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Automating Manufacturing Systems - Process Control and ...

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plc serial - 27.3<br />

50 ft<br />

In<br />

RS-232c<br />

Txd<br />

com<br />

Rxd<br />

Out<br />

3000 ft<br />

RS-422a<br />

In<br />

Out<br />

3000 ft<br />

RS-423a<br />

In<br />

Out<br />

Figure 27.2<br />

Serial Data St<strong>and</strong>ards<br />

The RS-422a cable uses a 20 mA current loop instead of voltage levels. This<br />

makes the systems more immune to electrical noise, so the cable can be up to 3000 feet<br />

long. The RS-423a st<strong>and</strong>ard uses a differential voltage level across two lines, also making<br />

the system more immune to electrical noise, thus allowing longer cables. To provide serial<br />

communication in two directions these circuits must be connected in both directions.<br />

To transmit data, the sequence of bits follows a pattern, like that shown in Figure<br />

27.3. The transmission starts at the left h<strong>and</strong> side. Each bit will be true or false for a fixed<br />

period of time, determined by the transmission speed.

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