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 andIT 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 contain01 in bit-positions 6 and 7 on the
channel; receive-type commands contain10 in bit
positions 6 and 7; and control commands contain 11
or00 in these bit positions. The "Sad" ("Set
Address") commands (e. g., Sadzer) are classified
as exception commands and used only by the2702. 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-TypeCommands 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 the2703 terminates the operation). Since the four bytes
or code characters are requested by the2703 each
time its MDW-l needs service, the average data
transfer rate from the channel to the2703 is deter
mined by the bit rate of the communicationsfacility". The Write command is accepted, but ended
immediately withChannel 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 theChannel End, Device End, and Unit Check status
bits, as well as a particular bit in the sense field.
These are: parity on BusOut, which will set the
BusOut Check; a VRC Check, an Echo Check, or
Break signal received on the appropriatetylJe of
line, which will cause the DataCheck 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 withChannel End and Device End
status.
WrapOn acceptance of this command, the 2703 wraps the
output of the addressed line to the input of the line
with the lowest address in the2703. This command
is executed in an identical manner to a Write, and
the error conditions are the same.
BreakOn acceptance of this command, a continuous Space
signal is transmitted on the addressed line. Bytes
are transferred from the channel to the2703 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, aCCW count of ten
causes a space ofO. 94 seconds. These bytes may
have any bit configuration. The Break command is
valid for Telegraph TerminalC'1ntrols Type I and
Type II, the World Trade Telegraph TerminalCon 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 the2703 goes to receive mode until either
a reply is received or a three-second timeout is
completed.
If a negative reply is received, the2703 asks for
the next polling characters and continues on in this
manner. If, however, a positive reply is received,
the2703 sets Channel End, Device End, and Status
Modifier, and the channel chains immediately to2- Read command. The Poll command is valid only for
the IBM TerminalControls Type I and II.
In actual operation, thePoll command causes the 2703 to request three characters (two in the case of
the1030) 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 addresschar'3.cters from the channel,
25
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.
stored commands (see also Figure 7) are valid to the
Start/Stop Line Bases Type I and
transmit-type commands; receive-type commands;
and control commands. The groups are identified
by the low-order bit positions: transmit-type
commands contain
channel; receive-type commands contain
positions 6 and 7; and control commands contain 11
or
Address") commands (e. g., Sadzer) are classified
as exception commands and used only by the
more segments of a message block together.
Transmit-Type
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
or code characters are requested by the
time its MDW-l needs service, the average data
transfer rate from the channel to the
mined by the bit rate of the communications
immediately with
from the communications line on this address.
Various other unusual ending conditions also set the
bits, as well as a particular bit in the sense field.
These are: parity on Bus
Bus
Break signal received on the appropriate
line, which will cause the Data
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
Intervention Required bit in the sense field is set.
If the data transfer is completed normally, the
command is ended with
status.
Wrap
output of the addressed line to the input of the line
with the lowest address in the
is executed in an identical manner to a Write, and
the error conditions are the same.
Break
signal is transmitted on the addressed line. Bytes
are transferred from the channel to the
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
causes a space of
have any bit configuration. The Break command is
valid for Telegraph Terminal
Type II, the World Trade Telegraph Terminal
equipped with the Type I Terminal Interrupt feature.
invalid.
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
a reply is received or a three-second timeout is
completed.
If a negative reply is received, the
the next polling characters and continues on in this
manner. If, however, a positive reply is received,
the
Modifier, and the channel chains immediately to
the IBM Terminal
In actual operation, the
the
station address and the third being a count character
(any graphic). The
transmits the two address
25