3 rs-485 description, 3 rs-485 description -17 – Watlow 12LS Controller User Manual
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3.5.3 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.
The maximum signaling rate used by the ANAFAZE 12LS 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.
3.5.3.1 CABLE RECOMMENDATIONS
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 _ for a properly
terminated line with a reasonable margin for noise. Thus the following recommendations for distance and wire gauge
should be observed:
|------------------------------------------------------|
¦ Distance ¦ Wire Gauge ¦ Recommended Cable ¦
|------------------------------------------------------¦
¦ 4000 ft. ¦ 24 AWG ¦ Belden 9729/9842 ¦
¦ 6000 ft. ¦ 22 AWG ¦ Belden 9184 ¦
|------------------------------------------------------|
See 3.1.3 para. 7 and 3.1.4 for further recommendations.
The use of a shield depends on the noise environment and grounding considerations. The above cables are shielded.