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Line Characteristics
Logic and Control Definitions
Telegraph Interface
Logic Definitions
Data Terminal Ready. The IBM 3704 and 3705 generate signals on this circuit to
control the switching of the modem to the communication channel if communica-
tion is over a switched facility. Data Terminal Ready must be ON to allow the
modem to answer calls and must remain ON to maintain an established connec-
tion. The OFF condition should cause the modem to disconnect from the commu-
nication channel. Data Terminal Ready is an EIA RS-232 voltage mode circuit.
The interface circuits associated with the signal lines consist of cable drivers and
terminators that operate into a coaxial cable that has a characteristic impedance of
90 to 120 ohms. The terminator impedance should be 100 ohms resistive. Any
open circuit or driver power OFF should be identified by a terminator as OFF.
The sleeve of each interface line's coaxial lead is connected to Signal Ground,
except those sleeves designated as EIA RS-232 circuits.
1. Logical 1 and Control OFF, or Mark: In this condition, the driver output
current must be less than 5.0 mA into 100 ohms.
2. Logical 0 and Control ON, or Space: Driver output current will be greater than
23 mA, when terminated by 100 ohms and biased negative at least -0.8 volts
(but less than -1.3 volts). The IBM 3704 and 3705 terminator open circuit
input voltages will be between -0.8 volts and -1.3 volts.
The IBM 3704 and 3705 Communications Controllers can be connected to a
telegraph line through a cable that provides two twisted pairs of wires for connec-
tion to two separate telegraph circuits. The maximum cable length is 50 feet.
Figure 4 shows the IBM 3704 and 3705 protective input network for a telegraph
line. The interface can be used for 20 mA, 40 mA, or 62.5'mA single current
telegraph line termination. Card jumpers are used to select the proper circuitry
for detecting signals on 20 mA, 40 mA, or 62.5 mA telegraph loops.
Notes:
I.
When transmitting, the distortion generated by the IBM
3 704 and 3 705 does not exceed 2 %.
2.
The maximum allowable receive distortion is 40%.
3.
An echo check is signaled as a modem error.
1. Logical 1 or Mark: Current (nominal 20 mA, 40 mA, or 62.5 mA; to be
adjusted by common carrier).
2. Logical 0 or Space: No current.
A 300 volt Zener diode limits the surge voltage across the telegraph line to 300
volts maximum.
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IBM 3704 and-3705 Communications Controllers Original Equipment Manufacturers' Information
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