MAX3485 modules for MODBUS

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SpookyAwol

626 posts

Ultimate Geek

#305789 4-Jun-2023 09:39

Any electrical experts can confirm these are for the same purpose?

The bottom module (red) works for a MODBUS interface, but interested in using the top one (blue) if suitable due to smaller form factor

MAX3485 module TTL to RS485 Usart communication

MAX3485 Module TTL To RS485 Module MCU

Using the same pinouts doesnt work and the top one has an additional pin I cant find documented "EN" - well not that I can find anything on this module
When I say doesnt work, I get a TX but no RX response
Ive tried swapping RX/TX and also A/B

dasimpsonsrule

147 posts

Master Geek

#3084586 4-Jun-2023 10:07

On an RS485 bus only 1 device can transmit at a time, so the enable pin is needed to enable the driver driving the bus, and it must be disabled after transmitting otherwise no other device can transmit on the bus. The RS485 driver chips usually have 2 enable inputs, one which enables the transmitter, and one which enables the receiver. These are usually setup in the opposite polarity, so that if they are wired together then the driver can be used as a receiver or and transmitter by the state of the pin. The other option is to pull the receive enable to ground and have the receiver enabled all the time, the disadvantage to this is that the receiver will receive what is being transmitted. On your module at the top, they are probably tied together. The red board has a lot more components, they are probably using a resistor network to produce a 1 on the RS485 bus, and then enabling the transmitter to produce a 0 on the bus. This is not as reliable as the proper driving of the enable signal, but it works if you have only a few devices, with a slow baud rate, and a short cable

SomeoneSomewhere

1804 posts

Uber Geek

Lifetime subscriber

#3084592 4-Jun-2023 10:13

Per the datasheet (p7), pin 2 is:

Receiver Output Enable. RO is enabled when RE is low; RO is high impedance when RE is high. If RE is high and DE is low, the device will enter a low-power shutdown mode.

So tie EN to ground.

Both of these are little more than a breakout for the IC with some passive components - the red ones also have termination resistors, but that's probably only necessary on longer cables. Possible more ESD/surge, protection too.

If space is a premium, designing the actual IC onto your own PCB along with other components is probably going to give you a much smaller form factor.

SpookyAwol

626 posts

Ultimate Geek

#3084657 4-Jun-2023 11:15

I guess thats why the slight confusion, some use RO RE etc, others RX, TX etc

RO RE version
It seems when using this, RE and DE are tied together and linked to D5 on a Wemos ESp8266

RS485-TTL Module [4519] : Sunrom Electronics

Below is my version which doesnt use D5 and only RXD and TXD to pins D6 and D7
Still using ground, 3.3v of course
Buy MAX3485 TTL TO RS485 CONVERTER MODULE | iFuture Technology
I think I did try EN to ground, but Ill try again

SpookyAwol

626 posts

Ultimate Geek

#3084707 4-Jun-2023 11:52

This is what Im using for the red MAX3485 (And works well)
WiringDiagramMAX3485

As opposed to a MAX485:

WiringDiagramMAX485

I assume the blue module is more aligned with the 3485 than the 485 (exactly the same pins and naming), but has the inclusion of the extra EN pin

neb

11294 posts

Uber Geek

Trusted

Lifetime subscriber

#3084717 4-Jun-2023 13:07

Man, I know this place is called Geekzone, but I still didn't expect that many experts on obscure data bus hardware...

SpookyAwol

626 posts

Ultimate Geek

#3084769 4-Jun-2023 14:45

neb: Man, I know this place is called Geekzone, but I still didn't expect that many experts on obscure data bus hardware...

😂

Can only hope ;)

SomeoneSomewhere

1804 posts

Uber Geek

Lifetime subscriber

#3084780 4-Jun-2023 15:47

I'm pretty sure both the parts you linked are based on the MAX3485, not the MAX485. So you should be able to just trace pads on the one that works and compare with the other one to see what the difference is.

Not really familiar with 485 behind the scenes, but there's a lot of stuff built on it.