Longer HP-IL cable runs

[Paul Berger (Canada)]

(02-18-2018 08:05 PM)rprosperi Wrote:  

(02-18-2018 06:58 PM)Paul Berger (Canada) Wrote:  Gigabit ethernet uses all 4 pairs in the cable, and CAT 5/5e/6 cables is unshielded. Ethernet uses differential signalling to reduce the effects of noise.

So, do you think these (CAT5/5E/6) are usable for HP-IL, or are you saying not suitable? Also, I think you mean the individual pairs are unshielded, not the entire cable, right?

I was going to mention Gbit Ethernet and thought "well, very few folks use this at home anyhow so why bother" but even that is increasingly untrue. With prices for Gbit routers and devices dropping every day, it pays to use them.

I think Cat5 like cables would be fine and as you said you can use two pairs in the cable for the connections in each direction if you happen to have a long lobe on the loop. HPIL uses differential signalling so has some built in resistance to noise. The characteristic impedance of Cat 5 is specified as 100 ohms which is the same as HP-IL cables according to "The HP-IL Interface Specification". This manual also gives information on testing cables.

No the entire cable is unshielded it is simply 4 twisted pairs inside a plastic tube. In ethernet you do not get into shielded cables until 40 and 100 Gb wiring, but at those speeds optical fibre would seem to be more common. Cables like this are designed to be used in systems that use differential signalling over a pair of wires. In differential signalling the receiver is looking at the difference in signal level between the two wires in the pair, unlike something like RS-232 where the receiver is looking at the the signal level on a single wire relative to ground. Another common differential system is RS-422. The main advantages of differential signalling are 1. It is not ground referenced so ground shift between sender and receiver is of no concern and 2. because the conductors in a pair are in close proximity, noise impulses will affect both conductors equally without changing the difference in signal level between the two conductors in the pair, giving it very good noise immunity. The place where I have seen people get into trouble with cabling like Cat 5 is when they try using it for connections using single ended (ground referenced) signalling such as RS-232 where the lack of shielding makes it subject to induced noise plus when the two wires in a twisted pair are carrying different signals, cross talk between wires can cause problems. This is why a RS-232 cable is shielded and not twisted pairs plus it uses a connector that has a pin designated for grounding that shield. I have even seen RS-232 cables that have a foil shield around each individual conductor and then a braided shield around the whole bundle.

In my experience you can hardly buy anything these days that supports wired ethernet that is not Gb capable. This is likely due to chip manufacturers only building Gb and higher chips these days. I have seen instances where Gb controllers where used, but only 100Mb was supported due to the PHY used not supporting Gb.

Paul.