8

®

ISO485

1 we can see that the specified attenuation figures given

agree with those obtained by measurement; approximately

–1.3db/100ft, at 30Mbit/s (15MHz).

pattern is clear. In practice a precision decade resistance box

As the data rate is increased we can see from Figure 3 how

the signal distortion also increases. From the graph in Figure

TYPICAL ELECTRICAL CHARACTERISTICS

Capacitance

ALPHA WIRE CORP.

CDR. to CDR.

CDR. to (CDR. AND SHIELD)

VP.

NO.

pF/ft

(pF/m)

pF/ft

pF/m)

%

Ω

6072C

8.7

(28, 5)

5.9

(52, 2)

80

150

6073C thru 6079/27C

12.5

(41, 0)

22.0

(72, 5)

80

150

TYPICAL ATTENUATION VALUES

AT 20°C

10.0

5.0

1.0

0.5

1.0

5.0

10.0

50.0

Frequency (MHz)

6073 - 6079/27

6072

FIGURE 1. Cable Characteristics.

PARAMETER

EIA-232

RS-432-A

RS-422-A

RS-485

Mode of Operation

Single-Ended

Single-Ended

Differential

Differential

Number of Drivers and Receivers

1 Driver

1 Driver

1 Driver

32 Drivers

1 Receiver

10 Receivers

10 Receivers

32 Receivers

Maximum Cable Length (m)

15

1200

1200

1200

Maximum Data Rate (bps)

20k

100k

10M

10M

Maximum Common-Mode Voltage (V)

±25

±6

6 to –0.25

12 to –7

Driver Output

Loaded

±5

±3.6

±2

±1.5

Levels (V)

Unloaded

±15

±6

±5

±5

Driver Load (

Ω)

3k to 7k

450 (min)

100 (min)

60 (min)

Driver Slew Rate

30V/

µs (max)

External Control

NA

NA

Driver Output Short Circuit

150 to GND

150 to GND

150 to GND

Current Limit (mA)

250 to –7 or 12V

Driver Output

NA

NA

NA

12k

Resistance

Power on

High Z state (

Ω)

Power off

300

60k

60k

12k

Receiver Input Resistance (

Ω)

3 to 7

4

4

12

Receiver Sensitivity

±3V

±200mV

±200mV

±200mV

TABLE I. Summary of EIA Interface Standards.