# Difference between of 2 wire current loop and rs 485 communication

#### FANDHI

Joined Dec 6, 2019
10
Hi...

Can anyone explain the difference in communication between 2 wire current loop and rs 485?

Thanks

#### SteveSh

Joined Nov 5, 2019
105
This is pretty basic stuff. Just GOOGLE "current loop" and "RS-485", or go directly to Wikipedia. There's a lot more there than I could reasonably put into a response here. If you have any specific question after reading that background material, post a new message here.

#### FANDHI

Joined Dec 6, 2019
10
Ok steve. Thanks

#### FANDHI

Joined Dec 6, 2019
10
Hi .. Steve,
I have read the 2 Wire Current Loop Communication System.
There I need 2 classifications namely active and passive.
and in one system 2 Wire Current Loop Communication there is only 1 which is called active and the other is passive. What does it mean ?

As a case in point, I want to be connected to an existing 2 Wire Current Loop Communication System.
I have a microcontroller and a circuit board (2 Wire Current loop to Serial converter) and in the microcontroller I have installed a program that only works to read the transmitted data. and communication parameters such as baudrate, parity and protocol are appropriate.
The results I got were:
Before I connected my controller, 2 Wire Current Loop Communication was running normally, but when I connected my controller, 2 Wire Current Loop Communication stopped and I did not get the data that I wanted.
I think that the possibility of my controller included in the active classification, is that true and how do I know it (active or passive)?
and is it possible to change my controller to passive and how?

#### LesJones

Joined Jan 8, 2017
3,483
The active item is the one that provides the current for the loop. The passive item does NOT provide the current for the loop. The passive end probably just consists of 2 opto isolators (One for transmit and one for receive.) Post a schematic of how it was originally wired and how it was wired after you connected something to it. I suspect you have connected something in parallel with the loop rather than in series with it. You also have to make sure that the voltage supplied by the active end is enough to deal with the extra voltage drop caused by the added receiving device inserted into the loop. It does not make sense to have more than one transmitter in a loop.

Les.

#### FANDHI

Joined Dec 6, 2019
10
The active item is the one that provides the current for the loop. The passive item does NOT provide the current for the loop. The passive end probably just consists of 2 opto isolators (One for transmit and one for receive.) Post a schematic of how it was originally wired and how it was wired after you connected something to it. I suspect you have connected something in parallel with the loop rather than in series with it. You also have to make sure that the voltage supplied by the active end is enough to deal with the extra voltage drop caused by the added receiving device inserted into the loop. It does not make sense to have more than one transmitter in a loop.

Les.
Here is the schematic

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#### LesJones

Joined Jan 8, 2017
3,483
I have never seen a single current loop used for two way communication. Normally there is a loop for sending for each direction the data is sent. To use a single loop for two way data will require a protocol that avoids both ends transmitting at the same time.
The way you have connected your second controller is wrong. You no longer have A LOOP. The way a current loop works is there is either current round the loop or no current round the loop. (The static state is normally current flowing round the loop.) Think of the transmitter as a normally closed switch. Think of the receiver as an opto coupler. The current flowing through the LED part of the coupler causes the photo transistor in the coupler to be conducting. When there is nothing transmitting all the switches will be in the closed so there will be current through the loop. So any opto couplers in the loop will have current passing through them.
You need to break the existing loop and insert the second controller into the loop. (With the polarity correct.) There can only be one active item in the loop. On the equipment that I worked on (Terminals connected to computers.) there were switches or jumpers to set the devices to be active or passive.

Les.

