Core Electronics Forum

USB-RS485 Converter with Daisy-chain Topology

Hi YT
A quick couple of questions.
Are the wires single core or stranded.
I think I can see the Data collector terminal has screw connections. What are the others, I can’t really see.
A Rule!!! NEVER put soldered wires under a screw type terminal or crimp. Believe me it will not stay tight.
I can see now, I zoomed the image and they are screw terminals.
Single or stranded I think the safest bet will be bootlace ferrules.
Rule!!! NEVER put single (resistor lead) and stranded together under a screw terminal, always use a ferrule. If you use these ferrules right through here you will not go wrong and have good reliability.

What is the value of the terminating resistor at the “terminating end”. It doesn’t look much like 120Ω to me. The first band looks yellow which is 4 for a start.

And yes this is the way to go. But when using ferrules don’t twist the wires but keep them straight.
Jaycar have these ferrules under “Ferrule Crimp Connectors”
Cheers Bob

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Add on
What are the other terminals on the USB devices? Can we have a close up where any markings can be seen or a make and type so we can look it up. Some devices have a RS485 IN and OUT so they can be easily daisy chained. This removes the requirement to put 2 wires under the one connection. They are simply connected together on board. If this is the case the ongoing cable would connect to the “out” terminal and the terminating resistor connected the same on the last one.
Cheers Bob

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Hi Rob,

Those were 120Ω resistors I bought at Jaycar. Did you say the wires should be crimped with a ferrule and then screwed together in the connector? (As you can see, I absolutely have no experience with this :frowning: )
120_ohm

I am using stranded wires. Thanks for the advice, I will buy some ferrules from Jaycar. However, I am not too sure what you meant by “…removes the requirement to put 2 wires under the one connection”. If we do the connection like the picture below, wouldn’t there be two wires at the connector for each T (circled in the picture between wires 1 and 2)?

/YT

Hi YT
The circuit you have drawn is correct.

Probably the lighting made one of the bands look yellow instead of brown. My apologies.

Some RS485 devices have an “RS485 “IN”” and an “RS485 “OUT”” connection. These are connected together on board as a pass through connection exactly as you do by twisting the wires and you just connect the incoming cable to one and the ongoing cable to the other. The 120Ω termination can connect to where the ongoing cable would normally connect, “RS485 OUT”. This makes it a bit easier as you don’t have to connect 2 wires to the same point. If this is not the case then there is no alternative but to connect the wires together.

Yes but I hope I have explained above. If in doubt connect the 2 wires together (using ferrules) and you can’t go wrong.

Put the 2 wires to be joined (or the resistor and 1 wire in the case of the ends) in the same ferrule, crimp and put the ferrule into the connector. Do not twist the wires together.

There is a reason for not twisting the wires tightly as in most Electrician’s joints. If there is a wire protector in the connector it is not too bad but if a twisted wire is clamped under a bare screw there is a possibility of the wires being cut where the cross one another. If this occurs the connection will probably not remain tight and cause real problems. This same cutting action can occur in crimps so wires should never be tightly twisted, or tinned as the solder will “creep” and the connection loosens.
Cheers Bob
I have noticed in recent years most electricians using ferrules which can only be a good thing.

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Hi Robert and YT,

+1 on ferrules, While the advice on not using tinned wires is new to me, the consensus in the DIY 3D printer community is that ferrules are great.

-James

Hi James
Tinning of wires or anything else which is clamped is a definite no no where reliability is a concern. Which really should be at all times. In military or other high reliability applications it will be rejected out of hand. No grey areas.

Example. Computer Tote on Sydney racecourses many years ago. Engineer insisted 3 phase (large) mains cable connecting computer vans be tinned prior to fitting to connector (screw clamp). Advised this needed a brass ferrule soldered to the end so screw clamped on ferrule not on tinned cable. This was overruled. Result: A few meetings later this connector literally exploded and destroyed both cable end and fixed connectors.

Another high reliability situation. We had to tin some copper sheet earth connections to prevent dissimilar metal electrolysis problems. This had to be clamped with a bolt and nut through a solid copper block like washer. Before this was allowed we had to demonstrate that all of the solder had been removed just leaving a shiny tin-like finish and no excess solder remained which was likely to “creep”

Use of ferrules is pretty mandatory under screw type connections especially when a solid core has to reside with stranded wire as it would be very unlikely that the 2 conductors would be the same diameter so the smaller would probably be loose. With the Krone to any other insulation displacement connection it is a definite no no to use solid and stranded in the same connection (Krone allows 2 wires of the same size and type in the same connection).Even when only 1 wire is involved it is good practise to use ferrules in the final installation to prevent wire damage from causing problems further downstream.
Cheers Bob

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