Watlow Electric Revision 5 - Manual (Page 24)

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Multiple SYSTEM 32 units are connected in series. R+ is connected to the first
unit TX+ and TX- from the first unit is connected to TX+ of the next unit. These
serial connections are continued until the last unit is reached. The last unit TX- is
connected to the computer R-. T+ is connected to the first unit RX+ and the RX- is
connected to the next unit RX+. The last unit RX- is connected to the computer T-.
4.3 RS-485
The RS-485 is a voltage balance long distance multi-point transmission interface. It
may use 2 or 4 lines depending on system requirements.
4.3.1 RS-485 Description
The EIA Standard RS-485 specifies only the electrical characteristics of
generators (transmitters) and receivers for use in digital multi-point systems.
The specification of transmission lines, signaling rates, protocols, etc. is left
entirely up to the user. The transmitters and receivers selected by Anafaze
also meet the requirements of RS-422.
The following information is intended to make recommendations for the
application of the RS-485 interface to Anafaze equipment. This note covers
4 wire communication. Anafaze equipment will also support 2 wire
communication. Please contact the factory for recommendations.
The maximum signaling rate used by the Anafaze System 32 and associated
equipment is 9600 baud. Since this is far below the maximum signaling
rate covered by the specification, satisfactory performance may be expected
without strict adherence to all of the design rules. ANAFAZE has presented
conservative recommendations to insure reliable operation. If deviations
are necessary, please contact ANAFAZE.
4.3.2 Cable Selection
ANAFAZE recommends twisted shielded pairs for the RS-485 cables. The
transmitters and receivers specified in RS-485 are very tolerant of cable
characteristics, and some leeway is possible unless distances and signaling
rates push the specification limits.
One requirement is very important, as it impacts performance even down to
low frequency operation. The loop resistance of the transmission line [wire
only -- not terminating resistor] must not exceed 200 ohms for a properly
terminated line with a reasonable margin for noise. Thus the following
recommendations for distance and wire gauge should be observed: