status Bits is described in

the following sections.

All other Mode Commands are serviced internally by the RT. The Terminal Flag and Message Error Status Bits

and BIT Word contents are controlled by the RT; however the subsystem has the option to set the Message

Error Bit and to control the reset conditions for the Terminal Flag and Subsystem Flag Bits in the Status Word,

and the Transmitter Timeout, Subsystem Handshake, and Loop Test Fail Bits in the BIT Word.

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SCDCT1990 REV B 8/21/00 Plainview NY (516) 694-6700

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