e005056.pdf

COM PUTERS

Serial transceiver

Electrically isolated RS232

Design by A. Müller

This transceiver electrically isolates the TxD and RxD lines from the PC,

provides an isolated power supply and also looks after the bi-directional

conversion between TTL levels and symmetric RS232 levels.

Frequently, a

microcontroller

system should or

must have an electrically

isolated connection to the RS232 inter-

face of the PC. This can be necessary when

the target system works at a completely dif-

ferent voltage level, or when earth loops must

be avoided.

The circuit described here can isolate

potentials up to 2500 Veff or 3000 Vdc,

depending on the type of optocoupler used.

It uses only relatively easily obtained compo-

nents, and it does not include any homemade

inductors. Power for the circuit is provided by

a mains adapter. A Maxim MAX232 interface

driver is incorporated into the circuit to con-

vert signals to the PC from TTL to RS232 lev-

els (±12 V), and signals to the microcontroller

system from RS232 to TTL levels. All other

lines of the RS232 interface that would be

needed to support handshaking are not sup-

ported, but they can be added on without

great difficulty.

Separated

signals

A simple glance at the schematic

diagram in Figure 1 shows how the

electrical isolation is achieved. Two

optocouplers are used, along with a

DC/DC converter for the power sup-

ply. A fourth component, or better

said an isolating measure, can be

clearly seen in the circuit board lay-

out. A 5-mm wide copper-free strip

is located between the two voltage

regimes and underneath both opto-

couplers and the voltage converter.

This prevents leakage currents as

well as possible arcing with high

potential differences.

The Toshiba 6N137 optocoupler

used here (which are also available

from other suppliers) consists of an

infrared transmitter diode and a

photo-IC, which is made up of a

receiver diode, a Schmitt trigger and

a NAND gate with an open-collector

output (see Figure 2 ). The externally

available gate input (Enable) is per-

manently connected to +5 V in this

circuit, so that the optocoupler is

always enabled.

Elektor Electronics

5/2000

COM PUTERS

5V'

1µ

IC1d

470 Ω

IC4

63V

5V'

V+

C1+

IC1a

6 N13 7

IC3

1µ

63V

C1–

T1IN

T2IN

T1OUT

T2OUT

5V'

R1OUT

R2OUT

R1IN

R2IN

C11

C2+

IC2a

MAX232

100n

1µ

63V

C2–

V-

IC5

470 Ω

IC2d

1µ

63V

6N 13 7

IC1b

IC1c

IC2b

IC2c

IC1, IC2 = 74HC00

5V'

IC7

1N4002

7 805

>9V

C13

C12

IC6

NMV

C10

IC1

IC2

10µ

63V

100n

10µ

63V

100n

0505SA

10µ

63V

100n

000060 - 11

Figure 1. Two optocouplers and an integrated DC/DC converter provide the electrical isolation.

Both optocouplers are wired in

nearly the same manner. The output

of the microcontroller (TxD) is termi-

nated by resistor R1 and amplified

by two parallel HC NAND gates,

which provide the drive current to

the LED (limited by R2). Resistor R4

is the collector pull-up resistor. In the

other direction (RxD), the picture is

nearly the same, with the only

exception being that the termination

resistor is omitted. The gates and

optocouplers allow a data transfer

rate of more than 1 MHz.

The MAX232CPE interface driver

(IC3) limits the bandwidth to

115 kbit/s. If a higher level of input

protection against electrostatic dis-

charges is necessary, you should use

the MAX232ECPE instead. It guar-

antees protection up to ±15 kV.

voltage regulator (IC7, a 7805), in

combination with the reservoir and

decoupling capacitors C12 and C13,

provides a stable operating voltage.

Diode D2 provides reverse-polarity

protection. The primary operating

voltage is ﬁltered by C6 and used to

supply IC1, while the output side of

IC5 and the cathodes of the IR

diodes are powered by IC4.

