The RS-485 standard (ANSI/TIA/EIA-485-A or equivalently ISO 8482) specifies a 250-mA driver output current limit to prevent damage caused by data contention on the bus. That applies in the event that two or more transceivers drive the bus to opposing states at the same time. The SN65HVD3x family of devices includes current-limiting circuitry that prevents damage under these conditions.

In a specific combination of circumstances, a condition can occur in which current through the bus pin exceeds the 250-mA limit. This combination of conditions is not normally included in RS-485 applications:

• Loading capacitance on the pin is less than 500 pF
• The bus pin is directly connected to a voltage more negative than –1 V
• The device is supplied with V_CC equal to or greater than 3.3 V
• The driver is enabled
• The bus pin is driving to the logic high state

In these specific conditions, the normal current-limit circuitry and thermal-shutdown circuitry does not limit or shutdown the current flow. If the current is allowed to continue, the device heats up in a localized area near the driver outputs, and the device can be damaged.

Typical RS-485 twisted-pair cable has a capacitance of approximately 50 pF/meter. Therefore, it is expected that 10 meters of cable can provide sufficient capacitance to prevent this latch-up condition.

The –7 to +12-V common mode range specified by RS-485 is intended to allow communication between transceivers separated by significant distances when ground offsets may occur due to temporary current surges, electrical noise, and so on. Under those circumstances, the inherent cable needed to connect separated transceivers ensures that the conditions previously listed do not occur. For a transceiver separated by only a short cable length or backplane applications, it is unusual for there to be a steady-state negative common-mode voltage. It is possible for a negative power supply to be shorted to the bus lines due to miswiring or cable damage; however, this is a different root cause fault, and robust devices such as the SN65HVD178x family should be used for surviving power supply or miswiring faults.

The 250-mA current limit in the RS-485 standard is intended to prevent damage caused by data contention on the bus; that is, in the event that two or more transceivers drive the bus to different states at the same time. These devices are not damaged under these conditions because all RS-485 drivers have output impedance sufficient to prevent the direct connection condition stated previously. Typical RS-485 driver output impedance is on the order of 10 Ω to 30 Ω.