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?
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.