ADM205–ADM211/ADM213
Transmitter (Driver) Section
Enable Input
The drivers convert TTL/CMOS input levels into EIA-232-E
output levels. With VCC = +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 ADM205, ADM209, ADM211, and ADM213 feature an
enable input used to enable or disable the receiver outputs. The
enable input is active low on the ADM205, ADM209, ADM211
and active-high on the ADM213. Refer to Tables II and III.
When disabled, all receiver outputs are placed in a high imped-
ance state. This function allows the outputs to be connected di-
rectly 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
25.
The input threshold levels are both TTL and CMOS compat-
ible with the switching threshold set at VCC/4. With a nominal
V
CC = 5 V the switching threshold is 1.25 V typical. Unused in-
puts may be left unconnected, as an internal 400 kΩ pull-up re-
sistor pulls them high forcing the outputs into a low state.
3V
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 limit-
ing capacitor and the output impedance in the power-off state is
greater than 300 Ω.
EN
*
0V
T
T
DIS
EN
Receiver Section
The receivers are inverting level shifters which acceptEIA-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 also protected against overvoltages of up to
±30 V. The guaranteed switching thresholds are 0.8 V mini-
mum and 2.4 V maximum which are well within the ±3 V
EIA-232-E requirement. The low level threshold is deliberately
positive as it ensures that an unconnected input will be inter-
preted as a low level.
3.5V
0.8V
V
V
– 0.1V
+ 0.1V
OH
R
OUT
VOL
* POLARITY OF EN IS REVERSED FOR ADM213
Figure 25. Enable Timing
APPLICATION HINTS
Driving Long Cables
In accordance with the EIA-232-E standard, long cables are
permissible provided that the total load capacitance does not ex-
ceed 2500 pF. For longer cables which do exceed this, then it is
possible to trade off baud rate vs. cable length. Large load ca-
pacitances cause a reduction in slew rate, and hence the maximum
transmission baud rate is decreased. The ADM205–ADM211 and
ADM213 are designed so that the slew rate reduction with in-
creasing load capacitance is minimized.
The receivers have Schmitt trigger inputs with a hysteresis level
of 0.5 V. This ensures error-free reception for both noisy inputs
and for inputs with slow transition times.
Shutdown (SD)
The ADM205, ADM206, ADM211 and ADM213 feature a
control input which may 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 except
R4 and R5 on the ADM213 are put into a high-impedance dis-
abled state. Receivers R4 and R5 on the ADM213 remain en-
abled during shutdown. This feature allows monitoring external
activity such as ring indicator monitoring while the device is in a
low power shutdown mode. The shutdown control input is ac-
tive high on all parts except the ADM213 where it is active low.
Refer to Tables II and III.
For the receivers, it is important that a high level of noise immu-
nity be inbuilt so that slow rise and fall times do not cause mul-
tiple output transitions as the signal passes slowly through the
transition region. The ADM205–ADM211 and ADM213 have
0.5 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 ADM205–ADM211 and ADM213 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 120-kB/s under worst-case loading
conditions.
–10–
REV. 0
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