EastSideSquirrel
Member
Good day all. I have an RS-485 topology question. I am hoping for some guidance so that I can make a confident calculation/decision that this will work.
Here is what I understand.
1. Star networks are bad practice.
2. If you do need to use a star, keep stubs as short as possible.
3. Data transmission rates are going to suffer the longer the trunk is.
4. Data will degrade when adding longer stubs because of reflections.
5. Add terminating resistors, selection will vary depending on choice/length of cable.
I have read other forum posts on this matter and I have read a couple Texas Instruments papers.
The RS-485 Design Guide
Page 3 I do not understand the "driver's output rise time", do I need to contact the sensor manufacturer and ask them for the specific RS-485 chip within their equipment?
AN-1057 Ten Ways to Bulletproof RS-485 Interfaces
Here are some questions that I have.
1. What does "as short as possible" mean? Is this a couple inches, or a couple meters? The TI literature talks 7-38 ft
2. Does the consistency of stub length matter?
3. Is it normal to ask sensor manufacturers about the specific RS-485 chips within their devices?
4. Are stubs not more advantageous than daisy chaining (separate in and out terms on a device), because when a device fails, you can still communicate to other devices on the network?
Some context I plan on having 24 gas monitoring devices all with the same stub length of 12 inches. I plan on connecting the trunk from JB to JB on a terminal strip. From the strip, I would make the stubs through jumpered connections on said terminals. The trunk leaves on the strip as well. It is okay if we do not receive alarms from the device for 5 seconds, if the transmission rate needs to be that slow.
I initially made the design with stubs because one of the sensor manufacturers defined the connection that way. Looking at the TI design guide, they are constantly showing stubs as opposed to true daisy chains. It also seems more attractive with stubs. And I like saying stub. Stub, stub, stub, stub.
I can provide the trunk cable length if immensely relevant, I just need to do a little digging. It is roughly less than 350 feet.
Please let me know where I need to clarify things if I have described inadequately.
Thanks for your time and for your help!
Have a great weekend.
Here is what I understand.
1. Star networks are bad practice.
2. If you do need to use a star, keep stubs as short as possible.
3. Data transmission rates are going to suffer the longer the trunk is.
4. Data will degrade when adding longer stubs because of reflections.
5. Add terminating resistors, selection will vary depending on choice/length of cable.
I have read other forum posts on this matter and I have read a couple Texas Instruments papers.
The RS-485 Design Guide
Page 3 I do not understand the "driver's output rise time", do I need to contact the sensor manufacturer and ask them for the specific RS-485 chip within their equipment?
AN-1057 Ten Ways to Bulletproof RS-485 Interfaces
Here are some questions that I have.
1. What does "as short as possible" mean? Is this a couple inches, or a couple meters? The TI literature talks 7-38 ft
2. Does the consistency of stub length matter?
3. Is it normal to ask sensor manufacturers about the specific RS-485 chips within their devices?
4. Are stubs not more advantageous than daisy chaining (separate in and out terms on a device), because when a device fails, you can still communicate to other devices on the network?
Some context I plan on having 24 gas monitoring devices all with the same stub length of 12 inches. I plan on connecting the trunk from JB to JB on a terminal strip. From the strip, I would make the stubs through jumpered connections on said terminals. The trunk leaves on the strip as well. It is okay if we do not receive alarms from the device for 5 seconds, if the transmission rate needs to be that slow.
I initially made the design with stubs because one of the sensor manufacturers defined the connection that way. Looking at the TI design guide, they are constantly showing stubs as opposed to true daisy chains. It also seems more attractive with stubs. And I like saying stub. Stub, stub, stub, stub.
I can provide the trunk cable length if immensely relevant, I just need to do a little digging. It is roughly less than 350 feet.
Please let me know where I need to clarify things if I have described inadequately.
Thanks for your time and for your help!
Have a great weekend.
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