#### nsaspook

Joined Aug 27, 2009
8,655
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#### FANDHI

Joined Dec 6, 2019
10
I have never seen a single current loop used for two way communication. Normally there is a loop for sending for each direction the data is sent. To use a single loop for two way data will require a protocol that avoids both ends transmitting at the same time.
The way you have connected your second controller is wrong. You no longer have A LOOP. The way a current loop works is there is either current round the loop or no current round the loop. (The static state is normally current flowing round the loop.) Think of the transmitter as a normally closed switch. Think of the receiver as an opto coupler. The current flowing through the LED part of the coupler causes the photo transistor in the coupler to be conducting. When there is nothing transmitting all the switches will be in the closed so there will be current through the loop. So any opto couplers in the loop will have current passing through them.
You need to break the existing loop and insert the second controller into the loop. (With the polarity correct.) There can only be one active item in the loop. On the equipment that I worked on (Terminals connected to computers.) there were switches or jumpers to set the devices to be active or passive.

Les.
Hi.. Les..

I am trying to change the wiring scheme as attached, the results I get The first controller and the device can communicate normally (unlike the previous scheme), but the second controller still does not get the data transmitted between the device and the first controller. as information that connection parameters such as badurate, parity, etc are appropriate.
And the program code (using C language) in the second controller to read data as follows
Code:
// ===============================
char outmonitor;
int temp, j;

serBopen (5787L);
serBparity (PARAM_EPARITY);
b_rtson ();
for (j = 0; j <30000; j ++);
b_rtsoff ();
outmonitor = 0;
// this is the block for reading transmitted data
while (! outmonitor)
{
temp = serBgetc ();
if (temp! = - 1) {
printf ("% 02X", temp);
}
}

// =================
Temp variable never gets data, whereas my desire data can be received through these variables.
maybe this is because the first and second controllers are active items.

I really hope that someone can help me.

Thanks.

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#### MisterBill2

Joined Jan 23, 2018
9,238
If the current loop is a typical 4 to 20 milliamp loop then all of the devices must be in series. A parallel connection will never work correctly. If it is a digital communications loop, still it must be in series. There may be other problems, but current loops are series loops.

#### KeepItSimpleStupid

Joined Mar 4, 2014
5,066
Hold it! 4-20 mA is totally separate animal. it's analog. It allows a sensor to be powered and read with just two wires. 4 mA is sucked off to power the sensor. Active and passive go with this system.

The serial current loop is just called 20 mA current loop. There are other standards. The ASR33 teletype was natively 20 mA current loop. DEC (Digital Equipment Corps) console terminal was often Current loop.

The serial protocals like RS323, RS422, RS485. they are forgiving to one extent, you can flip all the wires all you want and no damage will occur. RS232 was easy with a "breakout box" Occasionally, some bozo doesn;t know how to use RTS and CTS properly or redefines DTR and DSR or uses DSR and DTR for CTS and RTS. Those were the good ole' days.

Current loop for data might be just 4 wires.

The serial protocol is master/slave. No such thing as two masters. RS485 I think has some multi-master modes.

I've tapped RS232 to do some monitoring.

#### LesJones

Joined Jan 8, 2017
3,483
I am VERY poor at "C" programming so I'm not sure I fully understand your program. (I find assembler much easier to work with.)
Can you confirm / answer the following.

1 My understanding is that with your schematic in post #9 the original communication between the "Device" and the original controller 1 works as it should. Am I correct.

2 Am I correct that controller 2 is only monitoring data and does NOT transmit data.

3 I am not clear if you are not seeing data from either the device or controller 1 or if you can only see data from one of them. Can you clarify this.

4 can you post the schematic of the interface parts of the device and controllers 1 and 2

Les.

#### MisterBill2

Joined Jan 23, 2018
9,238
Hold it! 4-20 mA is totally separate animal. it's analog. It allows a sensor to be powered and read with just two wires. 4 mA is sucked off to power the sensor. Active and passive go with this system.

The serial current loop is just called 20 mA current loop. There are other standards. The ASR33 teletype was natively 20 mA current loop. DEC (Digital Equipment Corps) console terminal was often Current loop.

The serial protocals like RS323, RS422, RS485. they are forgiving to one extent, you can flip all the wires all you want and no damage will occur. RS232 was easy with a "breakout box" Occasionally, some bozo doesn;t know how to use RTS and CTS properly or redefines DTR and DSR or uses DSR and DTR for CTS and RTS. Those were the good ole' days.

