mc68194 ON Semiconductor, mc68194 Datasheet - Page 4

• Manufacturer Part Number: mc68194
• Description: Carrier Band Modem Cbm
• Manufacturer: ON Semiconductor

1.3 MESSAGE (FRAME) FORMAT
one−at−a−time, the MAC or TBC is responsible for
combining the above defined MAC symbols into messages
(more correctly called frames). For the purposes of the
CBM, a simplified frame format can be used consisting of:
SILENCE || PAD−IDLE | START DELIMITER | DATA | END
DELIMITER || SILENCE
where:
Silence
Pad−Idle Pairs
Zero
One
Non−Data
Although the CBM only uses MAC symbols
Figure 1−4. Phase−Coherent Modulation Scheme
ND1
ND2
L
H
Mac−Symbol
L
H
ONE
ZERO
Figure 1−3. MAC Symbol Encoding
1 BIT TIME =
H
1/ BIT RATE
{off off}
{L L} {H H}
{H H}
{L L}
{H L}
{L H}
ND1
L
Encoding
L
1 BIT TIME
ND2
H
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In a multi−frame transmission, Pad−Idle is sent between
consecutive frames to separate them. If an End Delimiter
occurs within a multi−frame transmission its I Bit will = 1,
and the last end delimiter will have its I Bit = 0.
TBC and modulates the phase−coherent transmit signal
accordingly.
phase−coherent signal stream from the cable, decodes the
MAC symbols, and reports them. On transmission there is
a direct one−to−one correlation between MAC symbols
requested and the modulated signal; however, during
reception exceptions can occur. The CBM is allowed to
report Silence or the actual Zero/One pattern during
preamble which is done to allow the receiver to “train” to the
incoming signal. Also, if noise in the system has corrupted
the data, it may show up as an incorrect MAC symbol or the
CBM can report a BAD SIGNAL symbol if an incorrect
combination of ND symbols is detected (ND2 without an
ND1, ND2 followed by ND2, etc.)
1.4 SYSTEM CONFIGURATION
required for an IEEE 802.4 carrierband 5 Mbps or 10 Mbps
data rate phase−coherent FSK physical layer. The CBM
communicates with the MAC or TBC through a TTL
compatible serial interface that is consistent with the IEEE
PAD−IDLE
START
DELIMITER
DATA
END
DELIMITER
A single transmission can consist of one or more frames.
The CBM accepts a stream of MAC symbols from the
Figure 1−5 illustrates the CBM and peripheral circuitry
=
=
=
=
Conversely,
alternating {LL} {HH} pairs which must
occur in octets or groups of eight sym-
bols. Pad−idle provides a training signal
for the receiver and occurs at the begin-
ning of every transmission (and between
frames in a multiple frame transmission).
a unique pattern of eight symbols (one oc-
tet) that marks the beginning of a frame.
The pattern is:
ND1 ND2 0 ND1 ND2 0 0 0
where ND1 is the first symbol trans-
mitted.
octets of ZERO/ONE patterns that are the
actual data or “information” contained
within the frame.
a unique pattern of symbols that marks
the end of a frame. The pattern is:
ND1 ND2 1 ND1 ND2 1 {I=0/1} {0/1}
where ND1 is the first symbol trans-
mitted. Note that unlike the Start Delimit-
er, the last two bits of the End Delimiter
octet are not always the same. The
seventh bit of the octet is called the I Bit
or Intermediate bit which = 1 when there
is more to transmit and = 0 at the end of
a transmission.
the
CBM
receives
a