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  1. Standards Once apon a time in the 'good old days' there was a standard to ease interfacing of different equipment together. It was called 'RS-232' for simple and effective serial interfacing. The voltage was a given always a -12 to +12 volt swing, a safe voltage with reasonable noise immunity. Then one fine day in order to reduce parts count it was implemented as 0 to +12 volt. Still almost as good and cheaper. As terminals became PC's the parts count was reduced so the readily available 0 to +5 volt was used as a power source. Not quite as good but even less expensive. The lower voltage allowed for a higher communication speed. Sometimes one end of the Com line was one type and the other was a different type. This lead to problems but neither end manufacturer was 'responsible' so it continued on. The Standard called for a 25 pin 'D' sub connector allowing for plenty of future space. This configuration gave full hardware hand shaking, these hardware lines could be tied up or down to reduce the parts count. Tied lines were not as good but are even less expensive. With the reduced pin count it was possible to use a 9 pin 'D' sub connector. This reduced cost and space requirements. Luckily throughout all of this development there has always remained a 'constant' which is one 'transmit', one 'receive' and one 'ground' pin. Within the Standard 25 pin version pin 2 TX, 3 RX, 7 Gnd are used. Of course the Standard 9 pin version is different 2 RX, 3 TX, 5 Gnd. The 6 additional handshaking lines are also moved around. These are the 'Terminal' standards, the other end is a 'Modem' standard with the functions of pin 2 and 3 reversed. Naturally if it is for a PC to PC Com line this will not work and the infamous 'Null Modem' crossed cable is used. PC's are often supplied with male connectors which may require a female - female adapter. Often a male - male adapter will also be necessary and a 25 to 9 pin adapter is very usefull to complete the connection. A break out box is a very handy tool allowing individual pins to crossed as required until the connection works. In real life sometimes one or more of the pins are accidentally left floating this gives all sorts of intermittant trouble. Grounding two electrical machines together via the serial connector sometimes produces a ground loop which plays havoc with the system. To avoid this danger some manufactures don't connect the ground pin to the chassis! The original 'standard' had a cable shielding connection to the connecter shell but inexpensive connectors made of plastic defeated this function. So the 'RS-232' Standard implementation can be 25 or 9 pin, male or female, 12 or 5 volt, send or receive function, handshake or not, grounded or isolated. I should mention 'Standard' speeds, baud rate (bit) 50 and upwards, Mark and Space (1 and 0), Start and Stop bits, 5 to 16 bit per character, ' Standard' character coding, check sum controls, Com protocols, point to point or multi drop. So it may take a while to get a 'Standard' system online. a good read http://www.machine-information-systems.com/RS232_Pinouts.html and also https://en.wikipedia.org/wiki/RS-232 Regards Bob Clarke
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