I am trying to fashion a small module that would allow me to connect to an RS-485 system via Bluetooth. I have found that the potentially cheapest solution would involve an HC-06 TTL Bluetooth transceiver and an SN75176A RS-485/422 driver. However, in order to prevent echo on the BT side of things, I would need to somehow detect a signal coming from the HC-06 and use it to power both the R̅E̅ and DE pins, which would otherwise be held low. The HC-06 does not provide such a pin, having only Vcc, GND, RX, and TX. How can I interconnect these two devices?

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    \$\begingroup\$ Generally, with a microcontroller in between them \$\endgroup\$ – Scott Seidman Dec 5 '19 at 18:15
  • \$\begingroup\$ Unless this can be the master it will probably not work without something to run custom software between the Bluetooth and the RS-485. If it is the master, a time based circuit for driving the transmit enable (as sometimes seen for legacy PC's) could work. You'll receive your own transmissions but software on the other end of the BT link should be written to simply ignore those. \$\endgroup\$ – Chris Stratton Dec 5 '19 at 18:30

I have deleted my other answer. I was mistaken on the required interface, plus there were comments that the indicated chip was too expensive.

How about the MAX13487E/MAX13488E. They are made for half-duplex two wire RS-485 networks and have auto direction control. The cost is $2.65 at Mouser as of 05DEC19(Compared to $9+ for the MAX3162). Which is give or take $1.25 more than the SN75176A. The MAX13487E is slew limited(to cut down on EMI and is restricted to 500Kps) and the MAX13488E(allows up to 16Mps) is not.

Page 16 of the datasheet shows the auto direction circuit. There is also a Design Solution/App Note that deals with subject matter. It looks like at most it would require the MAX1348X, 6 resistors and 2 opto-couplers to interface it. And the opto-couplers are only required if the HC-06 can not deal with it's RX and TX being pulled up.

Note, there is a prior question that compared the MAX485 with the MAX13487.

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  • \$\begingroup\$ @ChrisStratton This one should be better. \$\endgroup\$ – GB - AE7OO Dec 6 '19 at 7:19
  • \$\begingroup\$ The optocouplers have nothing to do with auto direction but are for an entirely different sort of application. The logic side pulling resistor exists only because of the optos. The pulling resistors mentioned in the auto detect are rather those on the RS485 side. This chip may be an option, but the part proposed by the asker is too if used with the typical pulse stretcher. \$\endgroup\$ – Chris Stratton Dec 6 '19 at 14:14
  • \$\begingroup\$ Ooh, I never even knew such a chip was around! I chose a dumber chip mostly because I would much rather deal with a DIP package, but the autodetection is a huge selling point. Thank you, and RIP DIP \$\endgroup\$ – robbie Dec 7 '19 at 2:05

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