The interface operates at 31.25 (+/- 1%) Kbaud, asynchronous, with a start bit, 8 data bits (D0 to D7), and a stop bit. This makes a total of 10 bits for a period of 320 microseconds per serial byte.
Circuit: 5 mA current loop type. Logical 0 is current ON. One output shall drive one and only one input. The receiver shall be opto-isolated and require less than 5 mA to turn on. Sharp PC-900 and HP 6N138 optoisolators have been found acceptable. Other high-speed optoisolators may be satisfactory. Rise and fall times should be less than 2 microseconds.