HD26LS32P datasheet(10/15 Pages) RENESAS

10 / 15 page

HD26LS32

Rev.2.00, Jul.16.2004, page 10 of 14

This is because, since the termination resistance is normally matched to the transmission line characteristic impedance,

the value falls to several tens of hundreds of ohms, and the differential input pins are shorted by this termination

resistance. That is, the differential input voltage V

ID comes within the range

ID: –0.2 V < VIA – VIB < 0.2 V

and the output becomes undetermined.

To prevent this, resistance R

1 is inserted in series with the transmission line as shown in figure 6, minimizing the effect

of the termination resistance. Resistance R

2 is added to increase the current flowing between the termination resistance

and R

1, enabling the value of R1 to be kept small.

Inserting resistances R

1 and R2 in this way provides for the differential input voltage VID to become 200 mV or higher,

but the following points must be noted.

• Smallest possible R

1 value

If this value is large, the receiver input sensitivity will fall.

• Largest possible R

2 value

If this value is small, the load on the driver will be large.

Figure 7 shows experimental differential input voltages for variations in R

1 and R2.

"H"

Undetermined

Figure 5 Examples of Transmission Line Disconnection

Receiver

VCC

Driver

Figure 6 Method of Enhancing Fail-Safe Function

0.6

0.5

0.4

0.3

0.2

0.1

R1 (k

Ω)

100

kΩ

300

kΩ

R2 = ∞

VCC = 5 V

Ta = 25°C

VCC

VID

100

Ω

Figure 7 R

1, R2 vs. Differential Input Voltage

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