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Recommended RS485 connector for Cat 5 Cable - Not RJ45

K

Kevin

Jan 1, 1970
0
I want to build a RS485 network using Cat 5 cable. But since the RS485
specs don't specify connector types I'm at a loss to figure out what I
should use with (easily obtained) Cat5 cable.

Some of the remote nodes attach to equipment that is on wheels (ease
of moving for cleaning, maintainance etc). So I'm thinking that the
cables and connectors may get damaged over time. I would like to use
cables and connectors that are cheap and easily replaceable when
damaged. (Yes, I know the best thing is to ensure that they don't get
damaged in the 1st place, but this is the real world I'm talking
about).

This is half duplex RS485 communication, so it only requires 2 diff.
signal wires and ground. (A total of 3 wires).

I don't want to use a RJ45 connector on the Cat5 cable, as somebody
might think it's an Ethernet network cable and plug it into a laptop
or something.

It occurred to me that RJ11 cable has 4 wires, and might be the
solution to my problem.

What I was thinking of is to run Cat5 as close to each machine as
possible and connect it to a plain old surface mount modular RJ11
telephone box (you know, like what your phone cord plugs into). Then I
could use a standard RJ11 telephone cable (like what connects your
modem to the phone line) to hook the machine up to the network. (The
machine would also have a modular RJ11 telephone box which would be
connected to the RS485 transceiver of the electronics that I'll
design).

This might be more easily understood by this diagram:

Cat5 Cheap Phone Cable Inside Machine
---------[Phone Box]====================[Phone Box]--------------
Cat5 here?

The link cable (phone cable with RJ11 ends) and connectors (RJ11 phone
boxes) are cheap and easily obtained and replaceable.

Now I know that this will probably mess up the differential signal a
bit, since the RJ11 phone cable is not twisted pair. But say the phone
cable is less than 10 feet - will it make a difference?

Questions:

1. Is this workable? If not why? (Communication will be done at
9600bps).

2. What about if I use a slew-limited RS485 driver on all nodes to
help ensure signal quality (minimise reflections etc). Will I be able
to do this RJ11 thing then?

3. Are there any other cheap connectors and cabling that I can use for
Cat5 to run it directly to each machine? What type of connector do you
guys use when you do RS485 networks and how do you handle the physical
wiring arrangement? (Digikey #s appreciated if possible).

Thanks for any advice/suggestions.
 
A

Anders F

Jan 1, 1970
0
Kevin said:
I want to build a RS485 network using Cat 5 cable. But since the RS485
specs don't specify connector types I'm at a loss to figure out what I
should use with (easily obtained) Cat5 cable.

Some of the remote nodes attach to equipment that is on wheels (ease
of moving for cleaning, maintainance etc). So I'm thinking that the
cables and connectors may get damaged over time. I would like to use
cables and connectors that are cheap and easily replaceable when
damaged. (Yes, I know the best thing is to ensure that they don't get
damaged in the 1st place, but this is the real world I'm talking
about).

This is half duplex RS485 communication, so it only requires 2 diff.
signal wires and ground. (A total of 3 wires).

I don't want to use a RJ45 connector on the Cat5 cable, as somebody
might think it's an Ethernet network cable and plug it into a laptop
or something.

I like RJ45 as it enables me to use standard patch cables - and easily
terminate all conductors when making custom stuff... The connectors are crap
when lying around (the tab dies) so if one needs a more sturdy construction
I'd opt for the XLR-style RJ45 or perhaps the Harwin (or similar) industrial
ones.
It occurred to me that RJ11 cable has 4 wires, and might be the
solution to my problem.

What I was thinking of is to run Cat5 as close to each machine as
possible and connect it to a plain old surface mount modular RJ11
telephone box (you know, like what your phone cord plugs into). Then I
could use a standard RJ11 telephone cable (like what connects your
modem to the phone line) to hook the machine up to the network. (The
machine would also have a modular RJ11 telephone box which would be
connected to the RS485 transceiver of the electronics that I'll
design).

