RS-422 Serial Interface The receiver chip has 2 voltage inputs called A and B, used in combination to produce a logical 1 or 0 result internally. Input A is also called RX+, and input B RX-. Signals received on A and B must be at least 200mV (0.2 volt) apart in magnitude - the absolute values are not important, within limits. A > B = 1 B > A = 0 So for example: logic A B output +5 -5 A > B 1 -3 -5 A > B 1 0 -5 A > B 1 -2 +5 A < B 0 0 +5 A < B 0 RS-232 uses a + and - voltage swing on one transmit wire only, and this will easily match an RS-422 interface if the RS-422 input A (RX+) is tied to 0-volt. Input B (RX-) will simply track the +/- signal, always being greater or less than A. This will also invert the received RS-232 signal to recreate the original logical values (which are normally transmitted inverted by the RS-232 interface). A problem arises when trying to match a *TTL* signal to RS-422. TTL transmits only 0 to 5 volt. If the RS-422 receiver has its A input held to 0 volt (e.g. by a moulded RS-232 serial cable) and input B swings 0 to 5 volt, then B will always be equal to or more positive than A, producing a logical 0. Input B will never be less than input A, and the receiver chip will never produce a logical 1. (Voltage swing from a PIC chip output-pin can be +0.6 to +4.3 volt). One remedy is to use a small converter circuit, 1 or 2 components, to shift the 0 to 5 volt range into +/- volts. The Apple Newton provides +4.5 and -4.5 volts on its serial connector, so it should be possible to use these to make a simple TTL to RS-422 converter. One method is to use an opto-coupler (type: 4N25) and 3 resistors. Circuit details are in the heartbeat section, where it is used in the digital control-box. This TTL converter is very simple and has been used for sending TTL data to both Apple Newton and desktop PC. A regular moulded 8-pin DIN / 9-Way serial cable needs no modification.