9XTend™
OEM
RF
Module
–
Product
Manual
v1.2.4
©
2005
MaxStream,
Inc.
Confidential
&
Proprietary
10
2.1.3.
Flow Control
Figure
2.3.
Internal
Data
Flow
Diagram
(The
five
most
commonly
‐
used
pin
signals
shown.)
DI (Data In) Buffer and Flow Control
When serial data enters the XTend Module through the DI Pin (pin 5), the data is stored in the DI
Buffer until it can be transmitted.
When the RB and RO parameter thresholds are satisfied (refer to Transmit Mode section [p11] for
more information), the module attempts to initialize an RF connection. If the module is already
receiving RF data, the serial data is stored in the module’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 XTend Module).
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 (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 module, the module
will receive data from the host faster than it can transmit the data over-the-air.
2. If the module is receiving a continuous stream of RF data or if the module is monitoring data
on a network, any serial data that arrives on the DI pin (pin 5) is placed in the DI Buffer. The
data in the DI buffer will be transmitted over-the-air when the module 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 module de-asserts (high)
to signal to the host device to stop sending data [refer
to FT (Flow Control Threshold, p24) and CS (GPO1 Configuration, p21) 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 [p23]. This option only works with ASCII data.
DO (Data Out) Buffer & Flow Control
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.
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 module, the module will
receive data from the transmitting module faster than it can send the data to the host.
2. If the host does not allow the module 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, p31), data
will not be sent out the DO Buffer as long as
(pin 10) is de-asserted.
Software Flow Control (XOFF).
XON/XOFF software flow control can be enabled using the FL
(Software Flow Control) Command [p23]. This option only works with ASCII data.
