CT2579 datasheet(10/41 Pages) AEROFLEX

10 / 41 page

APPLICATION NOTE #108

Released 9/98

SIGNALS THAT INDICATE CHECKSUM FAILURE

For 1760 applications, the STATUS line indicates if a message has failed checksum. The

STATUS line will toggle up or down for each received data word as it is calculating the

checksum and is sampled on the falling edge of NCMDSTRB. The STATUS line can be tied

to any of the Status Word Bit inputs to set those bits in the event of a checksum fail. STATUS

is an open output line that will set the selected Status Word Bits for the Status Word

response in the current message. This is one of the great features for this product. The

subsystem does not have to verify the checksum in software to detect the error. The SmaRT

unit automatically does this in hardware and the unit is able to flag the error and set the a

Status Word Bit on the CURRENT Status Word response . This minimizes processor

overhead and reduces response time in notifying the Bus Controller that an error has

occurred.

For 1760 applications, the NVALCHK signal also indicates a valid checksum for the Receive

Command message. NVALCHK is a latched version of the STATUS signal and is updated

only on Receive Commands. It is valid on the falling edge of NCMDSTRB of a Receive

Command and remains stable until the next Receive Command message. A Transmit

Command message will not alter this signal because a Transmit Command does not require

an incoming checksum validation.

BLOCK TRANSFER LOGIC

The Block Transfer Logic (BTL) may be enabled for both Remote Terminal and Bus

Controller.

The BTL consists of a 32 word memory buffering the subsystem to the main RAM thus

guaranteeing data consistency for both transmit and receive transfers.

All reads and writes to the BTL are identical to read / write to the main RAM. The address

locations are the same. The only difference is that the BTL circuitry will intercept those read

T0-15

NEMPTY

NVALCHK

CMD WD

STATUS

VALID

NCMDSTRB

Latched Until Next Receive Command

8.5 µSec

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