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MFJ-1278B MULTI-MODE TABLE OF CONTENTS
Link Status Messages............................................................................................. 302
MFJ-1278B SPECIFICATIONS................................................................................... 305
GENERAL DESCRIPTION ......................................................................................... 306
DETAILED CIRCUIT DESCRIPTION ....................................................................... 307
Oscillator................................................................................................................ 307
Dividers and Baud-rate Generator.......................................................................... 307
CPU Complex ........................................................................................................ 308
Serial Interface ....................................................................................................... 308
Watch-dog Timer ................................................................................................... 309
Modem ................................................................................................................... 309
Power Supply ......................................................................................................... 309
RS-232C HANDSHAKING PROTOCOL.................................................................... 310
JUMPER FUNCTIONS................................................................................................ 312
PARALLEL PRINTER PORT PIN FUNCTION,J12................................................... 315
TTL INTERFACE PORT PIN FUNCTION,J5 ............................................................ 316
MULTI-LEVEL INTERFACE CONNECTOR,J13...................................................... 317
EXTERNAL MODEM INTERFACE CONNECTOR - J14......................................... 317
MODEM DISCONNECT HEADER, J4....................................................................... 318
HF TUNING INDICATOR .......................................................................................... 320
GENERAL TESTS ....................................................................................................... 321
Step 1: Power Supply............................................................................................ 322
Step 2: Obvious Problems..................................................................................... 322
Step 3: Assembly Problems................................................................................... 322
Step 4: Cabling Problems...................................................................................... 323
SPECIFIC SYMPTOMS............................................................................................... 323
Symptom: TNC appears dead ............................................................................... 323
Oscillator and Reset Circuits ............................................................................ 323
Digital Logic Lines ........................................................................................... 323
Symptom: Modem won't calibrate or key transmitter............................................ 324
Symptom: Uncopyable transmitted or received packets........................................ 325
TERMINAL INTERFACE TROUBLESHOOTING.................................................... 325
Symptom: MFJ-1278B won't sign on to the terminal............................................ 325
Symptom: The MFJ-1278B appears to be signing on but only gibberish is
printed on the terminal ........................................................................................... 326
Symptom: The MFJ-1278B signs on OK but won't accept commands ................. 326
Symptom: The MFJ-1278B appears to have "lock-up" i.e. not responding to any
commands .............................................................................................................. 326
EXPLANATION OF PROTOCOL............................................................................... 327
Physical Layer........................................................................................................ 327
Data Link Layer ..................................................................................................... 327
HDLC FRAMES........................................................................................................... 328
AX.25 LEVEL TWO
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329