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ACT3492 Datasheet(PDF) 7 Page - Aeroflex Circuit Technology |
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ACT3492 Datasheet(HTML) 7 Page - Aeroflex Circuit Technology |
7 / 49 page Aeroflex Circuit Technology SCD3492 REV B 6/26/01 Plainview NY (516) 694-6700 7 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, and high for a Synchronize with data word mode command (See Figure 9). 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 Figures 8 and 17 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. It should be noted that the RT expects the Vector word contents to be already prepared in a latch ready for enabling onto the internal highway when VECTEN goes low. If the subsystem has not been designed to handle the Vector word mode command, it will be the fault of the Bus Controller if the RT receives such a command. Since the subsystem is not required to acknowledge the mode command, the RT will not be affected in any way by Vector word circuitry not being implemented in the subsystem. It will however transmit a data word as the Vector word, but this word will have no meaning. Reset Mode Command Figure 7 shows the relevant signal timings for an RT receiving a valid reset mode command. Once the command word has been fully validated and serviced, the RESET signal is pulsed low. This signal may be used as a reset function for subsystem interface circuitry. Dynamic Bus Allocation This mode command is intended for use with a terminal which has the capability of configuring itself into a bus controller on command from the bus. The line DBCREQ (See Figure 7) cannot go true unless the DBCACC line was true at the time of the valid command, i.e. tied low. For terminals acting only as RTs, the signal DBCACC should be tied high (inactive), and the signal DBCREQ should be ignored and left unconnected. Use of the Busy Status Bit The Busy Bit is used by the subsystem to indicate that it is not ready to handle data transfers either to or from the RT. The RT sets the bit to logic one if the BUSY line from the subsystem is active low at the time of the second falling edge of INCLK after INCMD goes low. This is shown in Figures 14 and 20. Once the Busy bit is set, the RT will stop all receive and transmit data word transfers to and from the subsystem. The data transfers in the Synchronize with data word and Transmit Vector word mode commands are not affected by the Busy bit and will take place even if it has been set. It should be noted that a minimum of 0.5 µs subaddress decoding time is given to the subsystem before setting of status bits. This allows the subsystem to selectively set the Busy bit if for instance one |
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