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SECTION 4
INTERFACE
4.1
INTRODUCTION
The dotted box in Figure 4-1 illustrates the area of the
HyTerm covered in this section. I ncluded is information on
the connecting cable and plugs, signal names and functions,
input
and
output
level-converter
integrated
circuits,
voltages, timing, and data coding and format. Information
on the
modem must be obtained from the modem
ma nu fa cturer.
Information
concerning
the
USART
(Universal
Synchronous/Asynchronous
Receiver/
Transmitter) and the terminal logic will be found in
Sections 2 and 4 of the Maintenance Manual. The optional
current-loop interface is covered separately at the end of
th is section (see 4.10).
The standard interface between the HyTerm and the
modem
is electrically compatible with the Electronic
I ndustries Association (EIA) Standard RS-232-C and the
I nternational
Telegraph
and
Telephone
Consultative
Committee (C.C.I.T.TJ
Recommendation V.24. Those
portions of the standard/recommendation utilized by the
HyTerm are noted in this section: if further information is
ret.:luired, refer to the related documents listed in Section 1.
4.2
MODEM COMPATIBILITY
The HyTerm can be connected to any Bell type 1 03A or
113A modem, or an equivalent modem from another
manufacturer. Ti me Division Multiplexed (TOM) or Fre-
quency Division MUltiplexed (FDM) equipment may be
used in place of a modem, providing the equipment "looks
I ike" one of the above modems to the HyTerm.
4.3
FUNCTIONAL DESCRIPTION
The main purpose of the interface is to adapt the TTL
logic levels used inside the HyTerm to the 12-volt (nominal)
signals used by the modem so that data can be 'transferred
between the two. Since the EIA standard allows voltage
swings as far as ±25V, the HyTerm receiver circuits must be
capable of handling these levels. The HyTerm provides
signals in the range of 7 to 12 volts to the modem.
In addition to the data signals that are transferred
between the HyTerm and the modem, certain control
signals are required to notify each device that the other is
ready, so there will be no data loss. The input control
signals to the HyTerm are connected in a "fail safe" mode
so that an "off" or "inactive" indication will result if
th~
modem is powered down or if the connecting cable is
unplugged.
I
+12V
TO
-12V
I
OV OR
+5V
I
I
CONTROL
r
- -
--,
I
I
I
INPUT
I
LEVEL
TTL DATA
I
CONVERTER
I
(SERIAL)
MODEM
I
OR •
MUX
I
I
USART
TERMINAL
I
LOGIC
TELEPHONE
I
LINES
OUTPUT
LEVEL
TTL DATA
I
CONVERTER
I
(SERIAL)
I
-I·-J
L
- - -
I
I
CONTROL
I
I
Figure 4-1. Interface Functional Block Diagram
4-1
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