SBL2e and Half Duplex RS485

[Matt]

Hi everyone,

We're using SBL2es to tunnel serial data over ethernet connections.
We're not developing software for the boards but are using the boards exactly as they come from the factory.
The serial tunnel itself seems to work alright, TTL, RS232 and RS485 full duplex works excellent.

However I have issues getting the half duplex mode to work. I am using the MAX3079 just like on the SBL2e development board:

- TXP, RXP,/RE, GND to GND
- VCC, SLR, - to VCC (3.3 V)
- DI to U0TTLTXD
- DE to U0 3VRTS
- RO to U0TTLRXD
- H//F to SDA

I am using the Y and Z pins on the MAX3079 for my RS485 in/out, both side are terminated.
I can see the SBLe's pin 15 (SDA) toggle between high and low on the MAX3079 when I toggle between half and full duplex on the browser interface.
Full duplex is working well, but unfortunately that's as far as it goes.

As far as I understand the RTS should be used to control the driver, but when I look at it on the scope it's always high on both sides of the serial tunnel.
So practically the MAX3079s on both sides are constantly switched to transmit and thereby disable the RS485 receiver, so no data ever comes in on either side.
Obviously I am using UART0, because there's no RTS/CTS on UART1.

Has anyone ever successfully used RS485 half duplex on the SBL2es?

[Ridgeglider]

I think you need to invert the line that throttles direction, ie the pin 15 SDA port bit before it gets connected to the 3079 DE input. The schematic clip is from one of the Dev 100 boards...