RS485 Connector Splitter ???

Thread Starter

fieryfire

Joined Feb 14, 2017
150
Hello, Would anyone know/suggest a connector where i can interface 5x RS485 A and 5x RS485B through a connector like shown below. Basically Each RS485 A and B lines from each sensor would have to integrate to a bus line RS485 A and B that connects to a gateway

1636379385849.png
 

Ian0

Joined Aug 7, 2020
4,898
If each sensor has data collision avoidance software, then you can simply connect them together. Don’t forget that RS485 should be wired as a daisy-chain, terminated at each end, rather than as a star.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
If each sensor has data collision avoidance software, then you can simply connect them together. Don’t forget that RS485 should be wired as a daisy-chain, terminated at each end, rather than as a star.
Could you please suggest components on how to make this happen? because i do not know of existing devices or boards that make this happen.
 

Ian0

Joined Aug 7, 2020
4,898
If you post a little more information on the sensors, perhaps we may be able to help.
Do they send data in response to a request?
Do they send data on a regular timeframe?
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
If you post a little more information on the sensors, perhaps we may be able to help.
Do they send data in response to a request?
Do they send data on a regular timeframe?
If you post a little more information on the sensors, perhaps we may be able to help.
Do they send data in response to a request?
Do they send data on a regular timeframe?
Its pretty basic really, About 100m cabling between the gateway and the 5 PWAs which holds the RS485 transceiver each.
To answer to your question, the PWAs, send a data in response to a request.

1636555709966.png
1636555661567.png
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
Are the transducers addressable? I.e. they can each be set to a unique address to which they respond?
indeed, in fact there is only 1 bus RS485 and each sensor has its own address for request
the gateway will send the request to the sensor it wants to respond
 

Ian0

Joined Aug 7, 2020
4,898
All that is necessary is to wire them as a daisy chain with twisted-pair cable.
All the D+ Terminals connected together. All the D- terminals connected together.
The termination resistor (R3 in your drawing) goes across the D+ and D- terminals on the LAST transducer in the chain, and on the gateway.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
All that is necessary is to wire them as a daisy chain with twisted-pair cable.
All the D+ Terminals connected together. All the D- terminals connected together.
The termination resistor (R3 in your drawing) goes across the D+ and D- terminals on the LAST transducer in the chain, and on the gateway.
could all transducers have the 121Ohm termination resistor? would that be a problem. Also i for to mention, that 4 sensors would be at 100m away from the gateway, and 1 sensor would be quite close to the gateway. So would the sensor closer to the gateway be sufficient enough to be termination resistor on either ends?

Also for the twisted cable. thats for sure. And would you know of any good screw terminals that can accept 14AWG wires?
 

Ian0

Joined Aug 7, 2020
4,898
could all transducers have the 121Ohm termination resistor?
No, the transceivers can only drive 60Ω, 120Ω at each end of the line. Any other resistance is not matched to the line so will result in reflections and data errors.

would that be a problem.
Yes.

Also i for to mention, that 4 sensors would be at 100m away from the gateway, and 1 sensor would be quite close to the gateway. So would the sensor closer to the gateway be sufficient enough to be termination resistor on either ends?
It must be on the last one, the one with only one set of wires.


Also for the twisted cable. thats for sure. And would you know of any good screw terminals that can accept 14AWG wires?
I think that 14AWG is probably quite large. I usually deal in square mm. The usual size for data cables is 0.2mm^2, which ( I looked it up) is 24 AWG.

What is your data rate? Rs485 limit is about 1Mbaud for 100m.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
No, the transceivers can only drive 60Ω, 120Ω at each end of the line. Any other resistance is not matched to the line so will result in reflections and data errors.


Yes.


It must be on the last one, the one with only one set of wires.



I think that 14AWG is probably quite large. I usually deal in square mm. The usual size for data cables is 0.2mm^2, which ( I looked it up) is 24 AWG.

What is your data rate? Rs485 limit is about 1Mbaud for 100m.


