MCW-I for Start-Stop MCW-l for
Synchronous
MCW-2 for Start-Stop MCW-2 for
Synchronous MDW-I MDW-2
a
a
a
a
a AID-Assemble Disassemble Field 7 8
Char
Addr 10 12 15 16 I 7 18 19 20 21 23 "5 o .t E Mode Sequence Bit Count i-= I I I I I I 2: II II I I II II: : r
:u: Qi: 10 1 I M I I 0 I > I I E I lode Sequence: B
26 30 Command
31 32 33 34 35
u .,jf. >
u CI> 0 > .€ V'l ""0 Vi c: 'a; 0 w £ u -0 CI> e 0 et: I l -0: C>: : i-=: : i ! M ·e· I __ ____ ____ __ ____ ____________ : 4 ! 5! 7 8 10: 11 ! 12 15 16 17! 18 20! 21 24! 25 :26 30 31 32 33 34 35
LRC
Check Char only (Start-Stop)
Data Byte 000 Data Byte 100 7
7 8 15
Data Byte 00 1
7 8 15
Data Byte 101 18 Sense Byte
18
16 17 18
Char.
Count
Data Byte 010 16 17 18
Char.
Data Byte 110 Count
25 26 33 34 35 .c Status Byte ... Word Parity Indicator 25 26 33 35
25 26 33 34 35 0 .c Data Byte all 8 ... 25 26 33 34 35
Data Byte 111 .E ·t 0 e Figure 4. Main Control Words (MCW-l, MCW-2) and Main Data Words (MDW-l, MDW-2)
17
control, the data in the A/D field is transferred to
the MDW-1, or MDW-2, during a Read operation;
or, during a Write operation, the data in MDW-1,
or MDW-2, is transferred to the A/D field. This
counter ensures that each line in the 2703 never
requests data service in less than two character times,
for any synchronous operation.
For synchronous operations this counter is also
used for counting the time between Write commands
while in transparency. In addition, it is used during
receiving to count for the one-second and three­
second receive time outs .
Timeout. The timeout bits are set during a line
timeout and define the type of timeout condition
occurring.
Shift. The shift bit indicates an upper-case charac­
ter in the A/D field. It is set to ON when the A/D
field receives a. shift character or transmits a
character with a shift bit. This field is assigned as
an additional position in the mode field for synchron-
ous type operations. The mode bits indicate whether the associated
line is in text-out, text-in, poll, control, transparent,
or intermediate-block mode.
Sequence. The sequence field defines the operation
to be carried out on the character in the A/D field.
For synchronous operations, this field is extended
to four bits by assigning bIt 24 as part of this field.
IBM. This is used only for synchronous operations toj;dicate ,"vhen the line is in intermediate-block
mode. A Set Mode command turns this ON, and it
is reset by either a Disable command or by another
Set Mode command when the Bus Out-one position
is zero. Command. The command field stores the program
commands in abbreviated form (the five low-order
bits modified from the command, see Figure 7).
Data Service. The data-service bit signals the 1/0- interface circuits to transfer data bytes between
MDW-l, or MDW-2, and the multiplexer channel.
18
During a transmit operation, data service is set
for MDW-1 when the last character leaves MDW-1 (for start/stop operations). For synchronous opera­
tions, MDW-1 is set for data service when the first
character leaves MDW-2, while MDW-2 is set for
data service when the first character leaves MDW-l.
During a receive operation, data service is set when
MDW-1, or MDW-2, contains four data bytes (or
fewer, on end-of-opcration).
End. The end bit signals the I/O interface to send
the ending-status byte to the multiplexer channel.
Halt/Stack. The halt/stack bit indicates one of the
following has occurred:
1. The line has received a Halt I/O command
from the channel during a transmit operation.
2. The line has received a Halt I/O command or
a Stop signal from the channel during a re­
ceive operation.
3. The channel was unable to accept either the
initial- or the ending-status byte and has
requested that status be stacked.
This bit indicates a Halt I/O when the end bit is OFF, and it indicates Stack when the end bit is ON. Receive. The receive bit is turned ON when the 2703
is receiving data on the associated transmission line.
Parity. This bit is set to provide odd parity for MCW-1. Main Control Word 2 MCW-2 (see Figure 4) stores the check character,
sense byte, and status byte for its associated line. LRC Check Character. The LRC-check-character
field holds the check character as it is being developed
during data transmission. For synchronous
operations, this field is unassigned.
Sense Byte. The sense-byte field contains the sense
information to be presented to the channel upon
receipt of a Sense command. (See "Sense-Byte Conditions!! under "Commands. ")
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