• Code: 505
• Description: Polling XMT is rejected.
• Causes: No polling original is set.

• Code: 540
• Description: No response after transmitting 3rd CTC or DCN received.
• Causes: Incompatible interface.

• Code: 541
• Description: No response after transmitting 3rd EOR or received DCN.
• Causes: Line is faulty. MJR PCB abnormal.

• Code: 542
• Description: No response to the 3rd RR transmitted or received DCN.
• Causes: Remote unit is abnormal.

• Code: 543
• Description: T5 timer (60 sec.) elapsed without MCF.
• Causes: Remote unit is abnormal.

• Code: 544
• Description: Stopped Transmission after EOR Transmission.
• Causes: Line is faulty. MJR PCB abnormal.

• Code: 550
• Description: Timer between frames in phase C has elapsed.
• Causes: Defective remote station.

• Code: 554
• Description: Transmitted ERR after receiving EOR.
• Causes: Line is faulty.