Specifications

CHAPTER 3: Card Setup

3.3 RS-485 Enable Modes

RS-485 is ideal for multi-drop or network environments.

RS-485 requires a tri-state driver that will allow the

electrical presence of the driver to be removed from the

line. The driver is in a tri-state or high impedance

condition when this occurs. Only one driver may be

active at a time and the other driver(s) must be tri-

stated. The output modem control signal Request To

Send (RTS) is typically used to control the state of the

driver. Some communication software packages refer to

RS-485 as RTS Enable or RTS block mode transfer.

One of the unique features of the PCI Card is the ability

to be RS-485 compatible without the need for special

software or drivers. This ability is especially useful in

operating systems where the lower-level I/O control is

abstracted from the application program. You can

effectively use the PCI Card in an RS-485 application

with existing (that is, standard RS-232) software drivers.

Switches SW3 and SW4 are used to control the RS-485

mode functions for the driver circuit. The selections are

RTS Enable (silk-screen “RT,” switch position 4) or Auto

Enable (silk-screen “AT,” switch position 3). The Auto

Enable feature automatically enables/disables the