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Something I saw is 1/8 unit load. In that document, it states that RS485 will allow up to 256 transceivers on the bus by using 1/8 unit load. My question is, what is 1/8 unit load and how do I use it?

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RS485 interface electrical specifications says

standard Driver load impedance is 54 Ohms max standard Receiver input impedance >=12K ohms,

RS-485 specifies a hypothetical term of a unit load (UL), which represents a load impedance of approximately 12 kOhms.

Drive capacity is 32 unit loads, i.e., 32 12kΩ receivers in parallel( gives 12k/32 = 375 Ohms). For receivers of higher input impedance, the number of unit loads on one bus can be higher. Any number of receivers can be connected to the bus, provided that the combined (parallel) load presented to the driver does not exceed 32 unit loads (375Ω).

The driver load impedance is 54Ω maximum, which, in a typical 24AWG twisted-pair environment, is 32 unit loads in parallel with two 120Ω terminators ( Parallel combination of 375 Ohms and 60 Ohms ==> 54 Ohms (approx).

Here, your mentioned devive MAX1482/MAX1483 has 96 KOhms input impedance, which is 8 times UL input impedance. because of that no of Devices can be connected now will be 32*8 = 256.

There are very good app notes from maxim and Ti.

This app note has good information about the UL convept http://www.analog.com/static/imported-files/application_notes/AN-960.pdf

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  • \$\begingroup\$ @RedGrittyBrick : Load is determined by the current, Resistance is inversely proportional to Current. Because of that Higher impedance loads, will take lesser current hence we can able connect more devices. \$\endgroup\$ – user19579 Aug 1 '13 at 16:09

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