Datasheet

7–182
Solving for R, the equation becomes:
R +
t
0.4 C
T
For multiple transceivers on a bus:
R +
t
0.4 C N
Where:
C = individual component and trace capacitance
N = number of components connected to the bus
Assuming that there are two components connected to the bus, each with a capacitance C = 15 pF, requiring a maximum rise
time of 10 ns/V and t = 15-ns total rise time for the input (2 V), the maximum resistor size can be calculated:
R +
15 ns
0.4 15 pF 2
+ 1.25 k
This pullup resistor method is recommended for ac-powered systems; however, it is not recommended for battery-operated
equipment because power consumption is critical. Instead, use the bus-hold feature that is discussed in the next section. The
overall advantage of using pullup resistors is that they ensure defined levels when the bus is floating and help eliminate some
of the line reflections, because resistors also can act as bus terminations.
Bus-Hold Circuits
The most effective method to provide defined levels for a floating bus is to use Texas Instruments (TI) built-in bus-hold
feature on selected families or as an external component like the SN74ACT1071 and SN74ACT1073 (refer to Table 1).
Table 1. Devices With Bus Hold
DEVICE TYPE BUS HOLD INCORPORATED
SN74ACT1071 10-bit bus hold with clamping diodes
SN74ACT1073 16-bit bus hold with clamping diodes
ABT Widebus+ (32 and 36 bit) All devices
ABT Octals and Widebus Selected devices only
AHC/AHCT Widebus TBA (Selected devices only)
Low Voltage (LVT and ALVC) All devices
LVC Widebus All devices
Bus-hold circuits are used in selected TI families to help solve the floating-input problem and eliminate the need for pullup
and pulldown resistors. Bus-hold circuits consist of two back-to-back inverters with the output fed back to the input through
a resistor (see Figure 9). To understand how the bus-hold circuit operates, assume that an active driver has switched the line
to a high level. This results in no current flowing through the feedback circuit. Now, the driver goes to the high-impedance
state and the bus-hold circuit holds the high level through the feedback resistor. The current requirement of the bus-hold circuit
is determined only by the leakage current of the circuit. The same condition applies when the bus is in the low state and then
goes inactive.
Input
Figure 9. Typical Bus-Hold Circuit
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