While it goes against conventional wisdom, and can potentially cause a problem with circulating currents by grounding a shielded cable at both ends, this method is very effective at keeping induced lightening noise away from the communications lines. The method to eliminate differential mode interference is to add a bias resistor (matching resistance in the dome camera) to the circuit, and use twisted-pair wires; common mode interference is transmitted between the signal line and the ground, which is asymmetrical interference. Termination resistors also reduce electrical noise sensitivity due to the lower impedance, and bias resistors are required. Why is termination important in an RS485/RS422 network? Why is bias important in an RS485/RS422 network? To insure that a receiver stays in an inactive state, when no data signal is present, bias is generally added to a network at one or more locations. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven.
Another potential problem with large networks without isolation, is that severe damage can occur to your entire system, if a high voltage source is connected (accidentally or otherwise) to your communications lines. This ensures that all security-related information and incidents are transmitted quickly and with high reliability. When there is no data activity on an RS485 network (or in many instances RS422 networks, other than point-to-point), the communications lines are “floating” and, thus susceptible to external noise or interference. Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. When full-duplex is converted into half-duplex only one device at a time can transmit data. In the most common multi-drop RS-485 protocol, one computer is designated as a “master” and the rest of the computers or devices on the serial bus are designated as “slaves”. Some of the typical applications of RS-485 is point of sale equipment.
While the standard only supports low data rates and short line length (50ft.) it is still widely used and, very useful in many applications. An “Automatic Converter” (RS232 to RS485) will turn on the RS485 transmitter when data is detected on the RS232 port, and revert back into the receive mode after a character has been sent. By “isolating” sections of a large network, the accumulated noise on one isolated leg is not so likely to cause a data error that will propagate to another leg of the network. Even though some IC manufacturers offer light loading devices, that can accommodate 256 or even 400 nodes on one RS485 network, you may NOT want to build such a network for a few reasons. This cable is usually 24AWG. Category-5 cable will generally work well in most instances even though its characteristic impedance is 100 Ohms. Galvanic isolation will break a large problem into several small, but manageable ones. If you have any questions, leave them in the comment section below or use our forums and I will try my best answering them. Do play around with transfer speed on your own and give a try to other ethernet cable types too.
The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application. While RS422 is comparable to RS485, it is limited to unidirectional data traffic, and is terminated only on the end of the line opposite the transmitter. Twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line. Loaded driver voltage (32 nodes on the network and termination) should typically exceed ±1.5V. Unloaded driver output differential voltage can be as high as ±6V. With galvanic isolation the damage is generally limited to only one leg of the network, except in extreme cases of very high voltage (induced by lightening for example). 12V. A driver in the high impedence (off) state is able to remain in this state over the common mode range, whether power is applied or not.
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