e02ch31.pdf

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SMALL CIRCUITS COLLECTION
031
Real RS232 for Laptop PCs
F. Schröter
pin 3 = ground). Current consumption is a modest 4 mA or so
which has no noticeable negative effect on battery life.
The two interface signals TxD and RTS are taken from the lap-
top to the MAX232 driver inputs, pins 8 and 13. Inside the
MAX232, they are first shaped to proper TTL/CMOS levels and
then applied to the actual level converters. The resultant signal
then reaches +10 V, which is accepted without problems by
the interface inside the Metex DVM.
The PCB designed for the converter is small and single-sided.
SMDs are the only option when it comes to fitting it all inside
the adaptor housing. Do observe the polarity of the elec-
trolytics in this circuit, since an SMD circuit, once built, is dif-
ficult to troubleshoot and repair. Unfortunately, the PCB for this
project is not available ready-made.
Many Metex DVMs feature a serial interface (RS232) which
enables measured values to be copied to a PC for processing.
Although this works just fine with most desktop PCs, problems
may arise — as the author found out the hard way — when a lap-
top PC like the IBM Thinkpad 370C is hooked up to the Metex
DVM. The cause of the problems is the limited voltage swing of
just ± 5V on the 370C’s serial interface. This is simply not enough
for the Metex DVM, which will appear ‘deaf’ to the laptop.
The voltage swing on the serial interface lines is easily increased
with the aid of a dedicated interface driver like the MAX232.
With the resultant circuit designed in SMD, it is easily accom-
modated on a small double-sided PCB that can be fitted a 9-
way sub-D adaptor housing. The converter receives its +5 V
supply voltage from the PS/2/mouse interface (pin 4 = +5 V,
(014099-1)
COMPONENTS LIST
Miscellaneous:
K1 = 9-way sub-D socket
(female), PCB mount,
angled pins
K2 = 9-way sub-D plug
(male), PCB mount, angled
pins
2 solder pins
K1
K2
1
DCD
DSR
RxD
RTS
TxD
CTS
DTR
RI
GND
DCD
DSR
RxD
RTS
TxD
CTS
DTR
RI
GND
1
6
6
Capacitors:
C1-C4 = 10
2
2
µ
F 16V (SMD)
7
7
3
3
Semiconductors:
IC1 = MAX232-CSE (SMD)
8
8
4
4
9
9
5
5
SUB D9
SUB D9
C4
10 µ
16V
2
V+
1
+5V
C1+
16
C3
IC1
3
10
µ
16V
C1–
9
8
R1OUT
R1IN
11
12
10
14
T1IN
T1OUT
13
R2OUT
R2IN
+
7
T2IN
T2OUT
4
C2+
C2
0V
MAX232
15
5
10
µ
16V
C2–
V-
6
C1
(C) ELEKTOR
014099-1
014099-12
10 µ
16V
014099 - 11
66
Elektor Electronics
12/2002
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