Aeroflex Circuit Technology

SCDCT1995 REV A 11/21/01 Plainview NY (516) 694-6700

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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 CT1995 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.

Synchronize Mode Commands

Once the RT has validated the command word and checked for the correct address, the SYNC line is set low. The

signal WC4 will be set low for a Synchronize mode command (See Figure 16), and high for a Synchronize with data

word mode command (See Figure 15). In a Synchronize with data word mode command, SYNC remains low during the

time that the data word is received. Once the data word has been validated, it is passed to the subsystem on the

internal highway IH08-IH715 in two bytes using IUSTB as a strobe signal and H/L as the byte indicator (high byte first

followed by low byte). SYNC being low should be used on the enable to allow IUSTB to clock synchronize mode data to

the subsystem.

If the subsystem does not need to implement either of these mode commands, the SYNC signal can be ignored, since

the RT requires no response from the subsystem.

Transmit Vector Word Mode Command

Figure 14 illustrates the relevant signal timings for an RT receiving a valid Transmit Vector Word mode command. The

RT requests data by setting VECTEN low. The subsystem should use H/L to enable first the high byte and then the low

byte of the Vector word onto the internal highway IH08-IH715.