to send. When the poll is terminated, the 2703 is
commanded to read from the terminal that responded.
If the terminal fails to respond within approximately
two or three seconds (depending on the terminal), a
timeout is recorded, and the Read command is
terminated. COMMANDS The command code in the channel-command word (CCW) specifies to the multiplexer channel and the 2703 the operation to be performed. The following
stored commands (see also Figure 7) are valid to the
Start/Stop Line Bases Type I and IT of the 2703. These commands are classified into three groups-­
transmit-type commands; receive-type commands;
and control commands. The groups are identified
by the low-order bit positions: transmit-type
commands contain 01 in bit-positions 6 and 7 on the
channel; receive-type commands contain 10 in bit­
positions 6 and 7; and control commands contain 11
or 00 in these bit positions. The "Sad" ("Set
Address") commands (e. g., Sadzer) are classified
as exception commands and used only by the 2702. When any of these commands are received by the 2703, they are treated as an I/O No-Op. NOTE: Command chaining should not be used to link two or
more segments of a message block together.
Transmit-Type Commands Write
Bytes are transferred from the channel to the ad­
dressed communications line four at a time except
at the end of a message (when the channel signals a
stop after the message has been completed, or the 2703 terminates the operation). Since the four bytes
or code characters are requested by the 2703 each
time its MDW-l needs service, the average data­
transfer rate from the channel to the 2703 is deter­
mined by the bit rate of the communications facility". The Write command is accepted, but ended
immediately with Channel End, Device End, and Unit Exception status bits, if the 2703 is receiving data
from the communications line on this address.
Various other unusual ending conditions also set the Channel End, Device End, and Unit Check status
bits, as well as a particular bit in the sense field.
These are: parity on Bus Out, which will set the
Bus Out Check; a VRC Check, an Echo Check, or
Break signal received on the appropriate tylJe of
line, which will cause the Data Check or Intervention
Required sense bit to be set. Finally, if a data set
is in a power-off condition, or in test mode, or if
a telegraph line has not been "enabled, !, the
Intervention Required bit in the sense field is set.
If the data transfer is completed normally, the
command is ended with Channel End and Device End
status.
Wrap On acceptance of this command, the 2703 wraps the
output of the addressed line to the input of the line
with the lowest address in the 2703. This command
is executed in an identical manner to a Write, and
the error conditions are the same.
Break On acceptance of this command, a continuous Space
signal is transmitted on the addressed line. Bytes
are transferred from the channel to the 2703 to
provide control over the length of the Space signal.
Each byte causes one character of space to be
inserted on the communications line. For example,
a terminal operating at 74.2 bps requires about
94 ms per character. Thus, a CCW count of ten
causes a space of O. 94 seconds. These bytes may
have any bit configuration. The Break command is
valid for Telegraph Terminal C'1ntrols Type I and
Type II, the World Trade Telegraph Terminal Con­ trol, and for an IBM Terminal Control Type I
equipped with the Type I Terminal Interrupt feature. On all other lines this command is rejected as
invalid. Poll On acceptance of the Poll command, the 2703 requests
a certain number of characters from the channel.
(This number is determined by the terminal control
for that line.) These polling characters are trans­
mitted and the 2703 goes to receive mode until either
a reply is received or a three-second timeout is
completed.
If a negative reply is received, the 2703 asks for
the next polling characters and continues on in this
manner. If, however, a positive reply is received,
the 2703 sets Channel End, Device End, and Status
Modifier, and the channel chains immediately to 2- Read command. The Poll command is valid only for
the IBM Terminal Controls Type I and II.
In actual operation, the Poll command causes the 2703 to request three characters (two in the case of
the 1030) from the channel--the first two being the
station address and the third being a count character
(any graphic). The then generates a © and
transmits the two address char'3.cters from the channel,
25

Transmit Receive Control Commands (
WRITE WRAP I BREAK
\ ---
( - - I READ
\ \ PREPARE I ) I I'JH I BIT ( I ENABLE
\ DISABLE SENSE j I/o NO-OP P o 2 3 4 5 6 7
o o o o o o o o t o o o o o o
o o o o o o
__ 0_ .-2 _0_ 0 _ JJ _ 0 __ [ _0 _ 0 _ _ _ _ 0 _ _1_ [ I 0 II o II t:- : f'\ v
o II : o
o
o
o f'\ v
o
o
o
o
o
o
o 0 o 0 o 0 o 0 o
o
o 0 o o
o o o
o o
o o
o
o o o
o o
Bit Positions Telegraph Terminal Control Type I or Type II World Trade Telegraph Terminal Control IBM Terminal Control Type I (equipped with
Type I Terminal Interrupt feature)
i 8M Term inai Control Type I or Type II Autocall Feature Telegraph Terminal ContiOl Type I RESERVE _I _0 _ 0 0 0 1 \ - - - _ _ _ _ _ _ _ _ _ _ _0_ Two Processor Switch Feature
RELEASE 0 0 0 I SADZER SADONE o o o o
o o o
Exception \ ) _. - -w- ( o o o
o o o o Figure {'. Start/Stop Commands '.hUe buffering the cOlmt character. It then turns "lr('LEld and awaits a response from the terminal. , '1" P t ' l'e four possible responses, each of which ;"j it diffp ],pnt cti nn frnrn the 270:3: o o
o
o