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Serial communications
9XTend-PKG-R RS-232/485 RF Modem User Guide
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Two cases in which the DO Buffer may become full and possibly overflow:
1. If the RF data rate is set higher than the interface data rate of the modem, the modem will receive data from the
transmitting modem faster than it can send the data to the host.
2. If the host does not allow the modem to transmit data out from the DO buffer because of being held off by
hardware or software flow control.
Hardware Flow Control (RTS)
. If RTS is enabled for flow control (RT Parameter = 2), data will not be sent out the DO
Buffer as long as RTS (GPI1) is de-asserted.
Software Flow Control (XOFF)
. XON/XOFF software flow control can be enabled using the FL (Software Flow Control)
Command. This option only works with ASCII data.
Transparent operation
By default, XTend RF Modems operate in Transparent Mode. The modems act as a serial line replacement - all UART
data received through the DI pin is queued up for RF transmission. When RF data is received, the data is sent out the
DO pin.
When the RO (Packetization Timeout) parameter threshold is satisfied, the modem attempts to initialize an RF
transmission. If the modem cannot immediately transmit (for instance, if it is already receiving RF data), the serial
data continues to be stored in the DI Buffer. Data is packetized and sent at any RO timeout or when the maximum
packet size is received.
The modem operates as described above unless the Command Mode Sequence is detected. The Command Mode
Sequence consists of three copies of the command sequence character [CC parameter] surrounded by the before and
after guard times [BT and AT parameters].
If the DI buffer becomes full, hardware or software flow control must be implemented in order to prevent overflow
(loss of data between the host and modem).
API operation
API (Application Programming Interface) Operation is an alternative to the default Transparent Operation. The API is
frame-based and extends the level to which a host application can interact with the networking capabilities of the
module. When in API mode, all data entering and leaving the RF modem is contained in frames that define operations
or events within the modem.
Transmit Data Frames (received through the DI (Data In) pin) include:
•
16-bit address
Receive Data Frames (sent out the DO (Data Out) pin) include:
•
Showing a received RF packet (16 bits only)
•
Response to a TX (Transmit) packet
•
Showing events such as hardware reset, watchdog reset, asynchronous events, etc.
The modem will send data frames to the application containing status packets; as well as source, RSSI and payload
information from received data packets.
API operation option facilitates many operations such as the examples cited below:
•
Change destination addresses without having to enter command mode
•
Receive success/failure status of each RF packet