Signal Transmission and Maximum Cable Length

 

RS-232 Transmission

 

In TIA/EIA-232E specification, the recommended maximum cable length is 50 feet, or the total capacitance on the cable shall not be more than 2500pF when transmitting data at its maximum speed (20kbps was specified in the first specification, 1962). Much further distance can be achieved if we lower the transmission rate.*Table below shows the transmission rate and its achievable distance.

 

RS232 cable length according to Texas Instruments

Maximum cable length

Baud rate

Maximum cable length (ft)

Maximum cable length (m)

19200

50

15

9600

500

152

4800

1000

305

2400

3000

914

1200

4000

1200

 

Note *: Experimental results given by Texas Instruments

 

Calculation of transmission distance

 

The total capacitance of a conductor is given by

 

††† C = Cm + Cs

 

where Cm is the mutual capacitance between conductors and Cs is the capacitance to other conductors or shield or earth. Cs = 2*Cm for shielded cable and 0.5Cm for unshielded cable

 

Using popular CAT-5E (1000-BaseT) cable for RS-232/RS-485 data transmission is very common nowadays due to its attractive price point and good performance. According to TIA/EIA-568B specification (specification for 1000-BaseT), it specifies that the maximum cable capacitance shall not be more than 52pF/m (15.8pF/ft). As such, the achievable distance using UTP (unshielded twisted pair) CAT-5E cable for transmitting data at 20kbps is therefore 2500/(52 + 52/2) =32 m, which is almost double the distance shown in the table above. Therefore, when delivering data at 9600bps, which is the default baud rate of TOV-340, on CAT-5E cable, the achievable distance is ~ 300 m. (Note: this is only a theoretical value and has not been verified in our lab)

 

RS-485 Transmission

 

Unlike RS-232, which is a single-ended transmission, RS-485 is a differential transmission that exhibits very good common-mode noise and interference rejection and hence longer distance can be realized. EIA specifies that the maximum distance for RS-485 signal transmission is up to 1.2Km. Table below shows the RS-485 transmission rate versus distance, and its comparison with RS-232.

 

 

Baud rate

RS-485

RS-232

35Mbps

12m

-

100Kbps

1200m

-

20Kbps

-

15m

1Kbps

-

1200m

 

 

 

Composite Video (PAL/NTSC/SECAM) Transmission

 

The signal level of a composite video is specified at 1Vpp. To calculate the transmission distance, we need to know the link budget between the transmitter (e.g. camera) and the receiver (e.g. TOV-340). For example, the minimum signal level at TOV-340ís input is 0.5Vpp. This yields 6dB link budget between camera and TOV-340. The popular RG-59 cable has about 3dB loss per 100 m @ 10MHz (composite video occupies ~ 7MHz bandwidth). Using it for video transmission would give maximum transmission distance of ~200m. Much further distance can be expected if using better grade cable like RG-6 or RG-11.

 

Cable Type

Distance in ft

Distance in m

RG-59

750

230m

RG-6

1000

305m

RG-11

1500

457m

 

 

 

Cable Specifications and Usage

Table: American Wire Gauge, DCR and Current Handling

AWG

Conductor Diameter (mm)

DCR/Km (Ohms)

DCR/m

(Ohms)

Current Handling

Maximum Frequency for 100% Skin Depth

(Hz)

Application Example

Chassis Wiring

(A)

Power Transmission (A)

AWG30

0.26

338.60

0.3386

0.85

0.14

270K

Computer flat cable

AWG28

0.32

212.90

0.2129

1.40

0.23

210K

Computer flat cable

AWG24

0.51

84.22

0.0842

3.50

0.58

68K

CAT5, RS232

AWG22

0.65

52.94

0.0529

7.00

0.92

42K

 

AWG20

0.81

33.31

0.0333

11.00

1.50

27K

 

AWG18

1.02

20.94

0.0209

16.00

2.30

17K

 

AWG16

1.29

13.20

0.0132

22.00

3.70

11K

 

AWG14

1.63

8.28

0.0083

32.00

5.90

6.7K

Domestic power line

 

 

Table: Coaxial Cables for Video Transmission (75 ohms)

Cable Type

Manufacturer

Capacitance (pF/m)

Center Conductor DCR

(Ohms/Km)

 

Center Conductor Size

Cable Diameter

(mm)

Attenuation (dB) per 100m

Transmission Distance

(6 dB @ 10MHz)

1Mhz

10MHz

50MHz

100MHz

400MHz

RG-59

Belden 8241

67.3

160.7

AWG23

6.10

2.0

3.6

7.9

11.2

23.0

167m

RG-59

Belden 8241F

56.8

49.2

AWG22

6.15

1.0

3.0

6.9

9.8

21.7

200m

RG-59

Belden 1426A

53.5

32.8

AWG20

6.15

1.0

2.1

6.2

8.5

16.4

285m

RG-6

Belden 9248

53.1

21.0

AWG18

6.86

1.0

2.3

4.9

6.6

13..1

260m

RG-11

Belden 89292

52.8

8.2

AWG14

10.29

0.6

1.6

3.0

4.3

7.6

375m

Note: General usage of different types of coaxial cables: RG-59 for CCTV, RG-6 for cable television and modem, RG-11 for underground and long-drop transmission. The characteristic impedance is given by Zo = SQRT(L/C)

 

 

Table: Cables for Data Communication

Cable Type

Manufacturer

Mutual

Capacitance Cm (pF/ft)

Mutual

Inductance

(uH/ft)

Characteristic Impedance

(Ohms)

Capacitance between conductor and shield, or earth for UTP

Cs (pF/ft)

Conductor Size

DCR

(Ohms/Km)

 

Attenuation (dB) per 100m / cross talk

1Mhz

10MHz

100MHz

4 cores, shielded

Belden 9534

33

-

-

65

AWG24

82

-

-

-

2 twisted pairs, shielded

Belden 9502

30

0.188

75

50

AWG24

78

-

-

-

4 twisted pairs, UTP CAT-5E

Belden 1583A

15

0.170

100

330 per 100m

AWG24

94

2.0 / 62.3

6.5 / 47.3

22 / 32.3

 

 

 

 

 

Abbreviations

AWG

American Wire Gauge

bps

Bit per second (baud rate)

DCR

DC resistance

UTP

Unshielded twisted pairs

STP

Shielded twisted pairs

 

 

References

http://www.lammertbies.nl/comm/info/RS-232_specs.html#erro

http://www.lammertbies.nl/comm/info/RS-485.html

http://www.allkindscable.com/cocase.html

http://www.powerstream.com/Wire_Size.htm