IBM 2790, Руководство пользователя, Страница: 35 / 88

• EOT is transmitted if the 2715 detects an incorrect
message control byte.
• EOT is transmitted if the message received exceeds
126 bytes.
• EOT is transmitted to reinitialize control if the error
recovery procedure does not succeed in correctly
receiving a transmission.
Disconnect (DISC). The 2715 switched-network configu­
ration provides for a
2 0
-second timeout to prevent pro­
longed periods of data link inactivity. Upon receipt of
DISC (DLE EOT) or after 20-second timeout, the 2715 will
automatically go “on-hook.”
The 2715 transmits an EOT when it is either a master
station ending a message transfer or a slave station responding
to a master station transmission of EOT. After transmitting
the EOT, the 2715 initializes a 20-second timeout and re­
linquishes the right to bid for the line. If the 2715 does not
receive a response (ENQ, EOT, or DLE EOT) before the
end of the
20
-second timeout, it disconnects the data link
by dropping the data-set connection. If the CPU responds
with EOT before the end of the 20-second timeout, the
2715 responds with EOT (assuming it has no need to bid
for the line) and restarts the 20-second timeout. This con­
trol sequence allows the System/360 to override the 20-
second timeout if it desires.
Operator-Awareness Message. The 2715 provides two
operator-awareness messages to inform the user of commu­
nication problems. These two messages are:
• Data link not operational.
• Data link established.
“Data link not operational” is printed on the 2740 when
the retry count is exhausted. No other messages are printed
until one good message is transmitted over the BSC line;
when a good message is transmitted, “data link established”
is printed on the 2740.
Leading Graphics. The 2715 will not transmit leading
graphics. However, it will accept up to seven graphics pre­
ceding a slave-station response (ACK/NAK). The 2715
monitors the graphics for control characters only, and
otherwise will not act upon them.
Real-Time Clock (R TC )
This feature is standard on the 2715 Models 1 and 2. Func­
tions provided by this feature are:
1. Maintains a 24-hour clock that time-stamps all user
data atvthe transaction level (that is, transaction
header will contain time stamp). The time stamp
consists of four EBCDIC characters that specify
hours and minutes. For 2715 to System/360
messages, the time stamp contains the value of
the clock when the first data entry of the transaction
was received. For System/360 to 2715 messages,
the time stamp must contain valid numeric EBCDIC
characters. Messages to the 2740 or 1053 print the
time stamp preceding the text.
2. Broadcasts time of day about every four seconds to
all 2791 Area Stations’ visual display. At the user’s
option, this time will be maintained as either 24-hour
or
12
-hour clock.
Time Advance
A standard option allows the 2715 to synchronize the ad­
vance of the RTC from the user’s master-clock system. If
this option is not used, the RTC advance is synchronized
from the 2715 power source. The master-clock systems
used may be one of three types: three-wire self-regulating,
two-wire polarity-reversing, or two-wire single-impulse. The
duration of the ‘minute advance’ pulse must be at least 275
milliseconds. For the three-wire self-regulating and the two-
wire polarity-reversing master clocks, the duration of the
correction pulse must be at least 275 milliseconds. The
two-wire single-impulse master clock does not provide
correction pulses.
Interface information is provided in the IBM 2790 In­
stallation Manual-Physical Planning , GA27-3017.
Time Set
The 2715 CE/operator panel allows the operator to set and
display the RTC. Time must be set by the operator when­
ever the 2715 is powered on.
Day Number
The 2715 provides a three-byte EBCDIC field for storing
day number. The day number can be entered by the operator
at the CE/operator panel whenever the 2715 is powered on.
The day can also be entered or changed from the System/360.
When the 2715 transfers data to the System/360 or to
disk, it prefixes a message header to the data. The day is
included in the message header.
C E /O P E R A T O R P A N E L
The CE/operator panel (Figure
2 2
) is accessible to the opera­
tor and can be used for limited functions. Many of the
switches and lights are designed for Customer Engineering
Maintenance, and cannot be used by the operator.
Pushbutton Switches
Power On. Turns the 2715 power on.
ICPL. Loads the Initial Control Program.
Power Off. Turns the 2715 power off.
Lamp Test. Tests indicator lamps.
IBM 2715 Transmission Control Unit 35