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
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)
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 |
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-340’s 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 |
|
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 |
|
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 |
|
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 |
