Real RS232 For Laptop/Notebook PCs

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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 laptop 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 accommodated on a small double-sided PCB
that can be fitted a 9-way sub-D adaptor housing. The converter receives
its +5V supply voltage from the PS/2/mouse interface (pin 4 = +5V, pin 3
= ground). Current consumption is a modest 4mA or so which has no
noticeable negative effect on battery life. The two interface signals
TxD and RTS are taken from the laptop 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 electrolytics in this circuit,
since an SMD circuit, once built, is difficult to troubleshoot and repair.

C1-C4 = 10µF 16V (SMD)

IC1 = MAX232-CSE (SMD)

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