Not a problem if it's short. And definitely not at 9600 baud. If it's a
noisy environment you might miss the shield though....
2. What about if I use a slew-limited RS485 driver on all nodes to
help ensure signal quality (minimise reflections etc). Will I be able
to do this RJ11 thing then?

Always use slew rate limited parts when possible!
3. Are there any other cheap connectors and cabling that I can use for
Cat5 to run it directly to each machine? What type of connector do you
guys use when you do RS485 networks and how do you handle the physical
wiring arrangement? (Digikey #s appreciated if possible).

I use micromatch and ribbon cable!

/Anders
 
J

James Beck

Jan 1, 1970
0
I want to build a RS485 network using Cat 5 cable. But since the RS485
specs don't specify connector types I'm at a loss to figure out what I
should use with (easily obtained) Cat5 cable.

Some of the remote nodes attach to equipment that is on wheels (ease
of moving for cleaning, maintainance etc). So I'm thinking that the
cables and connectors may get damaged over time. I would like to use
cables and connectors that are cheap and easily replaceable when
damaged. (Yes, I know the best thing is to ensure that they don't get
damaged in the 1st place, but this is the real world I'm talking
about).

This is half duplex RS485 communication, so it only requires 2 diff.
signal wires and ground. (A total of 3 wires).

I don't want to use a RJ45 connector on the Cat5 cable, as somebody
might think it's an Ethernet network cable and plug it into a laptop
or something.

It occurred to me that RJ11 cable has 4 wires, and might be the
solution to my problem.

What I was thinking of is to run Cat5 as close to each machine as
possible and connect it to a plain old surface mount modular RJ11
telephone box (you know, like what your phone cord plugs into). Then I
could use a standard RJ11 telephone cable (like what connects your
modem to the phone line) to hook the machine up to the network. (The
machine would also have a modular RJ11 telephone box which would be
connected to the RS485 transceiver of the electronics that I'll
design).
We have an RS-485 network that uses CAT5 cables, and have yet to have
anyone plug a laptop into it (that I know of), and at 1000 plus networks
installed so far, that's pretty good. I think the most important thing
here is that I don't see that attaching an ethernet device to the net
would do anything other than not work and possibly drag down the RS-485
network. We use slew rate limited, failsafe, with short circuit
protection drivers and everything has been goin along great. I know it
is always a good idea to design the possibility of an idiot doing
something dumb out of a system, but in this case I really don't see it
as a problem. If you are REALLY worried about it have get some cable
tags and mark the CAT5's with RS-485 or maybe some scarely warning about
hooking to computers.

Jim
 
M

Mac

Jan 1, 1970
0
I want to build a RS485 network using Cat 5 cable. But since the RS485
specs don't specify connector types I'm at a loss to figure out what I
should use with (easily obtained) Cat5 cable.

Some of the remote nodes attach to equipment that is on wheels (ease
of moving for cleaning, maintainance etc). So I'm thinking that the
cables and connectors may get damaged over time. I would like to use
cables and connectors that are cheap and easily replaceable when
damaged. (Yes, I know the best thing is to ensure that they don't get
damaged in the 1st place, but this is the real world I'm talking
about).

This is half duplex RS485 communication, so it only requires 2 diff.
signal wires and ground. (A total of 3 wires).

I don't want to use a RJ45 connector on the Cat5 cable, as somebody
might think it's an Ethernet network cable and plug it into a laptop
or something.

I don't think this would harm anything. It definitely won't harm typical
RS-485 drivers, which are robust. I believe it is unlikely to harm any
ethernet device either, but I suppose there are no guarantees.
Remember, 100 baseT ethernet doesn't use all the wires, and 1000 baseT
requires transformer coupling on all pairs, so I would think you could
arrange a safe pinout for RJ-45.
It occurred to me that RJ11 cable has 4 wires, and might be the
solution to my problem.

What I was thinking of is to run Cat5 as close to each machine as
possible and connect it to a plain old surface mount modular RJ11
telephone box (you know, like what your phone cord plugs into). Then I
could use a standard RJ11 telephone cable (like what connects your
modem to the phone line) to hook the machine up to the network. (The
machine would also have a modular RJ11 telephone box which would be
connected to the RS485 transceiver of the electronics that I'll
design).

