TCP/IP Ethernet verses RS 485/232


Thread Starter

viju Pullan

This is in the context of Networking Fuel Dispenser to the Fuel Retail Outlet Automation Controller.

Q1. Wanted to know the advantages of TCP/IP over RS485/RS232,

Q2. What is the continuous length of the signal cable that can be used without boosters

Q3. I understand Multiple Dispensers can be connected serially on the same cable
1) RS-232 is point-to-point only and short distance, typically 20m. Not generally useful for 'multiple dispensers' without multiple serial ports and associated protocol and application software.

2) An appropriate comparison is ethernet to RS-485

100m distance, rates to 100Mb or 1Gb, hub and spoke topology, transformer coupled signals minimizes common mode issues.

Distances to 1000m, rates dependent on distance, can range from the low hundreds/Kb at long distance to 1.5Mb at short distances, daisy chain/multidrop topology, balanced differential line but subject to common mode and bias issues.

"Serial servers" are a class of devices that can convert and re-package 485 data for TCP/IP passage across ethernet.

If given the choice and distance is reasonable, I'll pick ethernet because of its robustness due to transformer coupling.

RS-485 can have a "bus" type multi-drop connection (multiple connections to the same cable), while modern Ethernet uses a "hub" type connection. While that sounds like an advantage for RS-485, a "hub and spoke" installation is often a lot easier in practice when you look at where you will run the cables. You can typically run a single Ethernet cable out to a switch located at a cluster of nodes, and then run another cable from that switch out to another switch at another cluster of nodes, etc. There's a limit to how many "hops" you can do that for, but whether that is a problem in practice depends on your physical layout.

RS-485 wiring and termination problems are a routine question that we get here, so the problems with RS-485 are probably quite common. Ethernet seems to be less prone to installation problems. However, if you are not using off the shelf Ethernet cables, make sure the wiring contractor connects the right wires to the right pins! They can sometimes hook it up wrong and things will still mostly work. The problems usually relate to mixing and matching wires from different pairs instead of keeping the pairs together.

As David mentioned, "serial servers" or "terminal servers" are a common way of connecting serial devices over Ethernet, so you can often use serial devices with Ethernet. If you are running your wiring a long distance outdoors, then you might want to look at fibre optic cable, as that may give you better resistance to lightning (you will need to ask someone else about that though).

If I was putting in a new installation today and I had a choice, I would prefer Ethernet and TCP/IP over RS-485. I suspect that RS-485 will eventually disappear from the market, while I expect Ethernet to be around for a very long time to come.