Current loop for data might be just 4 wires.

The serial protocol is master/slave. No such thing as two masters. RS485 I think has some multi-master modes.

I've tapped RS232 to do some monitoring.
OK, so now it is clear that this is a data communications loop, not an analog loop with occasional serial data. But still, it has to be a series loop, no stuff in parallel. And probably yes, one talker and others just listening. But in some varieties of loops there can be multiple talkers.

#### FANDHI

Joined Dec 6, 2019
10
I am VERY poor at "C" programming so I'm not sure I fully understand your program. (I find assembler much easier to work with.)
Can you confirm / answer the following.

1 My understanding is that with your schematic in post #9 the original communication between the "Device" and the original controller 1 works as it should. Am I correct.

2 Am I correct that controller 2 is only monitoring data and does NOT transmit data.

3 I am not clear if you are not seeing data from either the device or controller 1 or if you can only see data from one of them. Can you clarify this.

4 can you post the schematic of the interface parts of the device and controllers 1 and 2

Les.
Hi les..
1. Yes
2. Yes
3. I do not see any data from either device or controller 1
4. I don't have schematic of device, controller 1 and 2. however i attach a photo of current loop to serial interface board of Controller 2. May be this can give you information you need.

Thanks

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#### LesJones

Joined Jan 8, 2017
3,483
I assumed that you did not have the schematic of the 20 mA loop to RS232 converter board as you have not even posted the user manual. I only want to see the part of the schematic around the Opto isolator (4n35) so it should not be difficult to trace out. With no traffic on the loop can you measure the voltage between pins 1 and 2 of the 4N35. I expct a reading of about 1.5 volts. I am particularly interested in the polarity of this voltage. Do you have an oscilloscope to look at signals on the converter board ? I am wondering if there is data but it is not a protocol or baud rate that the device you are using to display it understands.

Les.

#### FANDHI

Joined Dec 6, 2019
10
I assumed that you did not have the schematic of the 20 mA loop to RS232 converter board as you have not even posted the user manual. I only want to see the part of the schematic around the Opto isolator (4n35) so it should not be difficult to trace out. With no traffic on the loop can you measure the voltage between pins 1 and 2 of the 4N35. I expct a reading of about 1.5 volts. I am particularly interested in the polarity of this voltage. Do you have an oscilloscope to look at signals on the converter board ? I am wondering if there is data but it is not a protocol or baud rate that the device you are using to display it understands.

Les.
Hi les..
Maybe the answer to my problem is controller 1 and controller 2 are the same type that is active current. I have tried connecting controller 2 with device without controller 1, after that I send data to the device and the result I get a response from the device.
Because in the current loop communication circuit there is only 1 active current, so when I connect controllers 1 and 2 with the device at the same time, controller 2 does not get data even though only monitoring it.
Now how do I change the current loop to serial board circuit to be passive current?
OR if anyone who has a schematic of a current loop to serial (RS232 or 485) converter with a type of passive current, I really hope to share with me.

Thanks

#### MisterBill2

Joined Jan 23, 2018
9,238
It may be that the second active controller can be switched to a passive mode, and it may even be that putting them all in series would be sufficient. Certainly you will need to understand the electrical functioning of the second controller to be able to connect it properly. Making it work once connected will be a separate challenge. But a current LOOP must be a single loop, not an arrangement with branches.

#### LesJones

Joined Jan 8, 2017
3,483
Without the manual or schematic all we are doing is guessing. Try to find the manual online for the THE MODEL of converter that you are using. I think it should have switches, jumpers or wire links that can change it's function between active and passive. One thing you could try if you won't do the test requested in post #15 or trace part of the schematic is to connect it as in post #9 and swap the RS232 connector from converter 1 to converter 2 and see if the original communication still works.

Les.

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