Real Time

Real Time 
A time which always increments without stopping or resetting.

HDLC ( High level Data Link Control)

HDLC ( High level Data Link Control) 
High level Data Link Control for synchronous communication between primary (master) and secondary (slave) as per standard defined. It is a bit-oriented protocol. 

Protocol

Protocol
A way of transmitting messages on a network by using a software for adding the additional bits like staring bits, headers, addresses of source and destination, error control its and ending bits. Each layer or sub layer uses its protocol before a message transmits on a network. 

RS232 Port

RS232 Port
A standard for UART transmission and reception in which TxD and RxD are at different voltage levels and handshaking signals, CTS,RTS, DTR, DCD and RT are at the TTL levels. 

COM Port

COM Port
A port at the computer where a mouse, modem or serial printer connects.

RxD

RxD
A line used for reception of UART serial bits. The 0 and 1 signals are at levels similar that for TxD line. 

TxD

TxD
A line used for transmission of UART serial bits. The 0 and 1 signals are at RS232C voltage levels when RS232C COM port is used, or at the TTL  levels in micro controllers.