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Automating Manufacturing Systems - Process Control and ...

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plc serial - 27.15<br />

part waiting<br />

box full<br />

feed part<br />

part waiting<br />

ST10:0 = "pickup"<br />

ONS<br />

Bit B3:0<br />

AWT<br />

Channel 0<br />

String ST10:0<br />

Length 6<br />

Figure 27.13<br />

A Box Loading System<br />

27.5 SUMMARY<br />

• Serial communications pass data one bit at a time.<br />

• RS-232 communications use voltage levels for short distances. A variety of communications<br />

cables <strong>and</strong> settings were discussed.<br />

• ASCII functions are available of PLCs making serial communications possible.<br />

27.6 PRACTICE PROBLEMS<br />

1. Describe what the bits would be when an A (ASCII 65) is transmitted in an RS-232 interface<br />

with 8 data bits, even parity <strong>and</strong> 1 stop bit.<br />

2. Divide the string in ST10:0 by the string in ST10:1 <strong>and</strong> store the results in ST10:2. Check for a<br />

divide by zero error.<br />

ST10:0 “100”<br />

ST10:1 “10”<br />

ST10:2<br />

3. How long would it take to transmit an ASCII file over a serial line with 8 data bits, no parity, 1<br />

stop bit? What if the data were 7 bits long?<br />

4. Write a number guessing program that will allow a user to enter a number on a terminal that<br />

transmits it to a PLC where it is compared to a value in N7:0. If the guess is above "Hi" will be<br />

returned. If below "Lo" will be returned. When it matches "ON" will be returned.<br />

www.PA<strong>Control</strong>.com

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