HS-245RH, HS-246RH, HS-248RH
Transmitter Operation
The transmitter alternately applies the current to each of the
two conductors in the twisted pair line such that the total
current in the twisted pair is constant and always in the same
direction. This current flows through either of the two 50V
terminating resistors at the receiver and returns to the
transmitter as a steady DC current on the transmission line
shield. The DC power supply return for the transmitter is
through the receiver terminating resistors (the transmitter
ground pin is only a substrate ground). Therefore, it is
essential that the shield be connected to the power supply
common at both the transmitter and receiver, preferably at
the integrated circuit “ground” pin. More than fifteen twisted
pair lines can share the same shield without crosstalk.
The transmission line used with the Intersil HS-245RH series
transmitter and receivers can be any ordinary shielded,
twisted pair line with a characteristic impedance of 100Ω.
Twisted pair lines consisting of number 20 or 22 gauge wire
will generally have this characteristic impedance. Special
high quality transmission lines are not necessary and
standard audio, shielded-twisted pair, cable is generally
suitable.
Since the necessary characteristics for various twisted pair
lines are not readily available, it may be necessary to take
some measurements on a length of the proposed line. To do
this, connect an HS-245RH transmitter to one end of the line
(100 feet or more) and an HS-246RH to the other end. The
rise and fall-times can be measured on the line at both ends
and the constant ‘‘K’’, for that line can be computed as
shown in Figure 5 so that the minimum pulse width can be
determined for any length of line.
Data rates of 2MHz have been obtained using 1,000 feet of
standard shielded, twisted pair, audio cable. Data rates of
15MHz are possible on shorter lengths of transmission line
(50 feet).
Receivers
The HS-248RH “party-line” receiver presents a high
impedance load to the transmission line allowing as many as
ten HS-248RH receivers to be distributed along a line without
excessive loading. Figure 3 shows a typical system of a
transmitter, a terminating receiver and a party-line receiver.
The transmission line is terminated in its characteristics
impedance by an HS-246RH or by a pair of 50Ω resistors
connecting each line to the ground return shield.
Electromagnetic Interference
Very little electromagnetic interference is generated by the
Intersil current mode system because the total current
through the twisted pair is constant, while the current
through the shield is also constant and in the opposite
direction. This can be verified by observing, with a current
probe, the total current through the twisted pair, through the
shield and through the complete shielded, twisted pair cable.
In each case a constant current will be observed with only
small variations. Small pulses may be observed if the
complementary inputs to the transmitter do not switch at the
same time. The current will decrease during the time both
inputs are high, and will increase during the time both inputs
are low. These switching pulses may be observed when
using the circuit shown in Figure 4. The amplitude and shape
of these pulses will depend of the propagation delay of G1,
and transition times G2 and G3. These pulses are generally
of no concern because of their small amplitude and width,
but they may be reduced by increasing the similarity of the
waveforms and timing synchronization of the complementary
signals applied to the transmitter.
In addition to generating very little noise, the system is also
highly immune to outside noise since it is difficult to
capacitively couple a differential signal into the low
impedance twisted pair cable and it is even more difficult in
induce a differential current into the line due to the very high
impedance of the constant current transmitter. Therefore,
differential mode interference is generally not a problem with
the Intersil current mode system. Large common mode
voltages can also be tolerated because the output current of
the transmitter is constant as long as the receiver
termination ground is less than 2V positive with respect to
Transmission Lines
The maximum frequency (or minimum pulse width) which
can be carried by a certain length of a given transmission
line is dependent on the loss characteristics of the particular
line. At low frequencies, there will be virtually no loss in
pulse amplitude, but there will be a degradation of rise and
fall-time which is roughly proportional to the square of the
line length. This is shown in Figure 5. If the pulse width is
less than the rise-time at the receiver end, the pulse
amplitude will be diminished, approaching the point where it
cannot be detected by the receiver.
150mV
LINE
VOLTAGE
AT TRANS-
MITTER
TTLH1
150mV
LINE
VOLTAGE
AT
RECEIVER
TTLH2
TTHL2
TTHL1
0V
TTLH2
TTHL1
0V
TTLH2 TTHL2
MINIMUM PULSE WIDTH
WIDE PULSE
TRLH2 = TTLH1 KL
2
TTHL2 = TTHL1 KL
2
Where: L is Line Length K is
determined by line loss
characteristics
FIGURE 5. TRANSMISSION LINE WAVE-SHAPING
5
INTERSIL [ INTERSIL CORPORATION ]
