Practical Industrial Data Networks:Design, Installation and ...

110 Practical Industrial Data Networks: Design, Installation and Troubleshooting7.4.3 Detailed troubleshooting7.4.3.1 Mis-wired communication cablingEIA-232 wiring for 3-wire point-to-point modeThere are various wiring scenarios such as point-to-point, multi-drop two-wire, multidrop4-wire, etc. In a point-to-point configuration, the physical standard used is usuallyEIA-232 on a 25-pin D connector, which means a minimum of three wires, are used.They are: on the DTE (master) side, transmit (TxD – pin 2), receive (RxD – pin 3) andsignal ground (common – pin 7). On the DCE (slave) side they are; receive (RxD – pin2), transmit (TxD – pin 3) and signal ground (common – pin 7).The other pins were primarily used for handshaking between the two devices for dataflow control. Nowadays, these pins are rarely used as the flow control is controlled viasoftware handshaking protocols. Details of these protocols can be obtained from thePractical Industrial Data Networks: Design, Installation and Troubleshooting bookreleased by IDC.With the advent of VLSI technology, the footprint of the various devices have atendency to shrink; even the physical connections of communications have been nowstandardized on 9-pin D connectors. The pin assignment that has been adopted is the IBMstandard in which the pin configurations are as follows. On the DTE (master) side,transmit (TxD – pin 3), receive (RxD – pin 2) and signal ground (common – pin 5). Onthe DCE (slave) side receive (RxD – pin 3), transmit (TxD – pin 2) and signal ground(common – pin 5).It follows that the cabling between the two devices must be straight through pin-for-pin.In this manner, the transmit pin of the master is directly connected to the receive pin ofthe slave and vice versa. Such cable is referred to as a straight-through cable. Thesecables are standard off-the-shelf products available in standard pre-determined lengths.Alternatively they can be fabricated to custom lengths, with ease.Master devices usually are the present-day IBM compatible computers, with a Modbusapplication installed on it, and therefore have the standard IBM EIA-232 port provided.The slave devices usually have a user selectable option to have either an EIA-232 or EIA-485 port for communication. Unfortunately some manufacturers, in order to force thecustomers to return to them time-and-again, employ a strategy of modifying thesestandards to their own advantage.Illustrated below are a couple of these combinations:Case IStandard RS-232 PortNon-StandardRS-232 CableNon-StandardRS-232 PortCase IIStandard RS-232 PortNon-StandardRS-232 to RS-485Converter & CableNon-StandardRS-232 PortFigure 7.13Typical customizations in EIA-232 cabling