1.6 Industrial interface transceivers  (Page 3/3)

You should typically start your project by determining how many bus nodes you need to connect to and across what distance the data link has to run before looking at a data sheet. Once you’ve got a rough idea, the most important parameters to look for are temperature range, data rate or driver rise and fall time, unit load or number of nodes, common-mode voltage range, supply current, and fault protection.

Temperature range

The wide range of applications for RS-485 transceivers has led to a number of different temperature ranges. Before evaluating data sheet parameters, make sure you have selected the correct temperature range for your application requirements. The specified operating temperature range is almost always given at the end of the device description on the front page of a data sheet.

Unit loads or number of nodes

For each bus line, RS-485 defines a unit load (1UL) as a 12-kΩ common-mode resistance between one of the bus lines and ground. The standard also specifies that a driver must be able to drive a total of 32 unit loads or 375 Ω on the bus. Thus, a transceiver with a 1/8 unit load has 8 times the input impedance of a unit load, which is 8 x 12 kΩ = 96 kΩ. Dividing 96 kΩ by 375 Ω yields 256, which is the maximum number of transceivers connected to the bus.

You can usually find either the unit load value or the maximum number of nodes listed on the front page of the data sheet. If not, turn to the receiver section of the data sheet and look for the specified common-mode input resistance. Divide this value by 375 to find your maximum number of transceivers.

Data rate or driver rise and fall time

As shown in Figure 5, you can determine your maximum data rate for a given bus length, or vice versa – a maximum bus length for a given data rate. The longer the cable length, the lower the data rate. Long-distance networks (1,000 feet to 4,000 feet) such as e-metering applications often use data rates of 10 kbps to 250 kbps. Low data rates also allow for longer stubs, the distance between the cable trunk and the actual transceiver terminals on the board. The maximum data rate is usually given on a data sheet's front page. If not, turn to the Driver Switching Characteristics section and find the maximum driver rise time, tr. Then calculate the maximum data rate: DR = 0.3 / tr.

Common-mode voltage range

A transceiver’s common-mode voltage range is usually listed in the Recommended Operating Conditions table as "voltage at any bus terminal (separately or common mode), VI." As mentioned earlier, standard-compliant transceivers provide a range of -7 V to +12 V. High common-mode transceivers allow for -20 V to +25 V. For higher common-mode requirements, choose isolated RS-485 transceivers.

Supply current

The supply or quiescent current is specified under no-load conditions for various driver/receiver on/off configurations. The most important configuration, however, is when the driver is disabled while the receiver is enabled, as this represents the main operating mode during a transceiver's lifetime. A low-power transceiver such as the SN65HVD3082 consumes only 600 μA in this condition, while a robust transceiver with 70-V standoff like the SN65HVD1785 requires a full 4 mA. The supply-current specification is commonly specified under Electrical Characteristics.

Fault protection voltage

This is a standoff voltage that allows your transceiver to survive should a broken bus cable come in contact with adjacent power cabling or other high-voltage contacts. This voltage is listed under Absolute Maximum Ratings as "voltage range at bus pins."

You can parametrically search TI's RS-485 transceivers here: http://www.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=545&uiTemplateId=NODE_STRY_PGE_T .