1
\$\begingroup\$

I'll use a shielded CAT5 cable for RS-485 line. The simplified bus structure is drawn in the picture.

picture

LDO is AP2204RA-5.0TRG1 with a 22uF electrolytic and a 1uF ceramic capacitor at the input. RS-485 IC is ADM2483BRWZ.

I have 130 slaves and their isolated part of RS485 IC is supplied from the supply placed at master. The cable length between slaves are about 0.5 meter. I tried it for 4 slaves and I'll increase it up to 130 slaves in the end. Is there any problem in this structure?

\$\endgroup\$
0
\$\begingroup\$

Some things to consider in the design:

  • Check the DC loading on the 9 V bus. Assuming 24 AWG CAT5 cable, you have 0.0842 ohm/meter with 3x parallel and 2x series. For a few hundred mA total, you should be fine on losses at about 70 meters of cable. It would be good to check that you have enough voltage on the last node after assembly.
  • Reducing the per-node input capacitance to 2.2-10 uF of electrolytic plus 0.1-1 uF ceramic would be advisable just to reduce your total, very large distributed capacitance. You might need to check the stability of your 9 V bus supply with the large capacitive load.
  • Ensure both ends of the RS-485 bus have the appropriate 120 ohm termination. The master probably would supply bias (see image below) and the far end of the cable would just have a 120 ohm resistor. This is needed to absorb reflections.
  • Your ADM2483 RS-485 transceiver is 1/8th unit load for 256 devices on the bus, so DC loading of the communication bus is not a problem.
  • From your comments, I don't expect EMI would be a problem inside the shielded cable. The transceiver has built-in slew rate limiting, so emissions aren't a big concern.

Wikipedia bias termination network for RS-485 https://en.wikipedia.org/wiki/File:Rs485-bias-termination.svg

\$\endgroup\$
  • \$\begingroup\$ I considered the termination resistors. I'll also try the 2.2uF-10uF capacitors. For now the project is in the prototype phase. I'll share my experience when i finish it. Thanks \$\endgroup\$ – Yaman Dec 3 at 6:17
  • \$\begingroup\$ @Yaman glad to help. Remember to consider upvoting useful answers and select an answer as accepted if it solves your problem. \$\endgroup\$ – user2943160 Dec 3 at 13:52
0
\$\begingroup\$

The only problem I can think of is that 130 LDOs with their input capacitors spread over 65m of relatively thin wire is an awful lot of a load on master supply.

Even though the individual power requirements of bus-side isolated part is probably small (you did not provide any references to actual components).

You may have to increase the supply voltage to something like 12V to get reliable power at the end of a chain, and you'll be wasting a lot of it on heat.

Other than that it should work.

UPDATE

With new information it looks OK. At 2 mA bus side and accounting for losses it could be somewhere around 0.5A load on power supply. Which is comparable to PoE.

The cable capacitance combined with 3000 uF should be manageable, although rather on high side. The datasheet for AP2204 recommends 1 uF input cap. I suggest you use that, maybe with additional 0.01 uF.

\$\endgroup\$
  • 1
    \$\begingroup\$ LDO is AP2204RA-5.0TRG1 with a 22uF electrolytic and a 1uF ceramic capacitor at the input. RS485 IC is ADM2483BRWZ. \$\endgroup\$ – Yaman Jul 8 '18 at 19:14
  • 1
    \$\begingroup\$ You need to add this info to the question where it has more chances to be seen. Preferably with links to datasheets, to save time for the people trying to help you. \$\endgroup\$ – Maple Jul 8 '18 at 19:19
  • \$\begingroup\$ I was focused on EMI issues of 3 parallel power cable pair next to RS485 cable pair when i asked the question. The supply voltage can be increased to 12V or more when needed. BTW thanks for your attention \$\endgroup\$ – Yaman Jul 8 '18 at 19:47
  • \$\begingroup\$ I see. No, that should not be a problem. It is DC after all, and you have all + wires twisted with grounds, not to mention the data line itself is a twisted pair. This is quite common use of CAT5/6 cables. Your LDOs have only 0.2V dropout, so voltage increase might not be necessary. Do the test and if you have 6.5~7V at the input of the last node then you are good to go. Do reduce the caps though. \$\endgroup\$ – Maple Jul 8 '18 at 20:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.