The COM1 serial port of the QuickPanel View, QuickPanel Control and DisplayPAC Operator Interface products can operate in either RS232, or in RS485 mode with either a 4 wire or 2 wire connection. RS232 and RS485 4 wire are both simultaneously available by default.

Use of COM1 in RS485 2 wire mode requires additional control of the port to ensure that the transmitter is turned off at the appropriate times to allow other connected stations to transmit and to avoid collisions of these transmissions that are possible since all the transmitters are tied together in 2 wire mode.

This function is normally carried out by the software driver included as part of the application software and is not part of the Windows CE operating system. Those who don't have the appropriate driver (DisplayPAC) or who wish to develop their own driver, will need to design their software driver to control the port appropriately in a 2 wire connection in RS485 mode.

In all QuickPanel and DisplayPAC models, the RTS (Request to Send) control signal is brought out on the COM1 connector but is also connected to the driver enable input (active low) of the RS485 transceiver device on COM1. When not transmitting, RTS is deasserted (turned off) to disable the RS485 transmitter and asserted (turned on) before transmission to enable the transmitter.

The RS485 receiver is hardwired to always be enabled. In 2 wire mode, the communication software will have to discard any received message which is returned to itself while transmitting (due to the 2 wire connection).

Activating RTS is possible using the standard Windows API for the COMM ports by appropriately setting the Device Control Block (DCB) for the port, then using the resulting handle to the port to manipulate this signal.

When relying on the Windows API to control RTS, be sure to write one entire message to the port at one time, not byte-wise, to ensure that RTS does not toggle in the middle of a transmitted message.