2070-6A Manual
9
2070-6Asm v2 Standard Manual A00886-A.doc
12 milliseconds after the RTS signal is
active the Modem to generates a
CTS
signal (an Output) which tells the Master
or local controller it can start to send
data. 12 ms is required because the
receiver Carrier Detect circuit requires 8
ms of Mark frequency before it will turn
the receiver on. If you transmit data
before the 8 ms, the receiver may chop
off the first portion of the message or not
ever turn the receiver on.
If you have a system that is broadcasting
continuously and you connect a Modem
on to the line, you may find that the
modem never receives any data. This is
because the receiver circuit never sees 8
ms of continuous MARK frequency.
When RTS drops, 10 ms of Soft Carrier
will be automatically transmitted. The
Soft Carrier Frequency is detected by the
Carrier Detect circuit and shuts down the
receiver. This helps prevent excess
energy in the system from causing extra
bits being generated in the receiver
section.
It must be remembered that the
transmitter section of the Modem is still
sending out Soft Carrier 10 ms after the
last data bit is sent. This is especially
important in a 2-wire half duplex system.
HOW FAR WILL THE SIGNAL GO?
This is an important question in higher
speed FSK Modem operation. Typically,
with a 0 dB launch signal you can expect
about 5-6 miles on 22 AWG wire. With
19 AWG wire, can go about 25% further.
If more range is required, turning the
transmitter output amplitude
potentiometer clockwise can increase the
transmitter output. These POTS are
labeled AMP A for channel “A” and
AMP B for channel “B”. An extra 10
dBm of output power is available. This
will give you another 1.5 to 2 miles. You
may have more crosstalk problems in
some cable systems with the increase
output.
Additional distance can also be obtained
by changing the modem input impedance
on the middle units of a system. The
input impedance of the Modem can be
changed from 600 Ohms to 4.75K with a
jumper located near the transformers for
each channel. The jumper is labeled 600
and 4K75. Move the jumper to the 4K75
position to increase the input impedance.
The Modems on the ends of the line
should be left to 600 Ohms. This will
significantly reduce the loading effects of
each addition modem on a string.
MAINTENANCE AND
ADJUSTMENTS
The 2070-6B will not normally need any
preventative maintenance. The Output
amplitude and frequency of the
transmitter may need to be adjusted or
checked if the error rate is becoming
excessive.
ADJUSTMENTS
There are 3 potentiometers for each
section of the 2070-6A modem for
making adjustments. The potentiometers
are all located on the topside of the PC
Board. The adjustments are as follows:
1)
Frequency adjustment. The
potentiometer adjusts the Mark and
Space frequency. The potentiometer
adjusts both the Mark and Space
frequency with the POT adjusted so