If you haven't designed the electronics yet, maybe you would consider
using LVDS instead of RS-485 levels. Then you can use RJ45 with no fear of
damaging ethernet should someone inadvertently plug-in their laptop.
This might be more easily understood by this diagram:

Cat5 Cheap Phone Cable Inside Machine
---------[Phone Box]====================[Phone Box]--------------
Cat5 here?

The link cable (phone cable with RJ11 ends) and connectors (RJ11 phone
boxes) are cheap and easily obtained and replaceable.

Now I know that this will probably mess up the differential signal a
bit, since the RJ11 phone cable is not twisted pair. But say the phone
cable is less than 10 feet - will it make a difference?

Not at 9600 baud.
Questions:

1. Is this workable? If not why? (Communication will be done at
9600bps).

Yes.

2. What about if I use a slew-limited RS485 driver on all nodes to
help ensure signal quality (minimise reflections etc). Will I be able
to do this RJ11 thing then?

Yes.

3. Are there any other cheap connectors and cabling that I can use for
Cat5 to run it directly to each machine? What type of connector do you
guys use when you do RS485 networks and how do you handle the physical
wiring arrangement? (Digikey #s appreciated if possible).

I like the idea of using Cat5 with RJ-45 or RJ-11. One thing you should
know is that you can plug a regular telephone cord into an RJ-45 socket.
This is what we do where I work (I mean the telephone jacks are RJ-45,
but we plug regular phone cords into them). So you could terminate the
CAT5 network in regular RJ-45 wall plates, and then go from the wall to
the RJ-11 in your device with ordinary telephone cords. You might want to
use the inner two wires for ground, and use the outer two wires for your
signal. This will avoid any conflict if someone plugs the cable into a
telephone. It would still allow the possibility of someone plugging a
laptop into the wall, but only if they have their own patch cord.
Thanks for any advice/suggestions.

You're welcome. I hope I helped!

--Mac
 
K

Kevin

Jan 1, 1970
0
I like the idea of using Cat5 with RJ-45 or RJ-11. One thing you should
know is that you can plug a regular telephone cord into an RJ-45 socket.

Really? Never tried this... interesting!
This is what we do where I work (I mean the telephone jacks are RJ-45,
but we plug regular phone cords into them). So you could terminate the
CAT5 network in regular RJ-45 wall plates, and then go from the wall to
the RJ-11 in your device with ordinary telephone cords. You might want to
use the inner two wires for ground, and use the outer two wires for your
signal. This will avoid any conflict if someone plugs the cable into a
telephone. It would still allow the possibility of someone plugging a
laptop into the wall, but only if they have their own patch cord.

Thanks. I will test this myself. But is it reliable? (after all the
RJ45 jack is larger than a RJ11 connector, so doesn't the RJ11
connector shift around or can potentially move while in the jack,
possibly breaking the connection?) What happens if someone pulls on
the RJ11 cable while inside the RJ45 jack? Does it pull out relatively
easily or sticks in the jack and the connector rips off the cable
first?

Here's another question: are there such things as *surface mounted*
RJ45 jacks (instead of wall plates)? By this I don't mean "surface
mount" as in "DIP vs SMT" package but RJ45 jacks that come in a
similar box like modular telephone boxes that can be screwed down to
any surface?

And how about topology? The recommended topology for RS485 is a bus,
but in the real world connections can sometimes be more conveniently
made if one uses a star topology (which from most RS485
documention/app notes discourages). How do you run RS485 in the real
world where things can be moved around somewhat and you need to have
some flexibility to rearrange cable as needed?
 
R

Rich Grise

Jan 1, 1970
0
I want to build a RS485 network using Cat 5 cable. But since the RS485
specs don't specify connector types I'm at a loss to figure out what I
should use with (easily obtained) Cat5 cable.

