SN75179B: RS485 TXR(s) Best Practice Questions

Wess Gates

Prodigy 60 points

Part Number: SN75179B
Other Parts Discussed in Thread: THVD1512, , THVD1424

Hello,

Many of the TI RS485 TXR(s) datasheets don't seem to provide typical application sections or recommendations on passive components i.e. filtering/bulk caps.. resistors etc... and many of the example circuits drawn out in TXR(s) notes or posts seem to show the TXR "bare bones" ... Is there anything I should look out for? See for example this "Top 10 RS-485 Design Tips" or even this "Going for distance or going for speed on the RS-485 bus?" or even the data sheet of our part of interest SN75179B.
Many of the RS485 discussions emphasize the "hall duplex" preferred nature when using RS485. Is there a drawback to using a full duplex RS485 TXR in application? Particularly point to point here i.e. not multi node.
We do not have a need for Driver/Receiver Enable functionality but many part numbers offer these pins. Is there any harm in keeping the pins always enabled if I end up needing one of those parts rather than the one identified so far? Other than perhaps some kind of "power cost" for keeping something enabled when not in use? We have no need for RTS/CTS etc... Just TX/RX .... everything else will be in software.
Our application is for use on a relatively small mobile robot where point to point cable lengths are sub 1 meter with plans on supporting 115.2/230.4/460.8kBaud tops. I currently planning on leaving the termination resistors off of our PCB based on what I have gathered so far. I realize its easy and inexpensive to drop a footprint so happy to do that with even a little bit of convincing?

Thank you,

Wess

over 1 year ago

0 Clemens Ladisch over 1 year ago

Guru 318050 points

1. All transceivers need a decoupling capacitor. Additional components to protects against voltage transients and noise might be necessary, depending on your environment.

2. Half duplex, by itself, is not preferred, but it is necessary when you cannot afford more than two signal lines, or when you have more than two senders on the bus.

3. Disabling the driver saves power, and is necessary to allow other drivers on the same line to be active. On a half-duplex bus, disabling the receiver is necessary if you do not want to receive your own data.

4. For unidirectional point-to-point connections, you need termination only at the receiver. Omitting the termination will generate reflections at the signal edges, which becomes worse with faster edges and longer cables. In your application, they might indeed not be necessary, but it would be a best practice to have at least the footprint.

5. The THVD1512 uses less power and is cheaper and more robust than the SN75179B. If you care about space, use a transceiver in a smaller package like the THVD1424.