DS232A
3 of 10
R1
IN
, R2
IN
: Receiver inputs. These inputs accept RS-232 level signals (±25 volts) into a protected 5 kΩ
terminating resistor. Each receiver provides 0.5V hysteresis (typical) for noise immunity.
R1
OUT
, R2
OUT
: Receiver outputs at TTL/CMOS levels.
C1+, C1-, C2+, C2-: Charge pump capacitor inputs. These pins require two external capacitors (0.1 μF
minimum, 10 μF maximum and should be the same size as C3 and C4). Capacitor 1 is connected between
C1+ and C1-. Capacitor 2 is connected between C2+ and C2-. Capacitor C1 can be omitted if +12 volts is
connected directly to V+. Likewise, C2 can be omitted if -12V is connected directly to V-.
DUAL CHARGE PUMP CONVERTERS
The DS232A has a two-stage on-board charge pump circuit that is used to generate ±10 volts from a
single +5-volt supply. In the first stage, capacitor C1 doubles the +5V supply to +10 volts which is then
stored on capacitor C3. The second stage uses capacitor C2 to invert the +10V potential to -10V. This
charge is then stored on capacitor C4. The ±10-volt supplies allow the DS232A to provide the necessary
output levels for RS-232 communication. The DS232A will operate with charge pump capacitors as low
as 0.1 μF. Larger capacitors (up to 10 μF) can be used to reduce supply ripple.
RS-232 DRIVERS
The two RS-232 drivers are powered by the internal ±10-volt supplies generated by the on-board charge
pump. The driver inputs are both TTL and CMOS compatible. Each input has an internal 400 kΩ pullup
resistor so that unused transmitter inputs can be left unconnected. The open circuit output voltage swing
is from (V+ - 0.6) to V- volts. Worst case conditions for EIA-232E/V.28 of ±5-volt driving a 3 kΩ load
and 2500 pF are met at maximum operating temperature and V
CC
equal to 4.5 volts. Typical voltage
swings of ±8 volts occur when loaded with a nominal 5 kΩ RS-232 receiver. As required by EIA-232E
and V.28 specifications, the slew rate at the output is limited to less than 30 volts/μs. Typical slew rates
are 20 volts/μs unloaded and 12 volts/μs with 3 kΩ and 2500 pF load. These slew rates allow for bit rates
of over 250k bits/s. Driver outputs maintain high impedance when power is off.
RS-232 RECEIVERS
The two receivers conform fully to the RS-232E specifications. The input impedance is typically 5
kΩ and can withstand up to ±25 volts with or without V
CC
applied. The input switching thresholds are
within the ±3-volt limit of RS-232E specification with an input threshold low of 0.8 volts and an input
threshold high of 2.4 volts. The receivers have 0.5 volts of hysteresis (typical) to improve noise rejection.
The TTL/CMOS compatible outputs of the receivers will be low whenever the RS-232 input is greater
than 2.4 volts. The receiver output will be high when the input is floating or driven between +0.8 volts
and -25 volts.
