AM3356: RS485 half duplex communication over UART

Hi Tashiro-san,

Does the customer running Linux on AM335x? Linux UART driver supports using the UART RTS pin as the RS485 DE pin.

Hi Bin,

The customer uses Linux SDK. Could you give more information about below feature?

Bin Liu said:

Linux UART driver supports using the UART RTS pin as the RS485 DE pin.

The customer guesses either below cases. Could you confirm it?
a) UART can be configured as half-duplex by HW.
b) RTS signal can be configured as output enable by HW.
c) UART driver controls RTS signal by software.
d) other

The customer also wants to know UART register configurations to support above feature.

Thanks and regards,
Koichiro Tashiro

Hi Tashiro-san,

Koichiro Tashiro said:

The customer uses Linux SDK.

Which version?

Koichiro Tashiro said:

a) UART can be configured as half-duplex by HW.
b) RTS signal can be configured as output enable by HW.
c) UART driver controls RTS signal by software.

Yes for all the 3 questions.

Koichiro Tashiro said:

The customer also wants to know UART register configurations to support above feature.

Customers don't need to know the registers, the UART driver handles it. The only thing need to do is to add the RTS pinmux in the corresponding UART RXD/TXD pinmux group. Then in the Linux user space program to set SER_RS485_ENABLED flag in calling TIOCSRS485 ioctl.

Hi Bin,

Bin Liu said:

Koichiro Tashiro said:

a) UART can be configured as half-duplex by HW.
b) RTS signal can be configured as output enable by HW.
c) UART driver controls RTS signal by software.

Yes for all the 3 questions.

The customer is confused with above answer.
a) and b) mean UART HW has a feature to use RTS pin as DE pin.
On the other hand, c) means RTS pin is controlled by software, i.e. HW does not have such feature.
Which one is correct?

Thanks and regards,
Koichiro Tashiro

Hi Bin,

Bin Liu said:

In Linux, kernel UART driver controls the RTS pin for RS485 DE. The AM335x UART module itself doesn't understand RS485.

I see. Thanks!

Bin Liu said:

Koichiro Tashiro said:

The customer uses Linux SDK.

Which version?

Their version is 05.01.00.11 which is a bit old one.
Could you check the RS485 support is available in this version?

Thanks and regards,
Koichiro Tashiro

Hi Tashiro-san,

Koichiro Tashiro said:

Their version is 05.01.00.11 which is a bit old one.
Could you check the RS485 support is available in this version?

Sorry, this old SDK is no longer supported. It is not trivial to confirm if its kernel has any bug in supporting RS485, I am unable to provide comments to SDK5.1.

Hi Bin,

I understood the version is old and no longer supported.
But I guess it is still possible to check if the RS485 support was already implemented in that version or not.
Could you check that point?

Thanks and regards,
Koichiro Tashiro

Hi Tashiro-san,

It probably is not much of effort to check if RS485 is supported by the SDK5.1 kernel, however, it is not easy to tell if the driver has any critical bug in RS485 support just by reviewing the kernel source code.

If the customer only wants to know if RS485 is supported or not in SDK5.1, I can download and review its kernel code tomorrow and let you know.

Hi Bin,

Thanks a lot!

Thanks and regards,
Koichiro Tashiro