I REJECT CODE
4.4.6.4 ROB/TAPE MARK BLOCKS
This situation is the same as that described in Section 4.4.5.4 except that tape motion is bacKward and, when the command is completed, the read head is positioned relative to the ISG preceding the tape marK blocK.
4.4.7 Write a Data Block (WRT) Command
4.4.7.1 NON-CACHED OPERATION
The WRT command causes, (1) tape to be moved in the forward direction, (2) the ending portion of the preceding ISG to be generated, (3) the data blocK to be written, (4) the data block to be read and checked for validity, and (5) the beginni~g
portion of the next ISG to be generated. The data block 1S
written in the format as determined by the Density Status/Select lines and the Density Select switch.
Non-data characters of the data blocK are automatically generated, encoded, formatted, and written. Data characters to be written are transferred serially across the interface,
automatically encoded, formatted, and written.
4.4.7.1.1. Signal Sequence
After the conmand ini.tiation, the MTS first asserts RECV, signifying that the Bi-Directional Data bus (DATA) is under control of the user interface and that the MTS will receive data
transfer on D"ATA.
The MTS next asserts one initiating TREQ signal sequence. The user" interface must respond with assertions of data characters on DATA and by assertion of TRAK within the timing limitations of Figure 4-6. If the user interface does not respond accordingly, the MTS resets TREQ, asserts REJECT, and terminates the command.
If the user interface responds correctly, then the MTS starts tape motion. After the ending portion of the IBG is created, the preamble is written. The user interface must respond to TREQ by placing a data character on DATA and then asserting TRAK. The MTS
then acknow.ledges transfer of the data .character by resetting TREQ. At this time the user interface may change DATA and must . reset TRAK.
-TREQ
---1 J---:---1
---~---
----DATA
---1
S---
---TRAK
---=---1 S---
---L-Tl--J ---T2---.J -T3-j -T4--j Tl, T2, T3, T4,
=
~ 0Tl + T2 S 75 milliseconds
Figure 4-6. Write Commands Initiating TREQ/TRAK/OATA Timing (Applies to First Byte of Data)
Normal TREQ/TRAK/DATA response continues until the user interface signals STOP, signifying that the last character to be written is being transferred. The MTS then causes the remainder of the block to be formatted and written. The read-after-write checKs are performed and the beginning portion of the next rBG is generated. Ending status is asserted, BUSY is reset, and the
operation is completed. RECV remains asserted until a command other than a WRT or LWR comnand is .initiated. These user interface responses must meet the timing limitations given in Figure 4-7. The time between consecutive TREQ signals may not be uniform. No user interface timing snouid be developed based on the time between consecutive TREQ signals. If TRAK or SiOP does not respond according to the timing limitations, an internal MTS write data character buffer may be overrun. When this occurs, incorrect data encoding has occurred. The MTS then discontinues requesting data, finishes formatting the block, sets Overrun Status (QVRNS), and tape motion halts.
-TREQ
---1 --- I---1
----DATA
---1
I---~
---TRAK
---1
I---
---L---
T5---J
L- - - - T 2 - - - _J L- - - - T 4 - --_J L- T 1 _J
L- T 3 -J
50 IPS 2921 50 IPS 2922 100 IPS 2922
T 1 ~ 0 ~ 0 ~ 0
T2 ~ 766 nanoseconds ~ 1100 nanosecond ~ 550 nanoseconds
T3 ~ 0 ~ 0 ~ 0
T4 ~ 766 nanoseconds ~ ""nn I I uu nanoseconds ~ ~~I"I :J:JU nanoseconds T5 2.8 microsec nomin 3.0 microsec nomin 1.5 microsec nomin Note: Specified times for GCR are at -the 2920 MTS interface
connectors.
