RS 485

[oldsparky52]

Bennett pump has a requirement that metal conduit be used in order for their warranty to be honored. I am assuming that it has something to do with their communication circuits (for data) between the control console and the fuel dispensers.

PVC conduit is going to have to be used at this project. I don't really know much about a 485 communication circuit, but it appears it's a 2-wire communication circuit with a ground reference wire as the 3rd wire of a 3-wire communication circuit.

I'm asking y'all what would be the best way to minimize noise on the 485 circuit? A twisted-pair cable and a separate ground reference conductor? A 3-wire shielded cable? Would the 3-wire cable need to have all 3 conductors twisted, if so, what is the twist per foot recommendation?

Is it better to be an unshielded cable?

The manufacturer will not discuss this, they just say we have to use metal conduit. Since the comm wires are in with the dispenser power and control wires, the requirement for metal conduit confuses me.

 

[gar]

161031-1611 EDT

What I would use is a Belden cable consisting of a twisted pair with Beldfoil shielding. This provides a shielded cable with a non-shorting foil shield and drain wire ( drain wire is in electrical contact with the shield ). The drain wire provides your common wire. The non-shorting shield prevents a shorted turn around the cable. This is accomplished by using aluminum foil anodized on one side as the shield. Shielding reduces capacitively coupled noise. The twisted pair reduces magnetic coupling of noise.

If neither end of your communication path has galvanic isolation, then I would be concerned. Galvanic isolation means no DC path and very little AC coupling, possibly less than 100 pfd. The drain wire usually should be grounded at one end. If isolation is provided at one end only, then the shield should be only grounded at the non-isolated end.

A metal conduit may degrade your signal level. So it is a trade off between signal level and noise isolation. Short distances are probably OK.

.

 

[GoldDigger]

In theory the data lines in RS-485 use a fully differential signal, so noise coupled in by an electric field should cancel out from the differential signal between the two data lines.
And as long as the common mode voltage limit is not exceeded any common mode offset such as a difference in "ground" levels between the sending and receiving end should also cancel out.
But you do not want to connect the shield at both ends if there is a significant difference in ground (EGC, chassis) potential between the two ends or excess current may flow in the wire.
The situation is much different with RS-232 which uses a single ended signal and almost requires galvanic isolation between terminals in different areas.

 

[wallyworld]

485 on pump data or card readers?

 

[hbiss]

Aren't fuel dispensers considered a classified location that require GRC with seal off fittings? No PVC allowed.

-Hal

 

[wallyworld]

Aren't fuel dispensers considered a classified location that require GRC with seal off fittings? No PVC allowed.

-Hal

There is an exception for PVC. It has to stop 2 ft before emerging from ground, rigid from that 2 ft setback, seals, etc

 

[oldsparky52]

485 on pump data or card readers?

dispenser communication (no card readers)

POS will be supplied by a different vendor/manufacturer than is supplying the dispensers ..... lucky me, lol

 

[oldsparky52]

Aren't fuel dispensers considered a classified location that require GRC with seal off fittings? No PVC allowed.

-Hal

The diesel is allowed to be not classified if it is out of the classified area of the gasoline. This is on a floating dock. So PVC to the diesel dispensers and as soon as the gas conduit is out of the classified area, slap on a sealoff and pulling C and switch to PVC.

 

[wallyworld]

dispenser communication (no card readers)

POS will be supplied by a different vendor/manufacturer than is supplying the dispensers ..... lucky me, lol

I've got a Bennett on a dock with PVC conduit. No POS though. Pump is piece of crap if that helps

 

[oldsparky52]

:lol:

I've got a Bennett on a dock with PVC conduit. No POS though. Pump is piece of crap if that helps

Actually, I've seen the big squirt really give some great flow rates. Gilbarco and Wayne don't really have anything designed (at least nothing I've seen) for these flow rates. PMC down in GA does, but they are high end and pricey. The Bennett is a cost effective high flow unit. They use M1 meters and 1.25" - 1.5" internal piping. Heard of one getting 45 GPM with 2" flex pipe running about 500' from an A/G tank using a 2 HP high pressure FE Petro pump with a mechanical leak detector.

 

[wallyworld]

Wayne and gilbarco both make dispensers with LC meters, essentially the same meter as a fuel oil truck. I'd take either over bennett. It seems bennett are popular on docks, not sure why?

 

[oldsparky52]

Wayne and gilbarco both make dispensers with LC meters, essentially the same meter as a fuel oil truck. I'd take either over bennett. It seems bennett are popular on docks, not sure why?

Got some model numbers?

 

[wallyworld]

Got some model numbers?

Not familiar with gilbarco numbers , but have seen lc meters in encore 500s and encore 750s. last wayne I saw was hs1v or hs3v. Narrow body high hose. They may not make those now as they changed from Vista to ovations and some other catchy name. But they still make a hi speed diesel with LC meters in them. You can get them with satelites, so you have a master with a sat. or just a master. You can also get Def dispensers attached to either wayne or gilbarco. I just looked at wayne site, I guess they still make HS Vistas

 

[K8MHZ]

The manufacturer will not discuss this, they just say we have to use metal conduit.

Are you talking about the Bennett Pump in Muskegon?

If so, I am surprised to hear they won't try to work with you. I looked up the address to see if they were still a Muskegon company. It looks like they moved their offices from Muskegon Heights to Pontaluna Road in Spring Lake, to there may have been some management changes, too.

I am also saddened to hear that someone thinks a Bennett Pump is a piece of crap. Bennett used to be top quality all the way around. Remember the air pumps with the little handle you turned to get the PSI and it would stop at that PSI? They were called ECO Tireflators and were all made at Bennett in Muskegon from 1937 on. Those were tough little buggers.