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
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