Watlow 8LS Controller - Manual (Page 42)

34 8LS User’s Guide
Installation
RS-485 Description And Connections
The RS-485 is a voltage balanced long distance multi-point
transmission interface. It may use 2 or 4 lines depending on system
requirements. The 8LS uses four lines [two lines can be accommodated
on special order -- contact Watlow Anafaze].
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 Watlow 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 Watlow Anafaze equipment.
The maximum signaling rate used by the Watlow Anafaze 8LS 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. Watlow Anafaze has presented conservative
recommendations to insure reliable operation. If deviations are
necessary, please contact Watlow Anafaze.
Cable Recommendations
We recommend 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
Ω
for a properly terminated line with a reasonable margin for noise.
Thus the following recommendations for distance and wire gauge
should be observed:
The use of a shield depends on the noise environment and grounding
considerations. The above cables are shielded.
Distance Wire Gauge Recommended Cable
4000 ft. 24 AWG Belden # 9729/9842
6000 ft. 22 AWG Belden # 9184