GE IC754VSB06MTD, Hardware User'S Manual

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3-16 6" QuickPanel* ViewBasic, Monochrome, Round Bezel – September 2010 GFK-2327B
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Operating COM1 in EIA-485 Two-Wire Mode
Use of COM1 in EIA-485 two-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 transmissions.
This additional control avoids transmission collisions, which are possible in
two-wire mode because all the transmitters are tied together.
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 or who wish to develop their own
driver, will need to design their software driver to control the port appropriately
in the EIA-485 two-wire mode.
In all QuickPanel View 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 EIA-485 transceiver device on COM1. When not
transmitting, RTS is de-asserted (turned off) to disable the transmitter and
asserted (turned on) before transmission to enable the transmitter.
The receiver is hardwired to always be enabled. In two-wire mode, the
communication software has to discard any returned message that it receives
while transmitting.
Activating RTS is possible using the standard Windows API for the COM 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, one entire message should
be written to the port at one time, not byte-wise, to ensure that RTS does not
toggle in the middle of a transmitted message.