I am trying to figure out how to directly connect a GSM modem (say SIM800) with a MAX485. The purpose of this is to make an RS485 GSM modem. I am struggling how to wire it up, do I wire the CTS of the DCE (modem) to the RE/DE pin together?


1 Answer 1


I do think is not possible without an intermediate protocol with MCU. The RE/DE is toggled with RTS from MCU, only when a master MCU requests data. Like PTT button on HAM radio transceiver. But the GSM module hasn't this feature, so a MCU is needed.

Other way is to use RS422, point to point (RS485 is for multidrop: 1 master + multi slave), then you have TX and RX signals for longer cable distance, If you do need additional control signals than you have to add those as well.


According to this link: Timer sequence with delayed off and long trigger pulse

I have made similar schematics. The TX signal is delayed, so that TX_EN sets the RS485 transceiver in transmit mode by RE/DE switch.

When TX is high, the TX_EN becomes high at the same time as TX_delay. When TX goose low, it takes some time before TX_EN goes low (T=RC). This time should be adapted such, that is longer than the longest mark space duration of the TX UART signal - it is baudrate specific.

The schmitt triggers come from 74HC14, but you could use also NAND gate schmitt trigger (74HC132), if they better arrange on PCB.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Please do pardon my coming question if annoying. I've asked a few days ago if was possible and was told it was possible here (electronics.stackexchange.com/questions/480632/…). though the person who answered did not explain how to wire it up if possible. Would you kindly share your insight on this matter, i really appreciate it. \$\endgroup\$ Feb 18, 2020 at 8:54
  • \$\begingroup\$ The only thing you could try is to make an additional logic around TX pin, that will toggle RE/DE. This kind of "missing" RTS is used widely with ebay, ali,.. RS485 to TTL converters. You will notice the scrubbed IC on those boards, it could be a PIC MCU or something simlar. See my post on: eevblog.com/forum/beginners/… \$\endgroup\$ Feb 18, 2020 at 9:32
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    \$\begingroup\$ Look here at TI simple solution using a multivibrator to derive the RE/DE signal from TX: ti.com/lit/ug/tiduei7/tiduei7.pdf \$\endgroup\$ Feb 18, 2020 at 9:48
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    \$\begingroup\$ See this also: electronics.stackexchange.com/questions/213799/… \$\endgroup\$ Feb 18, 2020 at 10:01
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    \$\begingroup\$ It takes some time for MCU to receive, parse and send new query. You can introduce a minimal delay in the MCU program, so that you don't send new query before the TX_EN becomes low on the other side. For example, the Profibus master (industrial RS485 protocol) has a strictly defined a silence interval, this to overcome the reflected bus signal. Therefore upon receiving the message from the GSM module you should start a timer, then you decode the message and only when timer is expired, you are allowed to send a new query. This is a common way. \$\endgroup\$ Feb 18, 2020 at 14:02

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