I N T E R FA C E U SES AND A P P L I C AT I O N S

I N T E R FA C E U SES AND A P P L I C AT I O N S

Communications standards are used extensively in applications with a host PLC or with a computer in a network where one or more interfaces are used. Sometimes a PLC with an RS-232C or RS-422 communication interface must communicate with an RS-485 device. In this case, an RS-232C–to– RS-485 converter (or an RS-422–to–RS-485 converter) can provide this communication (see Figure 8-49). These converters provide electrical isola- tion, in addition to longer distance. Figure 8-50a illustrates one of B&R Industrial Automation’s interface converters, which can be used for com- municating between two PLCs (see Figure 8-50b) over a long distance (maximum of 500 m, or 16,500 ft). Each PLC starts its interfacing via RS- 232 (or RS-422) and transfers to RS-485 to achieve the required distance.

RS-232C

RS-232–to–RS-485

RS-422–to–RS-485

Figure 8-49. (a) RS-232C–to–RS-485 and (b) RS-422–to–RS-485 converters.

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S ECTION Components Special Function I/O and C HAPTER 2 and Systems

Serial Communication Interfacing 8

2 3 Installation/Grounding (DIN rail mount)

1–DIN rail (grounded)

2–RS-232 cable 3–Cable holder 4–Cable shield 5–Grounding clamp 6–Voltage supply cable 7–Cable holder

8–RS-485 cable

(e.g., twisted pair) 9–Grounding the 6 7 8 negative supply

(a)

RS-232C or RS-422

RS-232C or RS-422

Interface

Interface

RS-232C or RS-422

RS-485 to

to RS-485

RS-232C or RS-422

(16,500 ft) 5000 m Max

(b)

Figure 8-50. (a) B&R Industrial Automation’s interface converter and (b) an example of the convertor communicating between two PLCs.

Figure 8-51 shows the relationship between transmission distance and data rate for the RS-485 interface converter. This diagram is based on a cable with an impedance of 110 Ω , a capacitance of 41 picofarads/m, and a cable

ohmic resistance of 0.094 Ω /m. The converter is capable of driving a signal at rates of 115.2K baud at a distance of 1500 m (5000 ft). It is also capable of operating at a distance of 5000 m (16,500 ft) at a rate of 9.6K baud.

Figure 8-52 shows another application of serial communication. In this example, an isolated link coupler (1747-AIC) interface connects several Allen-Bradley SLC-500 PLC processors to a DH-485 network (RS-485– based). This link coupler provides a connection for each of the SLC-500

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S ECTION Components Special Function I/O and C HAPTER 2 and Systems

Serial Communication Interfacing 8

Baud Rate (Kbits/sec) 38.4 30

Distance (m)

Figure 8-51. Baud rates for transmission distances in a RS-485 converter.

CPUs in the DH-485 network. The DH-485 network also interfaces with a personal computer through an RS-232–to–DH-485 communication inter- face. The maximum length of the main trunk of the DH-485 network is 4000 feet at a rate of 19.2K baud. This type of subnetwork is very useful for remote programming and data acquisition links of up to 32 devices.

RS-232/DH-485 Interface Connector

DH-485 network SLC 500

SLC 500

SLC 500

1747-AIC Isolated Link Coupler

Figure 8-52. PLC processors connected to an isolated link coupler interface.

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S ECTION Components Special Function I/O and C HAPTER 2 and Systems

Serial Communication Interfacing 8

K EY ASCII I/O interfaces T ERMS BASIC module

cold junction compensation direct action I/O interface distributed I/O processing encoder/counter module fast-input interface fast-response interface fuzzy logic interface intelligent I/O interface lead resistance compensation network interface module proportional-integral-derivative (PID) interface resistance temperature detector (RTD) interface serial communication servo motor interface stepper motor interface thermocouple input module weight input module wire input fault module

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