ADM231L–ADM234L/ADM236L–ADM241L

Rev. C | Page 13 of 20

Transmitter (Driver) Section

The drivers convert TTL/CMOS input levels into 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

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.

3V

0V

3.5V

0.8V

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.