EOT (End of Transmission)-- <0 Indicates an end of message transmission, and
resets the LRC counters at both sending and
receiving stations. NOTE: LRC does not apply to the 1030 or to the 2740 (without
the checking feature installed). EOA (End of Address)-- @ Indicates the transmission of text data is to
follow. Indicates an end of address, with text data to
follow. This signal starts the LRC counter at
both the sending and receiving terminals. The EOA signal is not included in the following
LRC check. Indicates positive response to a poll from the
2703. EOB (End of Block) -- ® Indicates the end of a unit block of text. This
is followed by the LRC charader to provide an
LRC check at the receiving station
with the EOB character included in the check.
Yes (Positive Response)-- Ci? Indicates a positive response to an address. Indicates a positive to an EOB when the
checking feature is installed.
No (Negative Response)-- ® Indicates a negative response to an address. Indicates a negative response to a poll. Indicates a negative answer to an EOB; the
hyphen character is printed as an indication
of this condition. SOA (Start of Address) -- ® Indicates a start-of-station-identification
condition (not used with the 1050). Inquiry (Positive Answer)-- @ Indicates a positive answer to an EOB from the
terminal. At this time, the terminal switches
to receive status.
TRANSMIT OPERA TION (WRITE COMMAND)
The transmit operation is initiated when the 2703
accepts a Write command from the multiplexer
24
channel. Before transmission to the terminals
occurs, the transmission control requests the first
four bytes of data from the multiplexer channel.
These bytes are then transmitted on a bit-by-bit
basis to the terminal. Additional requests for four
more bytes of data are made each time the last byte
is being transferred, until the complete message has
been transmitted.
The four bytes of data are stored in the main data
word (MDW -1, see Figure 4) located in the 2703
magnetic-core storage. From there they are trans­
ferred byte-by-byte to the main control word (MCW-1) for that line. The transfer from the control word in
core storage to the line base is accomplished on a
bit-by-bit basis until the character has been sent.
Each bit is huffered in the line-base delay line. The
byte, while in magnetic-core storage circuitry, is
shifted one bit position toward the high-order bit
before the delay line is loaded. When a character
has been sent, the next byte is requested from the
data-word location. When the data word is emptied,
a request for the next four bytes is sent to the multi­
plexer channel. This is repeated until the operation
is ended.
RECEIVE OPERATION (READ COMMAND)
The receive operation is initiated when the 2703
accepts a Read command from the multiplexer
channel. On detection of a start bit, the transmission
control prepares to receive a data character. Before
each bit is put into the low-order bit position of the
serial-data field, the field is shifted forward one bit
position in the high-order direction. Each bit of the
character is received in this manner until the terminal
control signals stop time to the common controls.
Stop time occurs when the bit count equals X, which
is a number preset in the terminal control and defined
by the number of bits in a character. For example,
X is 7 for the IBM Terminal Control Type 1. At stop
time, the character stored in the main control word (MCW-1) is transferred to the appropriate main data
word (MDW-1). The 2703 requests data service when
the fourth character has been assembled, and Simultaneously prepares to receive the next character.
- ..... ,. Jl __________ __ ____ ' ...... DurIng the recelVe operanon Lne and-storage section of the 2703 (see Figure 3) pro­
vides a timeout of 28 seconds or less between data
characters, provided a Read or Search command has
been accepted by the 2703. The timeout process is
interrupted by a received start bit and is reset and
restarted at the following stop time. If the timeout
completes before a start bit is received, the Read or
Search command is tenllinated.
Timeout may also result if a terminal indicates an
intention to send data after being polled and then fails
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
Previous Page Next Page