Signal Waveform Distortion on Terminatorless Transmission Line of UART-CSMA/CD Control Network

Abstract

Regarding a control network for a building air-conditioning system, signal wave- forms on the transmission lines without terminators were calculated and compared with experi- ments. The calculation predicated the severe distortion in the signal waveform due to line end re∞ections in the cases of improper line connections such as loops or long branches. 1. INTRODUCTION Cost is one of the most important factors in control networks for building facilities such as aircondi- tioning or lighting systems. As the total length of a control network is very long such as a thousand meters, reducing installation cost is strongly demanded. We have proposed a low-cost media access control method UART-CSMA/CD (Carrier Sense Multiple Access with Collision Detection using Universal Asynchronous Receiver Transmitter) which enables to omit terminators for impedance matching on the transmission line (1). However, the afiection of signal re∞ections has been studied only in the case of a point-to-point connection using a lossless transmission line model (2). In this research, we use our new model for waveform calculations assuming lossy transmission line and complex line connections. The calculation results have been verifled by comparing with the experimental results. Our new model predicts the level of the returning re∞ection signal for a variety of terminatorless line connections. In the case of an improper connection such as a loop connection, our calculation and experiment indicate a wide distortion region in the front part of one bit pulse resulting in wrong bit sampling. We conflrmed that our calculation is useful for prior evaluation of actual line connections in the building. 2. UART-CSMA/CD CONTROL NETWORK Figure 1 shows the concept of a control network for air-conditioning facility. In a typical building airconditioning system, there are hundreds of controllers distributed throughout the building. Fig. 2 shows the structure of an UART-CSMA/CD controller connected to the transmission line. The total line length often reaches as long as approximately 1000m. Therefore, not only the costs of each controller but also cost of network installation should be decreased.