5

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.

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 50

Ω resistors

connecting each line to the ground return shield.

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.

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 100

Ω.

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).

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

TTLH1

TTHL1

LINE

VOLTAGE

AT

RECEIVER

TTHL2

0V

150mV

WIDE PULSE

TTLH2

TTLH2

TTHL1

TTLH2 TTHL2

MINIMUM PULSE WIDTH

LINE

VOLTAGE

AT TRANS-

MITTER

Where: L is Line Length K is

determined by line loss

characteristics

150mV

0V

FIGURE 5. TRANSMISSION LINE WAVE-SHAPING

HS-245RH, HS-246RH, HS-248RH