Concept
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (MNB and Generic RS-485 Devices)
According to the TIA-485A standard, a single unit load is equivalent to a 12 kohm impedance attached to the + and – data lines (connected to ground or supply). A 1/8UL transceiver would have an impedance of 96 kohm. The TIA-485A defined total network load limit of 32UL is based on a common mode load resistance of 375 ohm connecting both the + and – data lines to ground (or CMV source). The standard requires the RS-485 drivers be capable of driving a network load of 32UL along with a Common-Mode Voltage (CMV) difference of -7 V to +12 V and produce a guaranteed minimum of 1.5 V transmit signal level. Such a full UL load with severe CMV conditions exhausts the maximum drive current of 60 mA provided by all standard RS-485 drivers. The specified minimum of 375 ohm resistance for the common mode load is the resulting resistance seen when 32 transceivers with 12 kohm input impedance are placed in parallel (12,000 / 375 = 32).
The TIA-485A standard does not accommodate any special allowance or exclusion for the addition of bias resistors. If you add load to the data lines (for whatever purpose), it is part of the common mode load and must be considered in the calculation of unit load on the network. In the same manner that 375 ohm equals 32UL, 510 ohm resistance equals 23.5UL.
The accumulation of RS-485 node counts in excess of the standard defined limit of 32UL does not alone create a violation prompting immediate inoperability. The primary performance parameter affected by network load is the ability of the RS-485 transmitter to output the minimum specified signal level of 1.5 V. Reduction in output signal level starts with the addition of the first load connected and reduces further as additional load is added. The 32UL boundary is simply the standardized guaranteed specification limit where the manufacturers guarantee the signal level will not have reduced below 1.5 V. As mentioned above, the 1.5 V signal output is guaranteed not only with a 32UL load, but also with an elevated common mode voltage (CMV of -7 V to +12 V). As the network exceeds the 32UL limit, the tolerance for this CMV will decay. The following graph from the TIA-TSB-89-A standards document shows the expected reduction in CMV as a function of unit load on the bus.
Maximum number of unit loads versus common-mode voltage and REQ (From TIA TSB-89-A 485 application guidelines
The REQ legend in the figure above refers to the resulting parallel load resistance of two termination resistors. The common 120 ohm termination produces 60 ohm and is shown by the REQ=60 cyan colored line. The cyan line indicates a standard UL load limit of 32 with a CMV between -7 V and +12 V. Extra current is required from the transmitter to overcome the common mode voltage seen through the common mode load impedance (unit load). As node count elevates, the tolerance for CMV pulls in as seen in the graph.
For example, if we take the 24UL of the bias network and add a collection of 127 nodes with each rated at 0.18UL, we would have a total network load of 47UL. From the figure above, you can see that the specified minimum -7 V CMV will be reduced to around -2.5 V and the normal +12 V will reduce to about +6.5 V. When operating in a situation where CMV is avoided, the reduction in transceiver CMV performance can be better tolerated. An isolated RS-485 bus configuration (such as with the MNB BACnet devices) allows the nodes to be insulated from local ground voltage differences which are a main source of sustained CMV on the bus. The isolated bus allows each of the transmitters to move up/down to the idle voltage of the bus.
RS-485 Communications
Generic RS-485 Network Devices
MNB BACnet Devices