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Because they are produced in such large quantities, Cat 5 cables are relatively inexpensive, often less than half the price of specialty RS-485 cabling. The recorder offers a fast sampling rate within 200msec for all channels and high accuracy with up to 18 universal input channels to connect to a variety of sensors commonly used in lab environments including DC voltage output from specialty sensors and thermocouples/RTDs to log temperatures with extreme precision. Able to handle multiple inputs, the datalogger provides useful information for experiments involving the heating of various liquids at a variety of volumes. In these distributed processing networks, a variety of machines and instruments work locally, but communicate and share data or resources with one another globally using a single serial link. The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels. We have been using Microcontroller Development Boards like Arduino, Raspberry Pi, NodeMCU, ESP8266, MSP430, etc. for a long time now in our small projects where most of the times distance between the sensors and board is not more than few centimeters at max and at these distances, the communication between the different sensor modules, relays, actuators, and controllers can be easily done over simple jumper wires without us being worried about the signal distortion in the medium and the Electrical noises creeping into it.

All communication cabling to be installed in a daisy chain configuration, no T’s are permitted. In some countries, inserting two cables into the same screw terminal is permitted. Splicing of communications cable is not permitted. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled DGND) on the communications connector that are connected to the controller’s ground plane. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. In this case it is possible to connect the main inlet and outlet terminal directly to the terminals of an instrument without creating a branch. The end resistance must be used only at the ends of the main cable. Normally, this connection is made at one end of the main cable. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. This distance does not include the branches (which must nevertheless be short). The main cable must be no longer than 700 m!

If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable. Maximum length is 4000 ft. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. The maximum number of devices that can be connected to a main cable is 32, including the Master. A Modbus RS485 connects a Master device to one or more Slave devices. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. It can drive things 1200 meters, which RS - 232 is about 15 meters maximum. The WattNode® BACnet and Modbus® meters communicate using RS-485 (TIA/EIA-485-A) half-duplex, differential signaling at 9600, 19200, 38400, and 76,800 baud.

The capacitance is close to 12.5 pF/foot but there is a small impedance mismatch (100 ohms for Cat 5, 120 ohms for RS-485). 100 ohms, Cat 5 cables almost meets the electrical requirements for RS-485 cables. A 2-wire synchronous IIC (Inter-IC) bus provides multi-drop signaling at rates up to 100 Kbaud. Plus a lot of times the data rates are so slow on these bar code readers because they're only sending a few kilobits. Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data. The branches must be no longer than 1200 m! All the ‘A’ terminals must be connected together and all the ‘B’ terminals must be connected together respectively. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel.

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