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SUMMER CIRCUITS COLLECTION

041

Electrically Isolated RS232 Adapter

data flow from an external device to the microcontroller,

the solution is easy. Since the RxD input of the microcon-

troller works with a +5-V level, all that we need is an opto-

coupler (IC2) whose LED is directly driven by the TxD out-

put of the external device via resistor R2. This resistor also

limits the current through D4 when TxD is inactive and thus

has a negative level (usually around –9 V). During data

transmission, the level of the pulses changes to around

+9 V. The collector of IC2 is connected directly to the RxD

input of the microcontroller. Resistor R5 is needed if micro-

controller RxD input does not have an internal pull-up resis-

tor. In any case, the microcontroller side of IC2 thus works

with TTL levels.

If we now want to send data from the microcontroller to the

external device, the microcontroller level (+5 V) must be

converted into an RS232 level of at least +6 to +7 V. To

achieve this, the negative voltage present at the RS232 TxD

output is tapped off by D1 and buffered by C1, which acts as

a storage capacitor. IC1 is a CMOS 40106 IC containing six

inverting Schmitt triggers.

One of these Schmitt triggers (IC1a) is wired with C2 and R1

as an oscillator. It generates a frequency of around 1.5 kHz.

This signal is fed to the other ﬁve Schmitt triggers, which are

connected in parallel and act as a driver. They provide the

necessary output current. When the output is Low, C3

charges to the supply voltage level via D2 (less the voltage

drop across the diode). When the output changes to High,

the voltage on C3 rises and adds to the voltage already pre-

sent; D2 blocks and C4 is charged via D3 to nearly twice the

A. Schiefen

This circuit represents an interface con-

verter between the UART pins of a micro-

controller (with TTL levels) and a ‘stan-

dardised’ RS232 port with symmetric

IC1.B

3

4

1

IC1.C

IC1 = 40106

R1

10k

15-

V levels. In contrast to the commonly

used IC solutions such as the MAX232, it

also provides electrical isolation between

the two sides of the converter. This inter-

face converter inverts the signals, so the

usual inverters on the microcontroller

side can (and must!) be omitted.

In most cases, the data lines RxD and

TxD are all we need for communications

with microcontroller systems. Fortu-

nately, handshake signals are very sel-

dom exchanged. The related RS232

leads are thus interconnected in such a

manner that communications can take

place without any problems.

All that is needed for the electrical isola-

tion of a signal is an optocoupler. If the

5

6

1

IC1.A

IC1.D

C3

D3

+5V

R4

470 Ω

1

2

9

8

1

1

IC3

8

BAT85

4µ7

16V

IC1.E

6

2

11

10

1

C4

C2

D2

IC1.F

100n

4µ7

25V

13

12

BAT85

1

5

7

6N136

3

TxD

K1

R3

D1

1

6

R5

BAT85

2

RxD

7

RxD

R2

2k2

3

TxD

IC2

8

2

8

6

4

C1

7

D4

9

C5

IC1

14

5

100n

100µ

16V

1N4148

GND

3

7

5

Sub-D

GND

6N136

000082 - 11

76

Elektor Electronics

7-8/2001

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SUMMER CIRCUITS COLLECTION

supply voltage. A voltage of around +9 V is thus available

from C4, which is connected to the collector of optocoupler

IC3. In principle, the voltage level at an RS232 interface

should be +12 V to +15 V, but PCs can generally work with

signiﬁcantly lower voltages. Notebook computers in partic-

ular sometimes have a voltage of only +8 V.

The emitter of IC3 leads to the RxD input of the RS232 port

and is held at around –9 V by R3. When the microcontroller

transmits data, the pulses from the TxD output of the micro-

controller arrive at the LED of optocoupler IC3. The tran-

sistor of IC3 switched on and applies the positive voltage

to the RxD input of the external device.

Normal diodes (1N4148) can also be used in place of the

Schottky diodes, although the generated voltages will be

somewhat lower. The 6N136 optocoupler is a high-speed

type; normal optocouplers are not suitable. The circuit can

theoretically transmit data at up to 57,600 baud, but in prac-

tice microcontroller circuits only use 9600 baud. These data

transmission rates have been successfully used with both

older-model and more recent notebook computers.

(000082-1)

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