Serial Cable Maximum Length

Posted By admin On 26/11/21
  1. Rs232 Serial Cable Maximum Length
  2. Rs485 Serial Cable Max Length
  3. Rs232 Serial Cable Max Length
  4. Serial Cable Maximum Length Chart

At resolutions up to 2560x1600, your cable run length is limited to 5 metres. At present, there are no reliable booster technologies which can be leveraged for DVI-D Dual Link at higher-than-HD resolutions. However this is a conservative figure accounting for high serial speeds over high capacitance cabling, for example using a baud rate of 9600 (a common console speed) over typical Cat 5 cabling, the maximum length increases to 100 – 150 meters (328 – 500 feet). Maximum cable lengths Cable length is one of the most discussed items in RS232 world. The standard has a clear answer, the maximum cable length is 50 feet, or the cable length equal to a capacitance of 2500 pF. The latter rule is often forgotten. If you do all these thing, you can transmit 1 Mbit/s data on CAT5 cable for long distances, like 1000 meters. As you do less of these things, the usable distance will go down. If it's just board to board in the same chassis, for example, then you probably don't need to do much special. IIRC, the serial communication uses pins 1,7,8 at 19,200 bits/second (baud); slow enough that it should be generally forgiving of the usual issues that might come with long cable runs. My ISY994 is 30 feet from my PLM, extended using standard Cat5e off a bulk spool, via patch panels.

Serial cables are typically used for RS-232 communication.

A serial cable is a cable used to transfer information between two devices using a serial communication protocol. The form of connectors depends on the particular serial port used. A cable wired for connecting two DTEs directly is known as a null modem cable.

Maximum cable lengths[edit]

The maximum working length of a cable varies depending on the characteristics of the transmitters and receivers, the baud rate on the cable, and the capacitance and electrical impedance of the cable. The RS-232 standard states that a compliant port must provide defined signal characteristics for a capacitive load of 2500 pF. This does not correspond to a fixed length of cable since varying cables have different characteristics. Empirically tested combinations of bit rate, serial ports, cable type, and lengths may provide reliable communications, but generally RS-232-compatible ports are intended to be connected by, at the most, a few tens of metres of cable. Other serial communications standards are better adapted to drive hundreds or thousands of metres of cable.

See also[edit]

Wikibooks has a book on the topic of: Serial Programming:RS-232 Connections
  • LapLink cable (can be seen as a parallel equivalent to a serial null modem cable)
  • Rollover cable (also known as a “Yost” cable.)


Rs232 Serial Cable Maximum Length

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Practically speaking, you can go up to 4,000 feet (1,200m) at baud rates up to 76,800 baud. Above that, you need to do a little math and reduce the length. For example, at 115k baud, your cable should not be much longer than 2,500 feet.However, the higher the baud rate, the more sensitive the cable is to the quality of installation – issues like how much twisted pair is unwound at each termination start to become very, very important. Our advice: For longer networks with lots of devices, choose 38k400 baud over 76k800 baud and optimize using COV, separate networks and by setting the Max Master to a lower number.


Image Source: Ten Ways to Bulletproof RS-485 Interfaces, National Semiconductor Application Note 1057, John Goldie, October 1996.

What about BACnet MSTP?

Rs485 Serial Cable Max Length

This is how the 2004 version of the spec defines the MSTP medium:

Rs232 Serial Cable Max Length

'An MS/TP EIA-485 network shall use shielded, twisted-pair cable with characteristic impedance between 100 and 130 ohms. Distributed capacitance between conductors shall be less than 100 pF per meter (30 pF per foot). Distributed capacitance between conductors and shield shall be less that 200 pF per meter (60 pF per foot). Foil or braided shields are acceptable. The maximum recommended length of an MS/TP segment is 1200 meters (4000 feet) with AWG 18 (0.82 mm2 conductor area) cable. The use of greater distances and/or different wire gauges shall comply with the electrical specifications of EIA-485'. In other words, if it complies with the RS485 spec, it meets the BACnet spec.

Serial Cable Maximum Length Chart

The cable gauge only changes the cables impedance. You should already be selecting a cable whose nominal impedance is relatively independent of length such as Belden 3106a. If you are using the correct cable, then the gauge should have little effect on the carrying capacity, except at very high frequencies.