Product manual
XStream‐PKG‐R™RS‐232/485RFModem–ProductManualv4.2B
©2005MaxStream,Inc.Confidential&Proprietary 13
3.1.3. Flow Control
Figure3.2. InternalDataFlowDiagram
(Thefivemostcommonly‐used pinsignalsareshown.)
DI (Data In) Buffer and Flow Control
When serial data enters the XStream Modem through the DI Pin, then the data is stored in the DI
Buffer until it can be transmitted.
When the RO parameter threshold is satisfied (refer to Transmit Mode section [p14] and
command descriptions [p22] 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. 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 XStream RF Modem).
How 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 and parity setting used.
2. Interface at a lower baud rate (BD Command) than the fixed RF data rate.
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 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 ( ). When the DI buffer is 17 bytes away from being full; by
default, the modem de-asserts (high)
to signal to the host device to stop sending data [refer
to FT (Flow Control Threshold) and CS (DO2 Configuration) Commands].
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 and Flow Control
When RF data is received, the data enters the DO buffer and is then sent out the serial port to a
host device. Once the DO Buffer reaches capacity, any additional incoming RF data is lost.
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 ( ). If is enabled for flow control (RT Parameter = 2), data will
not be sent out the DO Buffer as long as
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.