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

48Practical Industrial Data Networks: Design, Installation and TroubleshootingProblemGarbled or lost dataFirst characters garbledNo data communicationsASCII data has incorrectspacingProbable cause of problemBaud rates of connecting ports may be differentConnecting cables could be defectiveData formats may be inconsistent (Stop bit/parity/number of data bits)Flow control may be inadequateHigh error rate due to electrical interferenceBuffer size of receiver inadequateThe receiving port may not be able to respond quicklyenough. Precede the first few characters with theASCII (DEL) code to ensure frame synchronizationPower for both devices may not be onTransmit and receive lines of cabling may be incorrectHandshaking lines of cabling may be incorrectlyconnectedBaud rates mismatchData format may be inconsistentEarth loop may have formed for EIA-232 lineExtremely high error rate due to electrical interferencefor transmitter and receiverProtocols may be inconsistent/intermittentcommunicationsIntermittent interference on cableMismatch between ‘LF’ and ‘CR’ characters generatedby transmitting device and expected by receivingdeviceTable 3.4A list of typical EIA-232 problemsbefore and the other if the system has been operating and for some reason does notcommunicate at present. New systems that have never worked have more potentialproblems than a system that has been working before and now has stopped. If a system isnew it can have three main problems viz. mechanical, setup or noise. A previouslyworking system usually has only one problem, viz. mechanical. This assumes that no onehas changed the setup and noise has not been introduced into the system. In all systems,whether having previously worked or not, it is best to check the mechanical parts first.This is done by:• Verifying that there is power to the equipment• Verifying that the connectors are not loose• Verifying that the wires are correctly connected• Checking that a part, board or module has not visibly failed