3

Design Information

Voltage Mode Transmission

Data rates of up to 10 million bits per second can be

obtained with standard TTL logic; however, the transmission

distance must be very short. For example, a typical 50 foot

low capacitance cable will have a capacitance of

approximately 750pF which requires a current of greater

than 50mA to drive 5V into this cable at 10MHz; therefore,

voltage mode transmitters are undesirable for long

transmission lines at high data rates due to the large current

required to charge the transmission line capacitance.

Current Mode Transmission

An alternate method of driving high data rates down long

transmission lines is to use a current mode transmitter.

Current mode logic changes the current in a low impedance

transmission line and requires very little change in voltage.

For example, a 2mA change in transmitter current will

produce a 100mV change in receiver voltage independent of

the series transmission line resistance. The rise time at the

receiver for a typical 50 foot cable (750pF) is approximately

30ns for a 2mA pulse.

An emitter coupled logic gate is frequently used for a current

mode transmitter. However, ECL gates are not compatible

with TTL and DTL logic and they require considerable

power. The Intersil HS-245RH is a TTL/DTL compatible

current mode transmitter designed for high data rates on

long transmission lines. Data rates of 15 megabits per

second can be obtained with 50 feet of transmission line

when using the companion HS-246RH receiver. Data rates

of 2 megabits per second are easily obtained on

transmission lines as long as 1,000 feet. The Intersil

transmitter and receivers feature very low power, typically

25mW for the transmitter and 15mW for the receiver.

Intersil Transmitter/Receivers

The Intersil transmitter/receiver family consists of a triple line

transmitter, two triple line receivers with internal terminations

and a triple party-line receiver. The general characteristics of

the transmitter and receivers are outlined in Table A.

FIGURE 3. TYPICAL APPLICATION

50

(NOTE)

-5V

-5V

(+)

+5V

IN

ENABLE

(+)

(-)

“PARTY-LINE”

OUTPUT

1/3

HS-248RH

RECEIVER

OUT

+5V

1/3 HS-246RH

1/3

HS-245RH

+5V

50

Ω

(-)

Ω

RECEIVER

NOTE: HS-245RH should be driven by open-collector

gates. (Totem-pole output may cause slight reduction in

“on” data current). For more detailed information, refer to

Design Information section of this data sheet.

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