ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 62

Fig. 3.8 Block diagram of the RS-232 connection system

Fig. 3.9 9 pin RS-232

sockets

DTE and input to DCE. In most of the transfer operations, DTE is a personal
computer and DCE may be either modem, embedded system, or another computer.
RS-232 standard used positive and negative voltage levels, rather than positive
voltage levels described by TTL or CMOS logic. Logic '1' (or high) is represented
by -12 V, while logic '0' (or low) by +12 V. These levels are chosen to avoid the
noise levels and attenuation of signals at the end of a long communication link. At
the receiver, a voltage greater than +3 V is regarded as a logic '0' (low) and -3V
as logic '1' (high), an undefined between these two.
For communication between RS-232 and real-world devices (which operate on
TTL logic or CMOS logic), a level converter IC is used in the transceiver circuits.
Most common ICs are MAX232 and MAX233 from Maxim Corporation. Further
details of serial communications can be obtained in [ 26 ].

3.5.2 Serial Communication Using 8051 Microcontroller-
Based DAS Card

The detail schematic of the hardware test setup for developed ECG acquisition
system is given in Fig. 3.11 . The ECG signal is generated by an arbitrary wave-
form generator (AWG) and model Agilent A33220A in conjunction with Agilent
Intuilink Waveform editor, fed with PhysioNet data [ 27 ].