Serial communications
9XTend-PKG-R RS-232/485 RF Modem User Guide
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Both the RF modem and host (PC) settings can be viewed and adjusted using Digi's proprietary XCTU Software.
Flow Control
The following diagram shows the Internal Data Flow Diagram (The five most commonly-used pin signals shown).
DI (Data In) buffer and Flow Control
When serial data enters the modem through the DI pin (Data In), the data is stored in the DI Buffer until it can be
processed.
When the RB and RO parameter thresholds are satisfied (refer to ‘Transmit Mode’ section for more information), the
modem attempts to initialize an RF connection. If the modem is already receiving RF data, the serial data is stored in
the modem's DI Buffer. The DI buffer stores at least 2.1 KB. 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 RF modem).
To eliminate the need for flow control:
1. Send messages that are smaller than the DI buffer size. The size of the DI buffer varies according to the packet size
(PK parameter) and the parity setting (NB parameter) used.
2. Interface at a lower baud rate (BD parameter) than the RF data rate (BR parameter).
Two cases in which the DI Buffer may become full and possibly overflow:
1. If the serial interface data rate is set higher than the RF data rate of the modem, the modem will receive data from
the host faster than it can transmit the data over-the-air.
2. If the modem is receiving a continuous stream of RF data or if the modem is monitoring data on a network, any
serial data that arrives on the DI pin (Data In) is placed in the DI Buffer. The data in the DI buffer will be transmitted
over-the-air when the modem no longer detects RF data in the network.
Hardware Flow Control (CTS)
. When the DI buffer is 17 bytes away from being full; by default, the modem de-asserts
CTS (high) to signal to the host device to stop sending data [refer to FT (Flow Control Threshold) and CS (GPO1
Configuration) Commands]. CTS is re-asserted after the DI Buffer has 34 bytes of memory available.
Software Flow Control (XON)
. XON/XOFF software flow control can be enabled using the FL (Software Flow Control)
Command. This option only works with ASCII data.
DO (Data Out) buffer
When RF data is received, the data enters the DO buffer and is sent out the serial port to a host device. Once the DO
Buffer reaches capacity, any additional incoming RF data is lost. The DO buffer stores at least 2.1 KB.
Parity
None (NB parameter = 0)
Number of Stop Bits
1 (NB parameter = 0)
Parameter Setting
XTend RF Modem Default Parameter Value