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THE REPRESENTATION OF INDIVIDUAL BITS

To represent individual bits the transmitter simply raises or lowers the voltage on the data wire between the transmitter and the receiver. A binary '1' is called a mark and is represented by any voltage in the range of -3 to -15 volts. A binary '0' is called a space and is represented by any voltage in the range +3 to +15 volts. Any voltage in the six volts between +3 and -3 is considered invalid. In the presence of a high degree of environmental noise, a transmitter that sends bits using a large voltage is more immune to noise.

For example, if noise could cause a 3 volt swing in the signal on the cable then a transmitter that sent mark and space as -5 and +5 respectively could be disrupted (pulled into the 6 volt "no mans land" between +3 and -3) by the noise. The noise could pull the mark signal to anywhere between -2 to -8 volts, depending on whether the 3 volt noise was of positive or negative voltage. This transmitter would be better designed to output a +/- 15 volt signal; it would be immune to the noise. It would also require a more powerful set of components in the output circuitry (called the line drivers) and a larger power supply. It's up to the design engineer to make these types of decisions when the circuit is manufactured.

The voltages sent are referenced to a common ground, or 0 volt point. This requires that a ground wire be connected between the transmitter and receiver, to provide the common 0 volt reference point. This means that both transmitter and receiver must have an earth ground (to derive "zero volts" as the reference for the common ground wire). If the actual value of the earth ground is different on the two sides then a problem exists. Consider the situation in the picture below.

Mismatched Ground 
Example

On the left, the transmitter has pulled the voltage differential between the transmit wire and ground to a value of -10 volts. At this point the value of the earth ground point is zero volts. At the other end of the connection (perhaps in the printer room, down the hall) the value of the earth ground (because of the way the building wiring is installed) is +15 volts. It's not uncommon for the common voltage to vary by ten's of volts between two points in an electrical wiring system. If this situation existed then the -10 volt potential between the transmit wire and the ground that was impressed at the left side of this circuit is perceived as -25 volts on the right hand side. This is because the ground potential is +15 volts on the right. Each RS-232 entity treats its ground wire as "zero volts" and measures all signal voltages relative to that voltage. On the left, the difference between ground and signal is -10 volts, but on the right, it is -25 volts. What was transmitted as a very strong (-10 volt) mark is received as an invalid voltage. An error has just occurred in the transmission of the data. This problem is referred to as a Common Mode Voltage problem. The "mode" is "earth ground", the "common"ality is the ground wire between the two stations. The problem is the variance between the common mode voltage on the transmit side versus the common mode voltage on the receive side.

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General Overview ] RS-232 Standard ] [ Bit Encoding ] Character Encoding ] Data Errors ] Physical Circuit ] DTE and DCE ] DB-25 Connection ] Control Signals ] Break-Out Box ] Directional Signals ] Connecting DTE's ] Async & Sync ] Other Signals ] Custom IC's ] RS-423 ] RS-422 ] Modems ]

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