SN65HVD78: SN65HVD78DRBT - How to detect cable deconnected ?

What exactly do you mean with "equipment is connected"?

At the electrical layer, it would be possible to detect the voltage changes caused by the termination resistor(s). The SN65HVD78 itself cannot do this; it would require an ADC.

Thanks for your answer. In fact, currently if our equipment / station is not connected to the Profibus network (cable not connected), the current driver sends us the information cable disconnected or "no echo in transmission". I'm not  experimented in elctronic, but ADC idear seems insterresting, but wich signals you use with ADC ?

Hello Parker, thanks very much for your solutions.  In fact we would like to have SN65 with same functionnalities to be compliant to our old driver. We added a small FPGA associated to SN65HDV to do treatments.  Our old driver detect a small underload voltage (150 mV) trouhgout  a resitor  when we send data (this underload indicate that station is branched), we detect also a small underload voltage 150 mV with SN65 when emitting data. So we would like to continusly monitor consumption of SN65, do you have advice/solutions or components compliant with SN65HDV to watch current level or voltage ? 

Regards, Bruno.

Hi Bruno,

I don't think it is possible without additional hardware to meet the goal that you are trying to meet - I have a few questions to try to better understand the application and what limitations we need to work with to see if there is a possible solution.

1. What supply voltage rails are available in the system?

2. What is the max datarate of the system?

3. What is the max number of nodes supported by the application?

4. What is the length of the differential bus? 

5. Does each node have a shared ground or do they have separate ground connections - and if known do you know the range of ground potential difference that can occur in the system (this will impact what voltage supplies can be used for certain components)?

6. Do you just need a binary output - i.e. if a 150mV change is detected a "flag" signal will be signaled or does your application need to know the the specific voltage drop - if it is the former the solution could be possibly simpler.

Please let me know so I can see if we have a good working solution and see if I need to loop in any other product experts to help with some advise!

Best,

Parker Dodson

Thank you for the time you devote to me. You are right, we must add components to detect underload 150 mV  during sending frame (we need this information only when emitting frames), time is about 2-10 micoseconds. Speed bus is 1 Mbits for this step. We have severals voltage available :  5V, 3V3, 2V5 and 1V2. Effectively we must detect flag signal that we will treat in our FPGA associated to SN75. We think about analog comparators , is it good ? Do you have TI components compliant to SN75 ?

Regards, Bruno

Hi Bruno,

Alright - so I was a bit worried about the supply voltages. So the RS-485 standard allows -7V to 12V common mode voltage on the differential bus - that means that the detection hardware either must be able to support the full common mode range (which I am not sure with the available voltage supplies is directly  possible. It might be possible to have an attenuation network before the comparator at the bus to help prevent EOS on the comparator.

I am looping in our comparator team as they have more expertise on those types of devices and they maybe able to select  a well suited device for the application. 

Best,

Parker Dodson

Hello Parker,

Thanks for your answer. To bring precisions on SN75, we don't care about pinout A and B (differential bus), we test only supply voltage VCC  ( 8 pinout)  and  D( 4 pinout ).

When cable is branched and  when we emitting frames there is small underload, we notice 150 mV on VCC pinout8, all power goes throughout a resistor we named Rsense, about 2 Ohms), and when there is no cable branched  on A et B  there is no underload on VCC that indicate us there is no cable. ( Precision : Rsense is not the terminal resistance/impendance 150 Ohms on diffential bus).

My question is : wich component ( analog comparator ?) we can use to detect the 150 mV drop on VCC supply ( Pinout 8). To be more clear on our aim function : during emitting frames D, we mus detect  drop 150 mV on VCC if cable is branched

Regards, Bruno.

Thank you Parker for looping us in.

Hi Bruno,

I just wanted to clarify your application. There is a 150 mV drop on your VCC supply and you would like to monitor this drop and alert your system when this occurs? IF that is the case, I would use a comparator with internal reference, such as the TLV40XX family comparators: TLV40x1 Small-Size, Low-Power Comparator with Precision Reference datasheet (Rev. C) (ti.com). Here is what I came up using the TLV4041R1 which has an internal ref of 1.2V. As VCC drops from 3.3V to 3.15V, the output triggers from high to low. Feel free to play with the sim below. 

What power requirements are you looking for? How fast do you want to detect this 150mV drop? 

TLV4021R1-e2e.TSC

Hi Chi,

Thanks for your schems, great we thing it suits to our project ! We have done the schematic (see image 1), it seems that it is advised to add INA185 for isolation, do you think it is good ? (see image 2)

Cordially, Bruno.