Timeline for What limits the maximum number of nodes of a RS422 bus? How to check if N>10 nodes is possible without repeater?
Current License: CC BY-SA 4.0
12 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| May 3, 2020 at 13:11 | vote | accept | Mister Mystère | ||
| Apr 30, 2020 at 19:07 | answer | added | Justme | timeline score: 3 | |
| Apr 30, 2020 at 18:11 | comment | added | Justme | @MisterMystère It is possible, under certain conditions. There are at least two issues. Connecting RS-422 driver outputs together requires that the output drivers are enabled only one at a time. Can you be sure that these encoders disable their output drivers when idle, and only enable when being clocked? If manual does not say it supports multi-drop, then it doesn't. Also, since one RS-422 transmitter only supports singly terminated bus, so it means the driver should be at the other end of the bus, not in the middle, so it basically is meant to have only one transmitter at one end of the bus. | |
| Apr 30, 2020 at 17:10 | comment | added | Mister Mystère | If someone could find a RS422 or RS485 chip for the master and explain based on technical details of the datasheet why it is suitable for this application, I think it would be very informative and it would answer the question. | |
| Apr 30, 2020 at 17:05 | comment | added | Mister Mystère | @Justme: RS422 is supposed to be multi-drop, so TX+/- lines of RS422 devices should be able to be tied together? (I have seen schematics which makes this explicit) | |
| Apr 30, 2020 at 17:00 | comment | added | Justme | The encoder clock inputs have 132 ohm RS422 termination. No RS422 driver can drive the clock to more than one encoder at a time. Also the encoders have RS422 outputs so they can't be tied together. So basically you don't have to worry about having max 10 listeners on the bus because you can't drive more than one terminated bus. And there must only be one transmitter on bus. | |
| Apr 30, 2020 at 16:53 | comment | added | Mister Mystère | I may be able to modify the master for a RS485 master (though the question still stands). @Mattman944: could you expand with a schematic in an answer? | |
| Apr 30, 2020 at 16:44 | comment | added | Andy aka | Make them all RS485 was my suggestion but I guess you can't modify the master. | |
| Apr 30, 2020 at 16:41 | comment | added | Mattman944 | One limitation: The differential receivers usually have a weak pull-up and pull-down on each input. The voltage divider is slightly different on the plus and minus inputs to guarantee a specific logic level on the output when the inputs are disconnected. Too many receivers in parallel will eventually load down the bus. | |
| Apr 30, 2020 at 16:40 | comment | added | Mister Mystère | Although this certainly works (can any RS485 transceiver drive these RS422 chips right out of the box?), could you answer the question? I would like to understand this limitation. | |
| Apr 30, 2020 at 16:18 | comment | added | Andy aka | Pick an RS422 chip and see what it says it can deliver in terms of nodes. Or use RS485. | |
| Apr 30, 2020 at 16:15 | history | asked | Mister Mystère | CC BY-SA 4.0 |