The primary and secondary sup-

ply voltages are isolated by an inte-

grated DC/DC voltage regulator,

which is a Newport NMV0505SA

(available from Farnell, among oth-

ers). This component is a member of

a family of 100-kHz switch-mode DC

voltage converters that can be used

to convert a DC input voltage of 5 V,

12 V 24 V or 48 V to a DC output volt-

age of 5 V, 9 V, 12 V or 15 V (suffix A),

or ±5V, ±9V, ±12 V or ±15 V (no

suffix). The input voltage is indicated

by the first two digits of the part

number, while the output voltage is

indicated by the second pair. All con-

verters are available in DIL (suffix D) or SIL

(suffix S) packages. The output power capac-

ity is 1 watt per output for all models. The DC

isolation voltage is 3 kV for all models. The

conversion efficiency is typically 70 to 80% at

full load.

I F

6N137

I CC

V CC

I F

V O

I E

GND

DC/DC converter

V E

000060 - 12

Power is provided by a mains

adapter (>9 V at 250 mA). A +5-V

Figure 2. The internals of a 6N137 optocoupler.

5/2000

Elektor Electronics

COM PUTERS

verter. The two electrolytic buffer

capacitors C6 and C9 support the

voltage converter. Each IC also has

an additional 100-nF local decou-

pling capacitor. LED D1 shows that

the serial transceiver is operational.

COMPONENTS LIST

OUT1

Resistors:

R1 = 100k

Ω

R2,R5 = 470 Ω

R3,R4 = 1k

IC1

IC7

Ω

Construction,

testing and more

The printed circuit board layout

shown in Figure 3 is designed for a

9-pin sub-D connector (K1). A sim-

ple, standard RS232 cable can be

used to connect K1 to the COM port

of a PC. On the microcontroller side,

we have done without this or any

other sort of connector, in favour of

simple solder terminals. This makes

for flexibility as far as the connec-

tions to the microcontroller and the

power source are concerned. If the

transceiver is to be powered by a

mains adapter, ﬁtting a low-voltage

connector to the enclosure is a good

idea.

When inserting the components

into the board, you should use only

sockets that explicitly have an isola-

tion voltage of 3 kV (1 TΩ isolation

resistance) for at least IC4, IC5 and

IC6. You cannot automatically

assume that inexpensive sockets

will meet this requirement. Pay

attention to the component

silkscreen markings and don’t forget

the small wire bridge next to C9.

After visually inspecting the fin-

ished board, you can perform a func-

tional test. For this test, connect the

two TTL pins of the transceiver to

each other and then connect the

transceiver via an RS232 cable to

either COM12 or COM2 of a PC. Start

the HyperTerminal program under

Windows 95/98, disable local echo

and handshaking, and check

whether arbitrarily entered text is

displayed in the receive window.

If you do not want to do without

RTS/CTS handshaking, you can nat-

urally add another pair of optocou-

plers to the circuit. There are suffi-

cient unused NAND gates and two

unused channels in the MAX232. Of

course, this will require a completely

new circuit board layout, which

must retain the generous earth

planes and the 5-mm wide separa-

tion between the two sides as found

on the original board. The hand-

shake pins of the RS232 jack (pin 7:

R6 = 680 Ω

IC5

IC4

Capacitors:

C1...C4 = 1 µ F 63V radial

C5,C7,C8,C10,C11,C12 = 100nF

ceramic

C6,C9,C13 = 10

IC2

F 63V radial

IC3

Semiconductors:

D1 = LED, high efficiency

D2 = 1N4002

IC1,IC2 = 74HC00

IC3 = MAX232

IC4,IC5 = 6N137

IC6 = NMV0505SA (Newport,

Farnell)

IC7 = 7805

Miscellaneous:

K1 = 9-way sub-D socket, PCB

mount, angled pins

RTS, pin 8: CTS) must not be con-

nected to each other. The current

consumption on the secondary side

must not exceed 200 mA.

Finally, there is one (superﬂuous?)

tip: you naturally cannot connect the

transceiver to any microcontroller

system that already includes a

RS232 converter (such as the

MAX232). The TTL logic at the input

and output can only handle TTL sig-

nals (0/5 V)!

(000060-1)

Figure 3. The electronics on the two sides is

separated by a 5-mm wide copper-free strip.

We thus have the following brief speciﬁca-

tions for the model NMV0505SA converter

used here:

input voltage: +5 V

output voltage: +5 V

output current: 200 mA

package:

SIL

On the secondary side we find IC2, the out-

put side of IC4, the cathode of the transmit IR

diode of IC5 and the MAX232 interface con-

Elektor Electronics

5/2000

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