You can get locking with the frequencies at different rates.
Check the output frequency of the 1814 matches the input frequency settings (usually a config option 44/48/88/96 on lexicon gear) of the Lexicon.
From the manual:
http://www.lexiconpro.com/Product_Downloads/MX300_v1.pdf
5. S/PDIF Digital In/Out
Digital input accepts 44.1kHz / 48kHz signals. When the S/PDIF digital input is selected
and active, the S/PDIF LED on the front panel lights.
Important: It is recommended that you do not connect any digital device to the MX300’s S/PDIF inputs that transmits at sample rates other than 44.1 kHz or 48 kHz (such as 96kHz). Doing so can cause unpredictable performance. Make sure the device you are connecting to the MX300’s S/PDIF In is set as the Clock Master (if that option is available) and transmitting at a sample rate of 44.1kHz or 48kHz only. As with any other connection, if you need to unplug the S/PDIF cable, it is recommended that you switch to the analog inputs (see Input Source in the System menus) or press the Program Bypass button before disconnecting the cable.
Studio – Digital Setup
S/PDIF Out
S/PDIF In
This setup is ideal if you are using the MX300 as a hardware plug-in in a digital recording
system. This setup requires that you are using an I/O device with S/PDIF In/Out, or a digital mixer with S/PDIF I/O option. The Input Source must be set to Digital, and the Clock Source should be set to Digital in the System menu (described on pages 30-31).
Input Source
The MX300 has both analog and digital inputs. Depending on which inputs you will be using, you need to select between analog or digital audio
sources. The MX300 offers three different input sources to select from: Anlg Mono L (Left), Anlg Stereo, and Digital. Input Source refers to whether you will be using the analog Input pair or the S/PDIF digital input. When only a single analog input is to be used, connect to the analog Input Left input only and select Anlg Mono L as the Input Source. All inputs (analog and digital) can remain connected simultaneously in the event you find yourself switching between using both. Use Knob A to select the appropriate Input Source you will be using
Clock Source
This option allows manual selection of either the internal or external clock source for the MX300. When set to 44.1kHz, the MX300 utilizes an internal clock with a 44.1kHz sample rate. When set to 48kHz, the MX300 utilizes an internal clock with a 48kHz sample rate. When set to Digital, the unit utilizes the S/PDIF input clock source, even if an analog audio source is used. Refer to The MX300 and Digital I/O section on page 12 for more information on using digital audio sources. Use Knob B to select the appropriate Clock Source you will be using.
Digital Out
The MX300 can send unprocessed digital audio out its S/PDIF Digital Output while monitoring processed audio through the analog outputs. This is useful when you want to record a dry performance digitally, but also hear effects processing while you are recording. Note: This feature is only available for signals brought into
the MX300 via the analog inputs. It has no effect on signals being received at the S/PDIF In. Digital in/out signals are processed normally regardless of whether the Digital Out settings are set to Dry. To send unprocessed audio out
the MX300 digital outputs, use Knob C to set the Digital Out option to Dry.
