Message Name:

RADIO SYSTEM BUS MEMBERS CANNOT TRANSMIT

The Radio Management Units (RMUs) and Aircraft Radio Systems cannot transmit data through the Radio System Bus (RSB).

• Very High Frequency (VHF) Communication Unit 1 (A1)
• Very High Frequency (VHF) Communication Unit 2 (A2)
• Very High Frequency (VHF) Communication Unit 3 (A226)
• Very High Frequency (VHF) Navigation Unit 1 (A3)
• Very High Frequency (VHF) Navigation Unit 2 (A4)
• Radio Management Unit (RMU) 1 (A25)
• Radio Management Unit (RMU) 2 (A26)
• Integrated Avionics Computer (IAC) 1 (A11)
• Integrated Avionics Computer (IAC) 2 (A10)
• Integrated Avionics Computer (IAC) 3 (A12)
• Radio System Bus (RSB) Termination Resistor
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

1. Disconnect RMU 1 connector (A25P1), RMU 2 connector (A26P1), VHF Communication Unit 1 connector (A1P1), VHF Communication Unit 2 (A2P1), VHF Communication Unit 3 connector (A226P1), VHF Navigation Unit 1 connector (A3AP1, A3BP1), VHF Naviagation Unit 2 connector (A4AP1, A4BP1), IAC 1 connector (A11BP1, A11CP1), IAC 2 connector (A10BP1, A10CP1) and IAC 3 connector (A12BP1, A12CP1). Perform a continuity check as follows:
   For the Primary RSB:

   From	To	Result
   A25P1-V	A1P1-10
   A25P1-W	A1P1-11
   A25P1-V	A2P1-10
   A25P1-W	A2P1-11
   A25P1-V	A226P1-10
   A25P1-W	A226P1-11
   A25P1-V	A3BP1-71
   A25P1-W	A3BP1-81
   A25P1-V	A4BP1-71
   A25P1-W	A4BP1-81
   A25P1-V	A11BP1-70
   A25P1-W	A11BP1-71
   A25P1-V	A10BP1-70
   A25P1-W	A10BP1-71
   A25P1-V	A12BP1-70
   A25P1-W	A12BP1-71
   A25P1-V	A26P1-V
   A25P1-W	A26P1-W

   For the left secondary RSB:

   From	To	Result
   A25P1-M	A1P1-74
   A25P1-N	A1P1-61
   A25P1-M	A226P1-74
   A25P1-N	A226P1-61
   A25P1-M	A3AP1-88
   A25P1-N	A3AP1-102
   A25P1-M	A11CP1-36
   A25P1-N	A11CP1-37
   A25P1-M	A12CP1-36
   A25P1-N	A12CP1-37

   For the right Primary RSB:

   From	To	Result
   A26P1-M	A2P1-74
   A26P1-N	A2P1-61
   A26P1-M	A4AP1-88
   A26P1-N	A4AP1-102
   A26P1-M	A10CP1-36
   A26P1-N	A10CP1-37

   1. If there is no continuity, repair defective wiring as required and do close out.
   2. If there is continuity, continue with next step.

2. Perform a continuity check as follows:
   For the Primary RSB:

   From	To	Result
   A25P1-V	Ground
   A25P1-W	Ground

   For the left secondary RSB:

   From	To	Result
   A25P1-M	Ground
   A25P1-N	Ground

   For the right Primary RSB:

   From	To	Result
   A26P1-M	Ground
   A26P1-N	Ground

   1. If there is continuity, repair defective wiring as required and do close out.
   2. If there is no continuity, continue with next step.

3. Perform a resistance check on the termination-resistors as follows:
   For the Primary RSB (R5 and R6):

   From	To	Expected Result	Result
   (Test Connector)J242-x	J242-y	50 Ohms

   For the left Secondary RSB (R9 and R38):

   From	To	Expected Result	Result
   (Test Connector)J241-p	J241-q	50 Ohms

   For the right Secondary RSB (R7 and R8):

   From	To	Expected Result	Result
   (Test Connector)J242-z	J242-w	50 Ohms

   1. If the resistance value is not as specified above, replace the defective resistor(s) and do close out.
   2. If the resistance value is as specified above, continue with next step.

4. Install the removed LRUs one by one. Perform a test every time an LRU is connected.

   1. If the fault comes back after re-installing an LRU, isolate the LRU to confirm the fault and replace as required and do close out.
   2. If the fault does not come back after re-installing an LRU, continue with next step.

5. Perform the operational test of the digital bus system (Refer AMM 31-44-00-710-801).

   1. If the fault is no longer present, do close out.
   2. If the fault is still present, continue with next step.

6. Replace defective IAC.
7. Do close out.