HD26LS32

Rev.2.00, Jul.16.2004, page 8 of 14

HD26LS32 Line Receiver Applications

The HD26LS32 is a line receiver that meets the EIA RS-422A and RS-423A conditions. It has a high in-phase input

voltage range, both positive and negative, enabling highly reliable transmission to be performed even in noisy

environments.

Its main features are listed below.

• Operates on a single 5 V power supply.

• Three-state output

• On-chip fail-safe circuit

• ±7 V in-phase input voltage range

• ±200 mV input sensitivity

• Minimum 6 kΩ input resistance

A block diagram is shown in figure 1. The enable function is common to all four drivers, and either active-high or

active-low input can be selected.

When exchange is carried out using a party line system, it is better to keep the receiver input bias current constituting

the driver load small, as this allows more receivers to be connected.

Consequently, whereas an input resistance of 4 k

Ω or above is stipulated in RS-422A and RS-423A, the HD26LS32 has

been designed to allow a greater margin, with a minimum resistance of 6 k

Ω.

Figure 2 shows the input current characteristics of the HD26LS32.

The shaded areas in the graph indicate the input current allowable range stipulated in RS-422A and RS-423A.

HD26LS32 output is LS-TTL compatible and has a three-state function, enabling the output to be placed in the high-

impedance state, and so making the device suitable for bus line type applications.

With an in-phase input voltage range of ±7 V and a ±200 mV input sensitivity, the HD26LS32 can withstand use in

noisy environments.

Also, since signals sent over a long-distance transmission line require a long transition time, it also takes a long time to

cross the receiver’s input threshold level.

Therefore, the input is provided with hysteresis of around 30 mV to prevent receiver output misoperation due to noise.

An example of input hysteresis is shown in figure 3.

The fail-safe function consists of resistances R connecting input A to V

This circuit provides for the receiver input section to be pulled up or down by a high resistance that prevents it from

becoming a driver load so that the output goes high in the event of a transmission line breakage or connector

detachment.

When the input pin is placed in the open state by the pull-up/pull-down resistance, the differential input voltage V

as follows:

V

and the output is fixed high.

However, if the receiver-side termination resistance remains connected despite a line breakage or connector detachment,

the output will be undetermined (figure 5).