MC26LS30

http://onsemi.com

11

SYSTEM EXAMPLES

(Pin numbers refer to SO−16 package only.)

Differential System

An example of a typical EIA−422−A system is shown in

Figure 17. Although EIA−422−A does not specifically

address multiple driver situations, the MC26LS30 can be

used in this manner since the outputs can be put into a high

impedance mode. It is, however, the system designer’s

responsibility to ensure the Enable pins are properly

controlled so as to prevent two drivers on the same cable from

being “on” at the same time.

The limit on the number of receivers and drivers which

may be connected on one system is determined by the input

current of each receiver, the maximum leakage current of

each “off” driver, and the DC current through each

terminating resistor. The sum of these currents must not

exceed the capability of the “on” driver (

≈60 mA). If the

cable is of any significant length, with receivers at various

points along its length, the common mode voltage may vary

along its length, and this parameter must be considered when

calculating the maximum driver current.

The cable requirements are defined not only by the AC

characteristics and the data rate, but also by the DC resistance.

The maximum resistance must be such that the minimum

voltage across any receiver inputs is never less than 200 mV.

The ground terminals of each driver and receiver in Figure

17 must be connected together by a dedicated wire (or the

shield) in the cable to provide a common reference. Chassis

grounds or power line grounds should not be relied on for

this common connection as they may generate significant

common mode differences. Additionally, they usually do

not provide a sufficiently low impedance at the frequencies

of interest.

Single−Ended System

An example of a typical EIA−423−A system is shown in

Figure 18. Multiple drivers on a single data line are not

possible since the drivers cannot be put into a high

impedance mode. Although each driver is shown connected

to a single receiver, multiple receivers can be driven from a

single driver as long as the total load current of the receivers

and the terminating resistor does not exceed the capability

of the driver (

≈60 mA). If the cable is of any significant

length, with receivers at various points along its length, the

common mode voltage may vary along its length, and this

parameter must be considered when calculating the

maximum driver current.

The cable requirements are defined not only by the AC

characteristics and the data rate, but also by the DC

resistance. The maximum resistance must be such that the

minimum voltage across any receiver inputs is never less

than 200 mV.

The ground terminals of each driver and receiver in

Figure 18 must be connected together by a dedicated wire

(or the shield) in the cable so as to provide a common

reference. Chassis grounds or power line grounds should not

be relied on for this common connection as they may

generate

significant

common

mode

differences.

Additionally, they usually do not provide a sufficiently low

impedance at the frequencies of interest.

Additional Modes of Operation

If compliance with EIA−422−A or EIA−423−A Standard

is not required in a particular application, the MC26LS30

can be operated in two other modes.

1) The device may be operated in the differential mode

ground and −5.25 V. Outputs in the low state will be

greater than that shown in Figure 6. The Enable pins will

operate the same as previously described.

2) The device may be operated in the single−ended mode

ground and −5.25 V. Outputs in the high state will be at a

voltage as shown in Figure 10, while outputs in a low state

Termination Resistors

Transmission line theory states that, in order to preserve

the shape and integrity of a waveform traveling along a

cable, the cable must be terminated in an impedance equal

to its characteristic impedance. In a system such as that

depicted in Figure 17, in which data can travel in both

directions, both physical ends of the cable must be

terminated. Stubs leading to each receiver and driver should

be as short as possible.

In a system such as that depicted in Figure 18, in which

data normally travels in one direction only, a terminator is

theoretically required only at the receiving end of the cable.

However, if the cable is in a location where noise spikes of

several volts can be induced onto it, then a terminator

(preferably a series resistor) should be placed at the driver

end to prevent damage to the driver.

Leaving off the terminations will generally result in

reflections which can have amplitudes of several volts above

overshoots/undershoots can disrupt the driver and/or

receiver, create false data, and in some cases, damage

components on the bus.