I dont see any symptoms of a bad SMPS yet, but if you want to adjust the output, you would see 5V =Vref on pin 8 and Vref/2=2.5V on the FB pin 2 when in regulation. By pullup FB up a bit with a resistor of suitable size, that pushes down pin 1 the comparator output more on average and that delays the Reset internally so that a higher output voltage is achieved.
more likely the communication problem is on pins 2&3 of the serial ports ( or control signals like RTS/CTS, DTR/DSR or whatever means is used. Start by checking any voltage for bipolar RS232 levels. then activity. then baud rate and check settings if any.
The correct cable either straight or crossover (null-modem) is needed to slave.
If you have a PC with a serial port, then you can monitor both directions with a breakout box on the interfaces and have 2 Rx, 1 to slave and 1 to PC. RS232 responds to 1.2V logic thresholds with bipolar levels..