INCOM versus RS-485

[Dave Loucks]

INCOM was invented by Westinghouse as a method of transmitting serial data down wires that would be in close proximity to high power conductors.  Characteristics of INCOM included:

• Galvanic isolation between nodes
• Frequency shift keying
• Polarity insensitive
• Tolerance for wiring with T's, Y's, taps and branches
• Essentially unlimited number of nodes per cable (1000 listed as tested max.)
• 10000 feet (3000 m) end to end distance without repeaters

The earliest 1200 bps INCOM networks operated by transmitting a 115.2 kHz tone to represent a binary 1 bit and absence of tone to indicate a binary 0 bit.  Since a relatively high frequency tone was chosen, a transformer was used to couple the board electronics to the network.  This provided ~1500 V isolation node to node and node to ground from the network.  A bandpass filter was used to detect the 115.2 kHz tone, which offered good noise rejection.  At the 1200 bps signalling rate, 96 cycles of 115.2 kHz represented one bit on the INCOM data packet, which offered good oversampling / redundancy, again increasing the signal to noise ratio.

Later, the network was upgraded to 9600 bps.  The faster signaling rate meant that only about 12 bits of the 115.2 kHz signal represented a single INCOM bit.  Since the tone oscillator had to be switched on and off, the natural ramp up and ramp down of the signal essentially meant that the first and last few cycles of the 110 kHz tone would not be exactly on frequency.  This reduced the amount of oversampling and reduced the reliability of the network at 9600.  The solution to this problem was to keep the tone oscillator switched on, but make it a voltage controlled oscillator (VCO) and have the binary zero represent a different tone sufficiently separated from the binary 1 tone, but not so far away as to make slewing to and from the binary 1 tone excessively difficult.  The second tone chosen was 92 kHz

• Binary 1: 110 kHz
• Binary 0: 92 kHz (9600 bps mode), no tone (1200 bps mode)
• Message: 33 bits
• Start Bit: 2 bits
• Control Bit: 1 bit
• Data: 24 bits
• BHC Check: 5 bits code

What is the difference between INCOM and EIA-485 (formerly known as RS-485), a more common physical layer?

	INCOM	EIA-485
Galvanic Isolation	Transformer	*
Number of Nodes	1000 per segment	32 per segment
Wiring T's, Y's taps permitted?	Yes	No

* The EIA-485 standard does not include galvanic isolation, but a separate dc/dc power supply and opto-isolation can be added to provide isolation.  485 driver chips can be purchased that include this isolation built in.

For a discussion of noise immunity differences, see this document:
http://pps2.com/communications/files/INCOM/485vsINCOM.ppt