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|  MIL-D-81347C(AS)
.
will send an External Function Request to the computer (the Output Data Request is kept low at this
time). When the computer responds with an External Function line, the Function word on the corn- ,
puter output lines will be transferred to the instruction storage register in the MTC and the status
storage of the MTC will be cleared or initialized unless the instruction was an 02 (Status Report).
For any Function Code the MTC will select the MTT specified by bits 17-18 of the Function word to
insure that the MTT status is appropriate for the operation to be performed. The Function word
address bits will be compared with the Address Selection switch of the MC P. If the Function word does
not specify an existing MTT or specifies more than one, the Illegal Instruction Status condition will be
set and the Normal Completion Status condition reset provided that the Instruction requires the selec-
tion of an MTT.
For any Function Code except the codes specifying a Rewind,
Satus Report, Master Clear or Loop Instruction where the packing density is not specified, the
Illegal Instruction condition will be set and the Normal Completion Status condition reset. If the se-
lected MTT does not respond with a select acknowledge and ready signal, the Fault Satus condition
will be set and the Normal Completion condition reset. If the Function Code specifies a Write opera-
tion and the MTT Satus is Write Lockout, the Fault Status and Write Lockout conditions will be set
and the Normal Completion Status condition reset. If the Function Code specifies a Reverse Direction
command and the MTT Status is Load Point, the Illegal Operation Status condition will be set and the
Normal Completion Status condition reset.
For any Function Code except the codes specifying the
Status Report or Rewind Instruction, if the MTT Status is Rewinding, the Normal Completion
Status condition will be reset. If the Function Code specifies a Rewind Instruction and the MTT
does not respond with a Rewinding Status within four to ten microseconds, the Normal Completion
Status condition will be reset. When in the idle state if the Normal Completion Status condition is
reset for any reason, the MTC will not proceed with the instruction and no direction command will be
sent to the MTT. If the Interrupt Enable from the computer is a logical `1" the MTC will send an
Interrupt to the computer and transfer all Status information to the computer input data lines. If
the Interrupt Enable from the computer is a logic "0", the MTC will revert to the idle state; however,
the Status signals will still be available for Status Report function.
Write Function - When a Write class of Instruction, Write or
3.5.2.4.1.6.2
Write Extended Interrecord Gap, is received by the MTC and the idle state is normal, the MTC will
send an Output Data Request to the computer. At the same time, the MTC will send Forward, Write,
and Read commands to the MTT and initiate the start time delays.
If the MTT is at load point, a delay time corresponding to a
nominal 3-1/2 inch distance between the load point marker leading edge and the first character
of the record will be timed. If the MTT is not at load point, the MTC will time a start distance
determined by the Instruction (normal or extended interrecord gap) and the original Status. The
MTC is now ready to write the Data words one frame (character plus designated parity bit) at a
time on the tape at the specified density.
If the computer has not responded to the Output Data Request with
an Output Acknowledge indicating there is a Data word on the computer output line to be written with-
in the gap time, the MTC will set the Timing Error Status condition and reset the Normal Completion
Status. If the computer does respond to the Output Data Request, the MTC will transfer the word into
storage and request the second word from the computer. The MTC will have requested and received
at least one and possibly two words from the computer before the MTT is ready to write the first
character on tape. The MTC will count and transfer the characters to be written. When a full word
has been written (five characters) an Output Data Request is sent to the computer.
The timing of Output Data Requests from the MTC will now be
determined by the density at which data is being written on the tape. The process will continue
until the computer does not respond to an Output Data Request within the time allowed to write
a word on tape at the requested density. The MTC will recognize a no data condition and will time a
111
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