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Recommendations for RS-485 LAN communications wiring

Issue

Recommendations for RS-485 LAN communications wiring

Environment

I/NET C-LAN, Sub-LAN

Cause

A review of some of the I/NET manuals yielded several instances of the following description:

The same communications cable will support a combination of 32 Application Specific Controllers (ASCs), Micro Regulators (MRs) and Door Processing Units (DPUs), connected together in a "multi-dropped" arrangement, as illustrated in Figure 2. The cable should be run in a contiguous node-to-node arrangement without stubs or branches.

Resolution

We recommend the RS485 LAN (Including the Host LAN, Controller LAN & Sub-LAN) be arranged in a contiguous node-to-node bus configuration without stubs and within the maximum 4000' (1210 m) / 5000' (1500 m) length with 24/22 AWG low capacitance twisted pair shielded cable.

7808 Repeater

For the RS485 Token Passing Controller LAN (C-LAN), the model Repeater can be used to branch (stub) off the main LAN bus with a C-LAN bus extension. The 7808 Repeater acts as a single RS485 node on both of the C-LAN segments with both being wired in a contiguous node-to-node bus arrangement. Additional 7808 Repeaters can be applied along the main LAN to distribute additional branches (if needed). The distribution of Stubs/Branches along the main LAN can continue up to a maximum of 32 RS485 nodes (in total) along any one copper segment. There can be another 32 RS485 nodes on the copper segment on the other side of the 7808 Repeater.

The 7808 Repeater provides the vehicle to -

  1. Stub off the main C-LAN
  2. Split the main C-LAN distance when length is greater than 4000' (1210 m)
  3. Split the main C-LAN cable length when inferior cable (higher capacitance) is introducing excessive distortion
  4. Divide the RS485 node count to a maximum of 32 RS485 nodes on any single segment of copper. I.e. the 7808 provides a separate copper segment on each of the two sides with full RS485 node count (32) and distance specifications

Repeated/Tiered Stubbing

The 7808 Repeater can further accommodate stubbing off of the stubbed C-LAN segment as long as no more than four 7808 Repeaters separate any two C-CLAN controllers on the controller LAN network. I.e. looking from any 77xx controller out to all of the other 77xx controllers on the same controller LAN, you must not have to go through more than four 7808 Repeaters to reach any of the other controllers (regardless of whether the application actually has any interaction with the other controllers).

Old Commentary on C-LAN Tolerance

Dating back to the first release and field applications of the Series 6000 followed by Series 7000 controllers, there have been many instances where our Branches and Partners have needed to use exiting wiring in a facility. Inevitably, either the cable characteristics were not as specified (low capacitance data grade twisted pair), or the cable characteristics were not known, or the cable's geographic routing was not conducive to a node-to-node bus configuration. The economic impact of using existing cabling was always a magnet that attracted the project engineer and had to be balanced with a perspective of risk vs. reward. In many cases the existing cables made a node-to-node bus difficult, or impossible to achieve, without rewiring the facility. In some instances, the bus could be accommodated but would inject a large additional copper length into the main bus with the cable run to and from the distant controller location, before proceeding along the bus. With the prescribed cable, the preference is for the bus. But in forced situations where the cable isn't what it should be, it may be better to have a 250' stub than inject 500' of bad cable in the middle of the main LAN segment, with the preferred accommodation being the addition of a 7808 repeater to handle (isolate) the stub. The 7808 Repeater was created specifically to resolve such installation problems, and in most cases it is indeed installed after the fact as remedy for a problem instead of a initial installation and design of the system.

Early in the product life this issue/question on use of existing cabling prompted investigation on my part regarding potential guidelines as to where deviations in the recommended LAN configuration could possibly be tolerated. At the 19,200BPS data rate we are running on the C-LAN, there is a significant amount of tolerance for reflected wave distortion from impedance and loading mismatches on the cable. This creates a scenario where the installer can frequently get away with "minor" violation of one of the specified cabling constraints, but doing such is exhausting at least some portion of the system's tolerance for communications distortion and degradation. The problem is that continued successful violation of one recommendation is sometimes molded into what is thought to be standard practice and frequently followed by other and sometimes more subtle violation(s) and the system becomes intermittent because the tolerable margin of error/distortion has already long been exhausted.

Standard LAN Installation Recommendations

  • Designated Low Capacitance Cable 
  • Cable Impedance of 100-150 ohms (120 ohm = RS485 standard and preferred)
  • Conductor Gage of 22 or 24 AWG - Data Grade twisted pair with shield
  • Specified maximum of 32 nodes on a copper LAN segment
  • Maximum cable length (4000' (1210 m) for 24AWG)
  • Node-to-node bus configuration without stubs
  • Pass through connection of the cable shield at each controller location
  • Earth ground connection on cable shield at one end only
  • Earth ground connection to each RS485 controller on the designated ground terminal/conductor

Any comments regarding I/NET and the subject of "Ts and Stars being allowed" has to be an interpretation of possible earlier commentary by myself and/or others at TCON, Schneider Electric regarding what the I/NET C-LAN system will "typically" tolerate. I.e. what you can probably get away with, if you must. The recommended RS485 wiring approach is that the LAN be configured in a node-to-node bus configuration using the specified cable characteristics, distance limits and node counts and the 7808.

Repeaters can be used when stubs or distance extensions or node count requires. When economics of rewiring a facility inject an elevated tolerance for risk, it has been said that the C-LAN system will typically tolerate up to 10 stubs of up to 250 ft ("when other specs are not being violated"). That is a guideline of typical C-LAN tolerance and not a system installation recommendation or approval. The 7808 Repeater has often served as an insurance policy for some who have found the economic need to stub, and in some instances ran into subsequent problems. In other words, they injected stubs when needed and then added Repeaters later if needed to resolve a problem caused by the stub(s).

Note that the 7808 Repeater is applicable only to the RS485 Host-LAN and C-LAN, and there is no such post installation solution for the Sub-LAN. The distances are typically shorter on the Sub-LAN and therefore typical tolerance in other areas is higher, but the recommendations remain the same.

Termination

In most cases the RS485 LANs on I/NET are not being terminated. This is also a tolerance associated with the I/NET RS485 protocol and speed and is a functional issue regarding the 7808 Repeater and the Sub-LAN's SLI (7790A or 7798). The 7808 Repeater depends on C-LAN biasing and is not tolerant of termination on the C-LAN. The C-LAN can indeed be terminated with a 120 ohm resistor at each end of the C-LAN "BUS", when 7808 Repeaters are not being used. This can be a first step in resolving a C-LAN communications problem prior to introducing 7808 Repeaters. The 120 ohm 1/2 watt termination will reduce distortion on the C-LAN and improve noise immunity.

The Sub-LAN will not tolerate 120 ohm termination due to the 2.2K ohm biasing of the Sub-LAN required at the 7790/7798 controller. You will have to use a 620 ohm 1/2 watt resistor on the Sub-LAN systems

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Last update:
‎2018-09-06 01:35 PM
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