ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 61

2. Data can be transmitted over long distance (e.g., 4,000 feel using RS-485
communication) with a good speed (e.g., 3.2 Gbps using IEEE-1394b or
FireWire).

Many of the DAQ hardware manufacturers use a serial communication inter-
face layer between the DAS card and the PC. This communication interface may
be one of the protocols like RS-232, RS-485, RS-422, Ethernet, PCI, I2C, 1-wire.
The RS-232 standard, recommended by Electronics Industries Association
(EIA) in 1961, is one of the simple and popular serial communication protocols
used. The terminology reflects the RS-232 as a standard for communication
between a computer terminal and an external device (CRO, modem, etc.) separated
by a distance limit of 130 feet. Currently published by Telecommunications
Industry Association (TIA), it defines the two devices connected with a serial cable
as the data terminal equipment (DTE) and data communication equipment (DCE).
The RS-232 standard defines the parameters, viz. electrical characteristic (i.e.,
signal levels), pin assignments, names and functions of signals, and mechanical
specifications. The original specifications use 25 lines of the communication
channel; however, most of the real-world devices use a reduced 9-line version, as
detailed in Table 3.1 .
The connection schematic for RS-232 standard is shown in Fig. 3.8 , where the
two devices are connected by a serial cable. To implement asynchronous serial
transfer, primary communication is accomplished using three pins, viz. the
transmit data pin, the receive data pin, and the ground pin. The commonly used
version of RS-232 connector is shown in Fig. 3.9 , called 'D' connectors.
Using the asynchronous protocol, each communication device uses its own
internal clock, resulting in bytes that are transferred at arbitrary times. So, instead
of using time as a way to synchronize the bits, a data format (called 'frame') is
used. A generalized data format for RS-232 protocol includes a 'low' start bit, 8
data bits, a 'high' stop bit, and a parity bit (optional) as shown in Fig. 3.10 .
In RS-232 standard, all signals are specified between the communicating
devices, viz. DTE, which receives the data and DCE that initiates communication.
However, the signal names are referenced to DTE. For example, TD is an output to

Table 3.1 Serial port pin and signal assignments

Pin Label Signal name Signal type

1 CD Carrier detect Control

2 RD Receive data Data

3 TD Transmit data Data

4 DTR Data terminal ready Control

5 GND Signal ground Control

6 DSR Dataset ready Control

7 RTS Request to send Control

8 CTS Clear to send Control

9 RI Ring indicator Control