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Electrical Characteristics
The OFF condition must cause the modem to be removed from the communica-
tion channel following the completion of any "in process" transmission. The OFF
condition must not disable the operation of circuit CE/125 (Call Indication).
When circuit CD/108.1 is turned OFF, it is not turned ON again until circuit
CC/107 is turned OFF.
Data Terminal Ready (CD/108.2). The IBM 3704 and 3705 generate signals on
this circuit to control the switching of the modem to the communication channel.
The ON condition must ( 1) prepare the modem to be connected to the communi-
cation channel, and (2) maintain an established connection.
If
the station is equipped for automatic answering of received calls and is in the
automatic answering mode, connection to the line occurs only in response to a
combination of a ringing signal and the ON condition of circuit CD/108.2 (Data
Terminal Ready). The IBM 3704 and 3705 may present an ON condition on
circuit CD/108.2
(1)
whenever it is ready to transmit or receive data, or (2) in
response to an ON condition on circuit CE/125 (Ring Indicator).
The OFF condition must cause the modem to be removed from the communica-
tion channel following the completion of any "in process" transmission. The OFF
condition must not
disabl~
the operation of circuit CE/125 (Ring Indicator).
In switched network applications, when circuit CD/108.2 is turned OFF, it is not
turned ON again until circuit CC/107 (Data Set Ready) is turned OFF by the
communication equipment.
Ring
Indicator (CE/125). The modem generates signals on this circuit to indicate
to the IBM 3704 and 3705 that a ringing signal is being received by the modem.
Data Signal Rate Selector (CH/111). The IBM 3704 and 3705 may generate
signals on this circuit that may be used by the modem to select between two
operating speeds, if the modem is equipped to operate at more than one speed.
The IBM 3704 and 3705 modem interfaces comply with the electrical require-
ments of RS-232C and CCITT V.24 as highlighted in the following paragraphs.
All voltages are measured at the connector with respect to Signal Ground (AB).
The output line delivers between 5 volts and 15 volts into a resistance of at least
3,000 ohms, but not more than 7 ,000 ohms. The input circuit responds to a
voltage between 3 volts and 25 volts. The polarity of the signals depends on the
following:
Polarity
+
Data
Space
Mark
Logic
0
1
Control
ON
OFF
The terminating impedance of the receiving end of the interchange circuit must be
at least 3,000 ohms but not more than 7 ,000 ohms. The capacitance measured at
the interface connector should not exceed 2,500 pF. The open-circuit voltage of
the input circuits should not exceed 2 volts of either polarity. A short circuit
between any two conductors (including ground) must not result in a current in
excess of one-half ampere.
8
IBM 3704 and 3705 Communications Controllers Original Equipment Manufacturers' Information
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