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Low Latency for Xtend and XStream Digi Radio Modules

Digi radio modems are extremely easy to use. The default mode of operation is a transparent mode allowing the modems to replicate an RS232 or RS485 serial cable between the connected devices. The only difference between the radio modems and a serial cable is that the modems must be configured with the correct UART settings (baud rate, etc.) and that they add additional latency to the communication.

In a cable, data transmitted into one end appears instantaneously at the other end. When data is transmitted into a radio modem at one end, there is a specific delay before it appears at the receiving end. The amount of additional time required to transmit the data is called "latency".

When transmitting a 32 byte packet, the 19200 XStream radio modems adds 54ms of latency to the transmission time. The 9600 baud XStream inserts 72ms of latency. The amount of latency varies by the number of bytes.  For lower latency and faster speeds you also can consider the Xbee line of radios or the XLR.

To restate this, the XStream modules require an additional 35ms channel initialization time in each direction by default. The default mode of operation is the most robust and interference resistant. If lower latency is absolutely critical to the application, the XStream modules can be configured to an operating mode that reduces the latency in each direction to about 12ms by setting the ATSY parameter to 0x14. This command reduces the synch time but causes the link to be more susceptible to interference.

Other options for reducing latency:
  • The 38400 baud XCite has a synch time of 12ms in frequency hopping mode or 5ms in single frequency mode.
  • The 115200 baud XTend has a synch time of about 6ms in frequency hopping mode.

For help in choosing the best solution to a latency question please provide the following information about your system design goals to the Digi RF-Xperts:

  • What does the system allow for the latency of the radio transmission?
  • What is the operating environment (line-of-sight or not)?
  • What is the desired transmission distance?
Last updated: Aug 23, 2017

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