ADM231L–ADM234L/ADM236L–ADM241L
Rev. C | Page 13 of 20
Transmitter (Driver) Section
The drivers convert TTL/CMOS input levels into EIA-232-E
output levels. With V
CC
= +5 V and driving a typical EIA-232-E
load, the output voltage swing is ±9 V. Even under worst-case
conditions, the drivers are guaranteed to meet the ±5 V
EIA-232-E minimum requirement.
The input threshold levels are both TTL- and CMOS-compatible
with the switching threshold set at V
CC
/4. With a nominal V
CC
=
5 V, the switching threshold is 1.25 V typical. Unused inputs can
be left unconnected because an internal 400 kΩ pull-up resistor
pulls them high, forcing the outputs into a low state.
As required by the EIA-232-E standard, the slew rate is limited to
less than 30 V/µs without the need for an external slew-limiting
capacitor, and the output impedance in the power-off state is
greater than 300 Ω.
Receiver Section
The receivers are inverting level shifters that accept EIA-232-E
input levels (±5 V to ±15 V) and translate them into 5 V TTL/
CMOS levels. The inputs have internal 5 kΩ pull-down resistors
to ground and are protected against overvoltages of up to ±30 V.
The guaranteed switching thresholds are 0.8 V minimum and
2.4 V maximum, which are well within the ±3 V EIA-232-E
requirement. The low level threshold is deliberately positive
because it ensures that an unconnected input is interpreted as
a low level.
The receivers have Schmitt trigger inputs with a hysteresis level
of 0.65 V. This ensures error-free reception for both noisy inputs
and inputs with slow transition times.
Shutdown (SD)
The ADM236L and ADM241L feature a control input that can
be used to disable the part and reduce the power consumption
to less than 5 µW. This is very useful in battery-operated systems.
During shutdown, the charge pump is turned off, the transmitters
are disabled, and all receivers are put into a high impedance,
disabled state. The shutdown control input is active high on all
parts (see Table 5).
Enable Input
ADM239L and ADM241L feature an enable input used to enable
or disable the receiver outputs. The enable input is active low on
the ADM239L and ADM241L (see Table 5). When the receivers
are disabled, their outputs are placed in a high impedance state.
This function allows the outputs to be connected directly to a
microprocessor data bus. It can also be used to allow receivers
from different devices to share a common data line. The timing
diagram for the enable function is shown in Figure 31.
T
EN
T
DIS
3V
0V
R
OUT
3.5V
0.8V
V
OH
– 0.1V
V
OL
+ 0.1V
00070-0-036
EN
Figure 31. Enable Timing
APPLICATION HINTS
Driving Long Cables
In accordance with the EIA-232-E standard, long cables are per-
missible, provided that the total load capacitance does not exceed
2500 pF. For longer cables that do exceed this, it is possible to trade
off baud rate for cable length. Large load capacitances cause a
reduction in slew rate; therefore, the maximum transmission baud
rate is decreased. The ADM236L to ADM241L are designed to
minimize the slew rate reduction that occurs as load capaci-
tance increases.
For the receivers, it is important that a high level of noise
immunity be built in so that slow rise and fall times do not
cause multiple output transitions as the signal passes slowly
through the transition region. The ADM236L to ADM241L
have 0.65 V of hysteresis to guard against this. This ensures that
even in noisy environments error-free reception can be
achieved.
High Baud Rate Operation
The ADM236L to ADM241L feature high slew rates, permitting
data transmission at rates well in excess of the EIA-232-E
specification. The drivers maintain ±5 V signal levels at data
rates up to 100 kbps under worst-case loading conditions.
