AO-51 Configuration

Due to restrictions in the Satellite, there can be no digi functionality on Echo at this time.

Explanation:
Random noise from a receiver that is fed through the demodulator and into the SCC (a sort of smart UART) will cause interrupts to be sent to the CPU. With Echo having 4 or 5 open receivers the number of interrupts was enough to cause the CPU difficulties in getting normal processing done in a timely fashion. Basically, the CPU was locked up trying to handle the interrupts.

Echo is operating its SCC ports in a mode that will only pass AX.25 packets beginning with a certain character, which is called 'block' mode. This configuration solved the interrupt problem. For Echo, with the call sign being "PACB or PECHO", only packets beginning with the character 'P' can get through the SCC and generate a CPU interrupt.

A digi packet has the call sign of the destination station first, followed by the satellite (relay) call sign, and then the originator call sign. Therefore, the digi packet will not get through the SCC unless the destination station call sign begins with the letter 'P'.

A great deal of effort was put into solving this problem. This solution is the only one at the present time that has worked well enough to implement in the Satellite.

ANS thanks Mike, KE4AZN for the above information

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