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One of my client wants RS485 data from my remote field instrument to their control panel which is around 3km away.

I know,

  1. RS485 won't work for such distance.
  2. Converting RS485 to optical signal will work for up to several km's.

Since, in this particular case, the data transmission is point-to-point (not MIMO like RS485), my mind has stuck on an idea of transmitting the data over a pair by switching the constant current for mark and space. E.g. for mark - 20mA and for space - 4mA over the two wire loop.

Can anybody think of the possibility or the feasibility, if I can use my own transceiver circuit (20mA mark and 4mA space on a 2 wire loop) on both ends for data transmission?

toolic
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Harish Kr Singh
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  • I've seen situations where the ground voltage between two buildings separated by less than a km was 7 kV. We solved it using optical fiber. You can certainly try current loop. – periblepsis Jun 26 '23 at 11:41
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    The 4/20mA current loop is a technology that is intended exactly for your use-case. – Stefan Wyss Jun 26 '23 at 11:48
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    What sort of data rate is required? – jonathanjo Jun 26 '23 at 11:58
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    Does a cable already exist between the two points? – winny Jun 26 '23 at 12:26
  • Why won’t RS485 work at 3km? My solution would be to use off the shelf ethernet to fiber converters and ethernet to RS485 modules. By the time you fiddle with your own home brew solution and trying to get that reliable, the initial cost of my suggest will be cheap in comparison. – Kartman Jun 26 '23 at 12:59
  • It may work with some cables and it won't work with other cables. What's the cable? – Andy aka Jun 26 '23 at 13:35
  • It may work with two repeaters. Of course you need power for the repeaters. Data rate up to 100 kHz for 4000 ft cable length. With more repeaters in shorter distance more than 100 kHz is possible. – Uwe Jun 26 '23 at 14:03
  • @Andyaka The cable shall be two core shielded 0.5sqmm (at least to reduce the line resistance) armored. – Harish Kr Singh Jun 27 '23 at 04:12
  • @jonathanjo I think by switching 4/20mA current pulses through two BJTs would easily do 9600bps, which is desired. – Harish Kr Singh Jun 27 '23 at 04:19
  • In general it's a good idea to keep the specification (9600 bps) separate from the system (current loop) from the solution (BJTs). You don't say whether the data is bidirectional. If you want a current loop, consider a ready-made module (eg Mikroe), a purpose-made chip (eg TI) or at the very least at well-tried circuit: there's no need to design anything here. – jonathanjo Jun 27 '23 at 07:23

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