Some of the remote nodes attach to equipment that is on wheels (ease
of moving for cleaning, maintainance etc). So I'm thinking that the
cables and connectors may get damaged over time. I would like to use
cables and connectors that are cheap and easily replaceable when
damaged. (Yes, I know the best thing is to ensure that they don't get
damaged in the 1st place, but this is the real world I'm talking
about).

This is half duplex RS485 communication, so it only requires 2 diff.
signal wires and ground. (A total of 3 wires).

I don't want to use a RJ45 connector on the Cat5 cable, as somebody
might think it's an Ethernet network cable and plug it into a laptop
or something.

I don't think this would harm anything. It definitely won't harm typical
RS-485 drivers, which are robust. I believe it is unlikely to harm any
ethernet device either, but I suppose there are no guarantees.
Remember, 100 baseT ethernet doesn't use all the wires, and 1000 baseT
requires transformer coupling on all pairs, so I would think you could
arrange a safe pinout for RJ-45.
It occurred to me that RJ11 cable has 4 wires, and might be the
solution to my problem.

What I was thinking of is to run Cat5 as close to each machine as
possible and connect it to a plain old surface mount modular RJ11
telephone box (you know, like what your phone cord plugs into). Then I
could use a standard RJ11 telephone cable (like what connects your
modem to the phone line) to hook the machine up to the network. (The
machine would also have a modular RJ11 telephone box which would be
connected to the RS485 transceiver of the electronics that I'll
design).

If you haven't designed the electronics yet, maybe you would consider
using LVDS instead of RS-485 levels. Then you can use RJ45 with no fear of
damaging ethernet should someone inadvertently plug-in their laptop.
This might be more easily understood by this diagram:

Cat5 Cheap Phone Cable Inside Machine
---------[Phone Box]====================[Phone Box]--------------
Cat5 here?

The link cable (phone cable with RJ11 ends) and connectors (RJ11 phone
boxes) are cheap and easily obtained and replaceable.

Now I know that this will probably mess up the differential signal a
bit, since the RJ11 phone cable is not twisted pair. But say the phone
cable is less than 10 feet - will it make a difference?

Not at 9600 baud.
Questions:

1. Is this workable? If not why? (Communication will be done at
9600bps).

Yes.

2. What about if I use a slew-limited RS485 driver on all nodes to
help ensure signal quality (minimise reflections etc). Will I be able
to do this RJ11 thing then?

Yes.

3. Are there any other cheap connectors and cabling that I can use for
Cat5 to run it directly to each machine? What type of connector do you
guys use when you do RS485 networks and how do you handle the physical
wiring arrangement? (Digikey #s appreciated if possible).

I like the idea of using Cat5 with RJ-45 or RJ-11. One thing you should
know is that you can plug a regular telephone cord into an RJ-45 socket.
This is what we do where I work (I mean the telephone jacks are RJ-45,
but we plug regular phone cords into them). So you could terminate the
CAT5 network in regular RJ-45 wall plates, and then go from the wall to
the RJ-11 in your device with ordinary telephone cords. You might want to
use the inner two wires for ground, and use the outer two wires for your
signal. This will avoid any conflict if someone plugs the cable into a
telephone. It would still allow the possibility of someone plugging a
laptop into the wall, but only if they have their own patch cord.
Thanks for any advice/suggestions.

You're welcome. I hope I helped!

Would it make any sense to use 6-pin RJ-12s? They're a different size than
either the RJ11 (4-pin) or the RJ45 (8-pin) - this could reduce the
likelihood of plugging it into the wrong jack.
http://www.google.com/search?q=6-pin+RJ+connector

Good Luck!
Rich
 
M

Mac

Jan 1, 1970
0
Really? Never tried this... interesting!

I didn't believe it at first, either. My company recently moved to a
brand-new building, and the guy who installed all of the telephone and
Ethernet cable and jacks said that he was going to use RJ-45 for the
telephone. I immediately tried it out, and sure enough, it works just
fine. If you inspect the connector and socket, you'll see that grooves
kind of hold the connector in the socket laterally, and ridges separate
the individual conductors. I don't think there are any mechanical or
electrical problems with doing this.
Thanks. I will test this myself. But is it reliable? (after all the
RJ45 jack is larger than a RJ11 connector, so doesn't the RJ11
connector shift around or can potentially move while in the jack,
possibly breaking the connection?) What happens if someone pulls on
the RJ11 cable while inside the RJ45 jack? Does it pull out relatively
easily or sticks in the jack and the connector rips off the cable
first?

