Comtech EF Data XPA-200-OD - Manual (Page 14)

XPA-200-OD Revision 1
Customer Commands MN/XPA200OD.IOM
11
2.3 RS-485 (
FULL DUPLEX
)
SUMMARY
:
Two differential pairs - one pair for controller to target, one pair for target to
controller.
Controller-to-target pair has one line driver (controller), and all targets have line-
receivers.
Target-to-controller pair has one line receiver (controller), and all targets have tri-
state drivers.
2.4 RS-232
This is a much simpler configuration in which the controller device is connected directly
to the target via a two-wire-plus-ground connection. Controller-to-target data is carried,
via RS-232 electrical levels on one conductor, and target-to-controller data is carried in
the other direction on the other conductor.
2.5 B
ASIC
P
ROTOCOL
Whether in RS-232 or RS-485 mode, all data is transmitted as asynchronous serial
characters, suitable for transmission and reception by a UART. The asynchronous
character format is fixed at 8 data bits, no parity, and 1 stop bit. Only two (2) baud rates
are supported: 9600 baud and 19200 baud.
All data is transmitted in framed packets. The host controller is assumed to be a PC or
ASCII dumb terminal, which is in charge of the process of monitor and control. The
controller is the only device that is permitted to initiate, at will, the transmission of data.
Targets are only permitted to transmit when they have been specifically instructed to do
so by the controller.
All bytes within a packet are printable ASCII characters, less than ASCII code 127. In
this context, the Carriage Return and Line Feed characters are considered printable.
All messages from controller to target require a response (with one exception). This will
be either to return data that has been requested by the controller, or to acknowledge
reception of an instruction to change the configuration of the target. The exception to this
is when the controller broadcasts a message (such as Set time/date) using Address 0,
when the target is set to RS-485 mode.