How to Configure the ICF-1150’s Pull High/Low Resistors
Moxa’s new ICF-1150 media converter provides users with a valuable tool for tuning the converter’s pull high/low resistors. The two rotary switches located on the converter’s front panel can be adjusted independently to one of ten resistance values to ensure that data is transmitted without error through an RS-485 multi-drop network. You can go to Moxa's website to check the table.
Tuning the pull high/low resistors will inevitably involve a certain amount of trial-and-error. The brute force approach is to start with the lowest value on the dial, test the integrity of the network, and then work your way up the dial if you find that your data is not transmitted properly. A more intelligent approach is to use basic electronics principles to derive a reasonable starting value for the pull high/low resistance values. In this tech note we give users a straightforward method of determining such a value.
RS-485 communication uses balanced line drivers and differential transmission over 2 wires to transmit data between nodes. The precise amount of power needed by the interface transceiver driver at each node of an RS-485 network is influenced by the number of nodes, which can range from just a few all the way up to 256. As more nodes are added, the equivalent internal load impedance of the network as a whole will increase proportionately. For example, if a device can handle 32 nodes on the bus then the equivalent internal load impedance of the network as a whole is generally 12 KΩ. Since the impedance increases proportionately to the number of nodes, if the device can handle 64 nodes then the equivalent internal load impedance would be 24 KΩ, and if it can handle 256 nodes then the equivalent internal load impedance would be 96 KΩ. Since on RS-485 networks most devices are connected in parallel, a larger resistance will have a smaller effect on the total impedance.
The voltage difference across the two signal lines of a balanced line receiver is used to indicate the value of a bit. If the voltage difference is between 200 mV and 6 V, then the bit value is 1. If the voltage difference is between -200 mV and -6 V, then the bit value is 0. Voltage differences between -200 mV and 200 mV are interpreted as unknown. In addition, a voltage difference greater than 200 mV or less than -200 mV is used to indicate an idle state (also called tri-state), which means that all drivers connected the network are inactive.
To read more detail, please visit Moxa's website to download the complete version.
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