Figure 2-8. External coder/decoder interface phasing.

TM 11-5820-803-12

exception of the LOS input, are provided at the site

See paragraph 2-20c for further details of switch

interface connector, J5, on the rear panel of the modem.

settings.

The LOS input is a BNC type connector also located on

(1) The Internal clock output is derived from

the rear panel.

Further information on interface

an internal reference clock generator. This internally

connections is given in paragraph 2-19.

generated clock, which is equal in rate to the setting of

the INPUT DATA RATE switches ±0.005%, is available

2-8.

Direct Digital Interface

to the digital user as an optional clock source.

(fig. 2-9)

(2) The standard data output is that data

When the PSK modem interfaces with a digital user

which is received and decoded by the PSK modem

located near the modem, a direct digital interface via

receiver.

balanced line drivers and receivers is employed. The

(3) The standard clock output is a clock

interface signals are listed in a and b below.

signal, which is synchronized with the standard data

output, and is internally regenerated by the PSK modem

a. Input Signals.

receiver.

(1) The standard data input accepts data from

(4) The alternate data and clock outputs are

the digital user for modulation, coding, and transmission

identical to the standard data and clock outputs.

over the communications link. The PSK modem is

The input and output characteristics of the data and

capable of synchronizing an internal clock to the data

clock signals are illustrated in figures 2-10 and 2-11. All

input for use in the coder and transmitter operation.

the input and output impedances are 75 ohms ±10%.

(2) Use of the standard clock input is optional.

The following factors must be considered in planning the

The PSK modem may be configured to use this input

direct digital user interfaces:

clock to control modulation, coding, and transmission of

(a) The use of 75-ohm balanced

the standard data input signal. An internal switch in the

transmission

cable,

such

RG-108A/U

PSK modem provides the capability of inverting the clock

recommended (although not necessary for short runs).

signal. See paragraph 2-20b for further details on setting

(b) All direct digital outputs should be

of this switch.

terminated with 75-ohm loads. Unused outputs should

b. Output Signals. All direct digital outputs may be

be terminated at the PSK modem rear panel connector.

inverted by internal switches in the PSK modem.

driven is a function of the data rate, the phase distortion

of the cable used, and the loss of the cable type used.

2-10