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What year and model Ford are you working on?
Check power and ground to the EEC relay under the hood. You should have 12 volts ( Battery voltage) on pins #1 and #5 and good ground on pin #2. Poor gound will cause the chatter and no communication.
Also inspect the PCM diode.
What have you done?
If you jump pins 3 and 5 at the relay, will the engine run? Also, check for power on pin #16 at the Data Link Connector under the dash where the scan tool connects. If no power there, the scan toll will not communicate with the PCM.
Check the resistance to ground for pin #2 of the relay. Make sure the resistance is reading 0.0 OHMS or really close. It is also the same ground for the PCM.
I think the PCM has failed internally since it will not start and you cannot communicate with it.