Only the width dimension differs. The retention tab in the center insures
that the plug will be lined-up correctly, and the individual ridges keep
the connector from wiggling around or coming out any more easily than it
would from an RJ-11 socket. Hold an Ethernet patch cable up right next to
a phone cord. You'll see what I'm talking about.
Here's another question: are there such things as *surface mounted*
RJ45 jacks (instead of wall plates)? By this I don't mean "surface
mount" as in "DIP vs SMT" package but RJ45 jacks that come in a
similar box like modular telephone boxes that can be screwed down to
any surface?

I don't know.
And how about topology? The recommended topology for RS485 is a bus,
but in the real world connections can sometimes be more conveniently
made if one uses a star topology (which from most RS485
documention/app notes discourages). How do you run RS485 in the real
world where things can be moved around somewhat and you need to have
some flexibility to rearrange cable as needed?

I honestly don't know. My gut feeling is that with 9600 baud, and if
the wires are not too long, you MIGHT get away with star routing. It
would be nice to simulate the whole network. For your purposes, the
cat5 cable could be simulated as two 50-Ohm transmission lines with a
common ground. The phone cable might be more like two 40-Ohm lines with a
common ground, but that is just a guess.

If you don't want to simulate it, and it has to work the first time,
it would be safer to either have one long transmission line with
short pigtails for each device, and termination only at the two ends, or
a star topology with a dedicated transceiver for each node. That would
only work if there is some central resource who can control the transmit
receive direction, and if the devices don't need to talk to each other,
but only to the central resource.

Oh, actually, if you don't want to use pigtails, then each device could
have an "in" and an "out." This would make it a "daisy-chain" type of
configuration. This would mean that every device would have to be on all
the time, unless you work out some kind of automatic bypass scheme.

--Mac
 
K

Kevin

Jan 1, 1970
0
If you don't want to simulate it, and it has to work the first time,
it would be safer to either have one long transmission line with
short pigtails for each device, and termination only at the two ends, or
a star topology with a dedicated transceiver for each node. That would
only work if there is some central resource who can control the transmit
receive direction, and if the devices don't need to talk to each other,
but only to the central resource.

In my case this is how the network would work. Master node requests
data from a slave node. Slaves don't need to talk directly to one
another (keeps things simple and reliable).

I understand the long transmission line concept - a long cable
(terminated at both ends) with a whole bunch of taps in the middle
(where individual node cables connect to), but what is a practical way
to actually connect nodes to the main Cat5 cable with this approach?

How do you actually make the drop connections to each slave node
without doing something like stripping the main Cat5 cable at various
points and soldering another Cat5 cable to make the drop connection?
(to make the solder connection I'd probably have to untwist the wire
which I guess wouldn't be a good thing if done a couple dozen times
along the wire). Actually the more I think about it I don't think it
would be realistic to even attempt to strip and solder Cat5 (at least
I don't want to do it!).

I guess this is where a daisy chain config would make life easier -
cable plugs into first piece of equipment, then another cable plugs
into the first and then into the second etc. That way, I would only
have to worry about making cables and not interfering with the main
cable (in order to make drop connections). The only thing with the
daisy chain approach is a bad or unplugged cable can bring down the
whole network since a fault effectively breaks the terminated
transmission line.

I need assistance with the realities of running Cat5 cable and making
connections - what works and what doesn't for RS485?
 
M

Mac

Jan 1, 1970
0
In my case this is how the network would work. Master node requests
data from a slave node. Slaves don't need to talk directly to one
another (keeps things simple and reliable).

How many total devices do you have? Would using a dedicated transceiver
for each device be feasible?
I understand the long transmission line concept - a long cable
(terminated at both ends) with a whole bunch of taps in the middle
(where individual node cables connect to), but what is a practical way
to actually connect nodes to the main Cat5 cable with this approach?