It would be either 9600 or 115200 bit rate communicating from each sensor. So i would need one termination at the last sensor. and one termination resistor at the gateway.

Would the 14 AWG be a problem, i need to use a thicker wire to prevent a higher drop in voltage from the power supply seperated 100m away. And the cable should have 4 conductors.
 

Ian0

Joined Aug 7, 2020
4,898
The data rate would be OK.
The thick cables would work. They may not quite be 120Ω, and if they were intended as power cables not data cables, then the dielectric losses would be higher, but you are well below the maximum data rate.
I think you can get combined data/power cables (can’t remember where I saw it) but the usual technique seems to be to use CAT5 and put the other three pairs in parallel for the power.

HoW much current will be required? If the transducers have a wide power supply range, a bit of volt drop in the cable will do no harm. I doubt itwill overheat the cable!
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
The data rate would be OK.
The thick cables would work. They may not quite be 120Ω, and if they were intended as power cables not data cables, then the dielectric losses would be higher, but you are well below the maximum data rate.
I think you can get combined data/power cables (can’t remember where I saw it) but the usual technique seems to be to use CAT5 and put the other three pairs in parallel for the power.

HoW much current will be required? If the transducers have a wide power supply range, a bit of volt drop in the cable will do no harm. I doubt itwill overheat the cable!
Well the Power supply is seperated by about 100m of cable of 1.66ohm (14AWG) right? and it also has another protection resistor of about 2.7ohm and the 2.9ohm fuse at input of each PWA . so through the cabling from the consumption of the board, the voltage available at the input of the board would be about 6.28V and there is an LDO of 5v at each of the PWA. so we still could go higher in AWG i suppose.

But the CAT5 cable with 24AWG wouldnt work cause the voltage at the PWA will drop all the way to 2V.
 

Ian0

Joined Aug 7, 2020
4,898
Well the Power supply is seperated by about 100m of cable of 1.66ohm (14AWG) right? and it also has another protection resistor of about 2.7ohm and the 2.9ohm fuse at input of each PWA . so through the cabling from the consumption of the board, the voltage available at the input of the board would be about 6.28V and there is an LDO of 5v at each of the PWA. so we still could go higher in AWG i suppose.

But the CAT5 cable with 24AWG wouldnt work cause the voltage at the PWA will drop all the way to 2V.
I don’t know - you haven’t said how much current the transducers need.
 

Ian0

Joined Aug 7, 2020
4,898
Resistance of CAT5 cable is 188mΩ/metre, so 18.8Ω for 100m.
18.8Ω x 80mA is 1.5V drop
Add another 3.7Ω for the PSU output and it drops 1.8V, plus 16mA*2.9Ω, and it has lost 1.85V in total. Just about enough for the LDO.
Three strands of CAT5 in parallel gets the 18.8Ω down to 6.3Ω, so 0.8V drop in total.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
150
Resistance of CAT5 cable is 188mΩ/metre, so 18.8Ω for 100m.
18.8Ω x 80mA is 1.5V drop
Add another 3.7Ω for the PSU output and it drops 1.8V, plus 16mA*2.9Ω, and it has lost 1.85V in total. Just about enough for the LDO.
Three strands of CAT5 in parallel gets the 18.8Ω down to 6.3Ω, so 0.8V drop in total.
1636977648741.png

Thats what i thought. but apparently thats not how it goes. i was trying to find a proper VI curve for the one i use, but the manufacturer doesnt have it. So picture the one shown above with the following specs i am using for my application -

9493-PS-C7

Uo=8V
Io=3.11A
Po=6.22W
Nominal Output Voltage = 6.6…7.7V (so at 300mA the output voltage would be 6.6V approx. and no-load 7.7V)
Nominal Output Current = 300mA (Max 550mA)
Equivalent circuit = 7.7V (minimum) with series resistance 3.7R (maximum)

So it goes Voltage at sensor, = 8-0.3-0.265A[Current consumption by 5 of my sensors]*(3.7+18.8) = 1.73V. So it drops all the way to 1.73V.

Even by using the 3 strands i would not have enough voltage at the sensors.
 
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