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CL4790 Hardware Integration Guide
Version 3.0
Note:
Many simple 2- or 4-wire converters do not provide hardware flow control capabilities and therefore
require you to disable handshaking by setting Handshaking to None in the Laird Configuration Utility.
Check with your converter manufacturer for any specific requirements.
Hardware Flow Control
Flow control refers to the control of data flow between the host and the CL4790. It is the means of handling
data in the transmit/receive buffer of the CL4790 interface and it determines the throttling of data flow
between the host and the CL4790. Often in serial communication, one device is capable of sending data
much faster than the other can receive. Flow control allows the slower device to tell the faster device to
pause and resume data transmission. Because flow control signals CTS and RTS are used by the CL4790 and
locally
its host
(rather than over the air), one CL4790 cannot tell the other CL4790 to slow down or speed up.
The CL4790 controls the Clear to Send (CTS) output to the OEM host. The state of the CTS pin is based on
the amount of data in the interface buffer. If the buffer is below the maximum limit, the transceiver holds
CTS logic Low to signal to the OEM host that data can be accepted over the serial interface safely. If the
buffer is full, then CTS transitions logic High to signal to the OEM host that additional data sent over the
serial bus has the potential to be lost due to buffer overflow.
Ready to Send (RTS) is an input to the CL4790 from the OEM host. When the
the software configuration of the CL4790, the transceiver checks the status of RTS before attempting to send
received RF data to the OEM host. If RTS is logic Low, the transceiver sends data to the OEM host. If RTS is
logic High, it does not send data to the host.
Note:
CTS is always enabled by default. RS485 Interface does not support Hardware flow control.
RTS is high by default on the CL4790. If RTS Enable is enabled, the CL4790 does not transmit data
out the serial interface unless the RTS line is driven low by the OEM host.
Tip
Can I implement a design using just Txd, Rxd and Gnd (Three-wire Interface)?
Yes. However, Laird strongly recommends that your hardware monitor the CTS pin of the radio.
CTS transitions logic High by the radio when its interface buffer is getting full. Your hardware
should stop sending data over the serial interface at this point to avoid a buffer overrun (and
subsequent loss of data).
You can perform a successful design without monitoring CTS. However, you need to take into
account the amount of latency the radio adds to the system, any additional latency caused by
Transmit Retries or Broadcast Attempts, how often you send data, non-delivery network
timeouts, and interface data rate. Polled type networks, where a centralized host requests data
from the surrounding hosts and the surrounding hosts respond, are good candidates for
avoiding the use of CTS. This is because no one transceiver can monopolize the RF link.
Asynchronous type networks, where any radio can send to another radio at any point in time,
are much more difficult to implement without the use of CTS.
Americas: +1-800-492-2320 Option 2
Europe: +44-1628-858-940
Hong Kong: +852-2923-0610
www.lairdtech.com/ramp
RTS Enable
11
option is selected in
CONN-HIG_CL4790
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