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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?

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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.

EDIT:

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.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\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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