ECG Acquisition in a Computer - ECG Acquisition and Automated Remote Processing - page 80

Synchronization achieved at end of D 7 Discarded

bits

Accepted bits

D0 D1 D2 D3 D4 D5 D6 D7 D0 D1 D2 D3 D4 D5 D6 D7

Start
bit

Stop
bit

Start
bit

Stop
bit

Header

Fig. 4.6 Principle of byte recovery from decoded frame

'checksum error' byte is generated by the MCU from each decoded packet of the
received data by sequentially adding all the 35 bytes (header, packet number, data
bytes, and checksum byte). For an error free communication, this 'checksum error'
byte should be zero for each received packet. The checksum byte (i.e., the
last byte) of each received packet is replaced by the corresponding 'checksum
error' byte while being delivered to the PC, keeping other bytes intact. So, each
received packet is custom-made to form a 'modified packet'. In the PC, an esti-
mation of communication error can be performed by checking the last byte (which
is 'checksum error' byte) of each packet. The algorithms steps for generating the
checksum error byte from received packets from the point of achieving synchro-
nization in the HES MCU is given in end of Chapter Appendix 3 .
The HES firmware incorporates an intelligence to recover from an occasional
link failure between the transceiver units. In such situations, the input line of HES
MCU becomes temporarily 'locked', without any transition, at a level (high or
low) for certain period which is much greater than LOW bit width. To counteract
this, a timer element is used as 'watchdog,' which reinitializes the MCU after a
preset threshold count of timer states. After this internal reset, the HES MCU again
looks for synchronization. So, after the link is resumed, synchronization is again
established when the next available 'header' is found. In case of long duration link
failure, the MCU is reset many times by this process, but the synchronization is not
permanently lost. However, some data are lost during the period of snapped link.
Finally, the HES module transmits each modified packet using RS-232 protocol to
the desktop PC for serial acquisition and automatic storage in text file.

4.3.3 ECG Transmission Using Standard Telephone

Public telephone systems utilize amplitude modulation techniques for the com-
munication of voice data for circuit switch messaging. Plain old telephone service
[originally post office telephone service/system (POTS)] designed for voice-grade
telephone service (in a band of 300-3,400 Hz). It is up graded to touch-tone
dialing electronics phone exchange and fiber optic communication into PSTN. The
rooms within a building and buildings within a complex now-a-days are also
networked with the concept of PSTN. Electronic private automatic branch
exchange (EPABX) acts as local exchange from where all these rooms are con-
nected (mostly through wires) to constitute the 'intercom' and users of intercom
can exchange their words through their private exchange. One or more trunk line