A data guard system (140, 200) for use with a MIL-STD-1553 data bus, the data guard system comprising: a first coupler; a second coupler; a third coupler; a first switch; a second switch; a guarded bus in signal communication with the second coupler and the second switch; a transparent bus in signal communication with the first switch, the second switch, the first coupler, and the third coupler, and a data guard connected to the MIL-STD-1553 data bus via the third coupler, the data guard comprising: a first 1553 controller in signal communication with the second coupler; a filter that includes a plurality of pre-defined data rule sets (642, 646, 646) configured to be applied to data traffic messages received from the MIL-STD-1553 data bus; a plurality of memory blocks (312a, 312b, 314a, 314b, 316a, 316b) directly accessible to the first 1553 controller; and a microprocessor in signal communication with the first switch, the second switch, the first 1553 controller, and the filter; wherein, when the data guard system is in an active mode, a plurality of data traffic messages from a protected remote terminal, RT, connected to the second switch is directed by the second switch onto the guarded bus and then to the second coupler, and a data traffic message of the plurality of data traffic messages is compared against the pre-defined data rule sets to determine if the data traffic message matches at least one data rule (611, 612, 613) of the pre-defined data rule sets (642, 644, 646), and wherein, when the data guard system is in a passive mode, the plurality of data traffic messages from the protected remote terminal, using the second switch, are directed onto the transparent bus and sent by way of the first coupler to the first switch and, using the first switch, the plurality of data traffic messages from the protected remote terminal are sent by way of the third coupler to the MIL-STD-1553 data bus, thereby bypassing the data guard. a first coupler; a second coupler; a third coupler; a first switch; a second switch; a guarded bus in signal communication with the second coupler and the second switch; a transparent bus in signal communication with the first switch, the second switch, the first coupler, and the third coupler, and a data guard connected to the MIL-STD-1553 data bus via the third coupler, the data guard comprising: a first 1553 controller in signal communication with the second coupler; a filter that includes a plurality of pre-defined data rule sets (642, 646, 646) configured to be applied to data traffic messages received from the MIL-STD-1553 data bus; a plurality of memory blocks (312a, 312b, 314a, 314b, 316a, 316b) directly accessible to the first 1553 controller; and a microprocessor in signal communication with the first switch, the second switch, the first 1553 controller, and the filter; a first 1553 controller in signal communication with the second coupler; a filter that includes a plurality of pre-defined data rule sets (642, 646, 646) configured to be applied to data traffic messages received from the MIL-STD-1553 data bus; a plurality of memory blocks (312a, 312b, 314a, 314b, 316a, 316b) directly accessible to the first 1553 controller; and a microprocessor in signal communication with the first switch, the second switch, the first 1553 controller, and the filter; wherein, when the data guard system is in an active mode, a plurality of data traffic messages from a protected remote terminal, RT, connected to the second switch is directed by the second switch onto the guarded bus and then to the second coupler, and a data traffic message of the plurality of data traffic messages is compared against the pre-defined data rule sets to determine if the data traffic message matches at least one data rule (611, 612, 613) of the pre-defined data rule sets (642, 644, 646), and wherein, when the data guard system is in a passive mode, the plurality of data traffic messages from the protected remote terminal, using the second switch, are directed onto the transparent bus and sent by way of the first coupler to the first switch and, using the first switch, the plurality of data traffic messages from the protected remote terminal are sent by way of the third coupler to the MIL-STD-1553 data bus, thereby bypassing the data guard.