How do you actually make the drop connections to each slave node
without doing something like stripping the main Cat5 cable at various
points and soldering another Cat5 cable to make the drop connection?
(to make the solder connection I'd probably have to untwist the wire
which I guess wouldn't be a good thing if done a couple dozen times
along the wire). Actually the more I think about it I don't think it
would be realistic to even attempt to strip and solder Cat5 (at least
I don't want to do it!).

No, I wouldn't use any solder. There are junction box type things out
there for dealing with cat 5 cable (I'm not a cable guy, so I can't give
you better guidance than that). The idea is that at each drop, you
cut the cable, and use screw terminals or something to join the
cable together. If you can't find RJ-45 versions, you can certainly
find RJ-11 versions of these types of junction boxes. I don't think
putting breaks in the wire and short (a few inches) untwisted portions
will significantly effect the transmission line impedance in your
application.
I guess this is where a daisy chain config would make life easier -
cable plugs into first piece of equipment, then another cable plugs
into the first and then into the second etc. That way, I would only
have to worry about making cables and not interfering with the main
cable (in order to make drop connections). The only thing with the
daisy chain approach is a bad or unplugged cable can bring down the
whole network since a fault effectively breaks the terminated
transmission line.

Yeah. That is the downside.
I need assistance with the realities of running Cat5 cable and making
connections - what works and what doesn't for RS485?

I don't have any practical experience which can help you, but my gut
feeling is that for 9600 baud, you can get away with pigtails of at
least 10 feet or so.

One way to look at it is that the time a signal takes to go down the
pigtail and reflect and rejoin the main line is around 4 times the
pigtail length/C. So for 10 feet, it is maybe 40 ns. If your rise time is
much greater than 40 ns, then you should be good. At 9600 Baud, you have
around 100 microseconds between edges, and you talked about using
slew-rate limited devices, so I would think you would be OK. If your rise
time was, say, 1 (or even 10) uS, and your total length of cable is only a
few hundred feet, it almost doesn't matter how you arrange it as long as
you have some kind of a termination scheme that doesn't overload the
driver at DC.

The above sketch analysis is more from the perspective of the main device.
If you look at it from the perspective of one of the peripheral devices,
then you have a short transmission line, which then forks to create two
approximately equal and terminated transmission lines. That is not too bad
a situation either, because the reflection comes back from the fork pretty
fast, and after that, it just looks like Rterm/2.

The bottom line is that at 9600 baud, the system should be pretty
forgiving of less than perfect cabling. ;-)

--Mac
 
Kevin said:
Here's another question: are there such things as *surface mounted*
RJ45 jacks (instead of wall plates)?

Yes. Mouser carries Leviton jacks like that: 503-41018-Q or
503-41038-IDA for about $5 quantity 1, or a DGS Pro-Audio 154-3009 for
about $3.50 quantity 1. These are like the common telephone jacks, in
that the connector is molded into the base plate, and there are
terminals for the wires. You can also get small surface-mount housings
that accept the "keystone" type jacks that normally snap into a wall
plate with several rectangular openings - see for example
http://www.levitonvoicedata.com/catalog/BuildPage.aspx?BuildPageID=48 .

Buy a few samples before you place a big order. Some of the cheaper
phone jacks use sheet metal screws directly into the plastic for the
wire terminations. These tend to strip out when tightened. Better ones
have metal inserts in the plastic for machine screws to thread into.

Standard disclaimers apply; I don't get money from any of the companies
mentioned above.

Matt Roberds
 
S

Sylvan Butler

Jan 1, 1970
0
Would it make any sense to use 6-pin RJ-12s? They're a different size than
either the RJ11 (4-pin) or the RJ45 (8-pin) - this could reduce the
likelihood of plugging it into the wrong jack.

No, an RJ12 (6-conductor) is the same physical size as an RJ11 (4
conductor) and are fully interchangable (if you only use 4 wires).
Which also means than an RJ12 will plug into an RJ45 jack and make 6
connections. Can be handy sometimes.

sdb
 
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