HD26LS31

Rev.2.00, Jul.16.2004, page 8 of 11

When termination resistance RT is connected between the two transmission lines, as shown in figure 7 the current path

situation is that current IOH on the side outputting a high level (in this case, the Y output) flows to the side outputting a

low level (in this case, the Z output) via RT, with the result that the low level rise is large.

If termination resistance RT is dropped to GND on both transmit lines, as shown in figure 5 the current path situation is

that the current that flows into the side outputting a low level (in this case, the Z output) is only the input bias current

from the receiver. As this input bias current is small compared with the signal current, it has almost no effect on the

differential input signal at the receiver end.

Figure 6 shows the output voltage characteristics when termination resistance RT is varied.

Also, when used in a party line system, etc., the low level rises further due to the receiver input bias current, so that it is

probably advisable to drop the termination resistance to GND.

However, the fact that it is possible to make the value of RT equal to the characteristic impedance of the transmission

line offers the advantage of being able to hold the power dissipation on the side outputting a high level to a lower level

than in the above case.

Consequently, the appropriate use must be decided according to the actual operating conditions (transmission line

characteristics, transmission distance, whether a party line is used, etc.).

Figure 8 shows the output voltage characteristics when termination resistance R

5.0

4.0

3.0

2.0

1.0

0

–20

–40

–60

–80

–100

V

CC

= 5.25

V

V

CC

= 4.75

V

Ta = 25°C

Figure 3 I

0.5

0.4

0.3

0.2

0.1

0

1020304050

Ta = 25°C

Figure 4 I