Figure 4-7. WRT Command TREQ, TRAK and DATA
(Applies to All Subsequent Bytes of Data'Transferred) 4.4.7.2 CACHED OPERATION (MULTIPLE BUFFERED RECORDS)
The WRT 'comnand causes the next data block to be transferred from the host to the cache memory. Generally, a series of WRT commands causes multiple records to be held in cache memory for
later transfer to tape. If an error is encountered when writing to tape, the cache control will backspace, erase a portion of
tape, and try to rewrite the record. This error recovery process is repeated the number of times selected, either 5, 10, or 15 times. If the data cannot be written to tape, REJECT is asserted on the next available command, and no other commands can be executed until a Sense command transfers the appropriate status
to the host and a reset is then given to the MTS.
4.4.7.2.1 Signal Sequence (Buffered Mode)
The TREQ and TRAK Jor STOP) interface signal sequence is the same for long-record mode, except that the timing requirements of Figure 4-7 can be relaxed for slower interfaces. T1, T2 and T4 are determined by the host system and can be longer or shorter than shown in the figure. T5 would change correspondingly. T1 plus T2 plus T4 cannot exceed 75.0 milliseconds on any data byte.
Of course, faster timing results in higher system throughput.
Table 4-14 shows the timing required to give the maximum transfer rate.
4.4.7.2.2 WRT/Early EDT (Buffered Mode)
When writing ~ith ihe Cache Buffer installed and operating in buffered mode, 'a number of records may be present in the buffer at any given time. When the MTS determines that the tape is approach i ng the end -of - record i ng are"a, the max i mum number of records in the buffer is reduced to 1 or 4 as indicated by the switches. This ensures that no more than 1 or 4 records will be written after the end-of-recording area is encountered without
th~ host's knowledge. After the end-of-recording area is reached, only 1 record maximum is allowed in the buffer so that status is not given to the host unti 1 this record has been written to tape.
404.7.3 WRT/BDT
When a WRT command is initiated with tape positioned at BOT, the MTS writes and checks the 10 area before proceeding to the WRT command. The 10 area is written and checked automatically within
the MTS, requiring no signal responses from the user interface.
For the model 2925, the data for the record is taken into the buffer, transferred to tape, and checked for validity before BUSY is unasserted and status is given to the host. Note that when operating in buffered mode, no record can be written or read that
is larger than the buffer.
If the 10 area cannot bewritten and read with validity, the operation is terminated. A data blocK is not written and the WRT command is not performed. Appropriate 10 BRST, REJECT, and DATA CHK signals are included in the ending status.
4.4.8 Loop Write-to-Read (LWR) Command
The LWR command operations provide a means of testing the read and write data circuit paths within the MTS. Read signals are derived (looped) within the MTS from the write circuits. There is no tape motion. LWR is not allowed if tape is loaded and positioned away from BOT. Data is transferred over the interface the same as during a Write command.
The signal sequence is the same as that described in Section 4.4.7.1 for a WRT command operation.
4.4.9 BacKspace a File (BSF) Command
4.4.9.1 DESCRIPTION
The BSF command causes tape to be moved ba\.Kward, passing over data blocks encountered until a tape mark block is detected.
Tape motion is halted with the read head positioned relative to the IBG preceding (on the BOT side of) the tape mark. Tape Mark Status is included in the ending status and the operation is completed. No data characters are checked for validity or transferred across the interface. BLOCK is not asserted for any data blocks passed ,over; BLOCK is only asserted at the Tape Mark.
Other than command initiation, no signal responses are required of the user interface.
404.9.2 BSF/BoT
If the 1D area is reached before finding a tape mark blocK,' the operation is terminated. Tape is positioned at BOT. DATA CHK and BOTS are asserted in the ending status. If BSF is initiated with tape positioned at BOT, the corrmand is invalid, REJECT is asserted, and the operation is terminated.
4.4.10 Backspace a Block (BSB) Command
4.4.10.1 DESCRIPTION
The SSB command operation causes tape to be moved backward, passing over data blocks until signaled to STOP by the user interface. When signaled to stop, the read head is positioned relative to the ISG preceding the last data block passed over.
No data characters are checked for validity or transferred across the interface.