Serial Communication Interface of ER400TRS

The growth of wireless communication technologies since it came into existence in 1901 [Razavi, 1996] prompted the emergence of multitude of new applications and standards [Morgado, 2007].  One of such standards includes the use of Radio Frequency (RF) waves such as that in the ER400 modules which consists of a transmitter, receiver and transceiver. Tracking data from a remote location requires wireless solutions. Cables or infrared signals when used could be tedious and cumbersome. If infrared signals or other optical means including lasers are used, good obstacle-free line of sight or expensive and delicate optical fibres would be required. Thus the solution stays in the RF domain such as the 433MHz frequency channel (the license-free ISM band in Europe). ER400TRS can be interfaced with a microcontroller to transmit data between two remote locations.

The Easy-Radio employs radio waves to communicate over air and use serial communication to connect to a host device (computers or peripherals) which implies that it could act as a RF device as well as a serial device. Serial transmission could be defined as the sequential transmission of characters in a sequence over a single line, rather than simultaneously over two or more lines, as in parallel transmission. This serial transmission of UART (Universal Asynchronous Receiver/Transmitter) depends on the mode, whether in synchronous or asynchronous operation. For synchronous data transmission,  a separate clock signal would be used for synchronising the transmitter and receiver clocks while asynchronous makes use of signal edges (start, data and stop bits) for synchronisation.  The data bits of the ER400 modules are Manchester encoded. Manchester encoded bits are characterized by the presence of a zero crossing in the centre of every bit period. This property permits both low cost timing recovery and very fast start-up from a no signal condition [Ducar, 1981].

For handling the serial communication, a digital component, called the UART is required. Gordon Bell invented the UART when he needed some circuitry to connect a Teletype to a PDP‑1, a task that required converting parallel signals into serial signals and vice versa [Santo, 2009]. Although UART would be needed to send and receive serial data, software alone can do this, a process commonly referred to as bit bang [Ganssle, 1999]. UART can be made to operate in either full duplex or half duplex. Full duplex can be defined by the ability of a UART to simultaneously send and receive data. Half duplex would occur when a device must pause either transmit or receive to perform the other. While most microcontroller UARTs are full duplex, most wireless transceivers such as the ER400TRS are half duplex due to the fact that difficulty could arise when two different signals are sent at the same time under the same frequency causing data collision. VHDL as a design language was used in [Fang, 2011] to achieve the transmitter, receiver and baud rate modules of a UART with stable and reliable results.

Serial communication standards such as the RS232 specification which define the physical layer specify other voltage levels as those provided by UARTs, are commonly implemented as Transistor-Transistor Logic (TTL) or Complementary Metal-Oxide-Semiconductor (CMOS) type digital circuits. In order to convert from the TTL/CMOS levels to the required voltage levels on the medium and provide additional amplification, electrical components called line drivers are used; examples of such driver components are the MAX232 Integrated Circuit [Guoxin, 2010].

References

Razavi, B. (1996), Challenges in Portable RF Transceiver Design, IEEE Circuits and Systems Soceity, pp 12 – 25, ISSN: 8755-3996.

Morgado, A. Rivas, V. J. Rio, R. Castro-Lopez, R. Fernandez, F. V. Rosa, J. M. (2007), Behavioural modelling, simulation and synthesis of multi-standard wireless receivers in MATLAB/SIMULINK, Integration, the VLSI journal, Volume 41, pp 269 – 280, ISSN: 0167-9260.

Ducar, R. (1981), Tevatron Serial Data Repeater System, IEEE Transactions on Nuclear Science, Volume 28, Issue 3, pp 2301 - 2302, ISSN: 0018-9499

Santo, B. (2009), 25 microchips that shook the world, IEEE Spectrum, Volume 46 Issue 5, May 2009, pp 34 - 43 ISSN: 0018-9235.

Ganssle, (1999), Bit Banging, Available: http://www.ganssle.com/articles/auart.htm.

Fang, Y. Chen, X. (2011), Design and Simulation of UART Serial Communication Module Based on VHDL, International Workshop on Intelligent Systems and Applications, pp 1 - 4, ISBN: 978-1-4244-9855-0

Guoxin, L. (2010), Wireless transmission of RS232 interface signal based on ZigBee, International Conference on Anti-Counterfeiting Security and Identification in Communication (ASID), July 2010, pp 239 - 241, ISBN: 978-1-4244-6731-0