RS-485 implementation on Lantronix XPort-485

I was always under the assumption that RS-485 was an electrical standard, not a protocol. However, I have never actually seen a real copy of the TIA/EIA standards

RS485 is a physical layer specification, half duplex..

The RS-485 spec only specifies the electrical characteristics of the bus. Not the data protocol. This can be anything like 5/6/7/8 bit async, SDLC, HDLC, whatever.

Meindert

Now that everyone has told you that RS-485 is an electrical spec and not a data protocol spec, I'll tell you what you really wanted to know. I don't know about the lantronix products, so this is a generalization, if the product supports 9 bit directly, it will be in the documentation, if not you use the MARK and SPACE parity to force address or data. MARK parity is for addresses and SPACE parity is for data.

Jim

No. RS-485 specifies no such thing. There are protocols used on top of RS-485 that work that way.

It probably isn't. Very few UARTs outside the 8051 family do that 9-bit mode.

They don't have a support phone number or e-mail address?

You are correct.

I have. It says absolutely nothing about the format of the data on the line.

Thanks to you all, especially to Jim Beck. I traced back, where I took the wrong direction: Application Note 774 from PIC Microchip talks about "The USART in many PICmicro devices has a 9-bit Address Detect mode, ..." within the chapter "RS-485 Implementation". That led me to the conclusion, the mode would be part of RS-485.

Kind regards Frank

Even a small PIC like the 16F627 can....

That's one UART design out of all the non-8051 designs that does. There are a couple others that do as well, but the vast majority (included the 16x50 series) don't. In the past few years I've used probably a dozen different non-8051 UARTs, and only of them had 9-bit UARTs.

Well, AVR can do that, and TMS24xx/28xx also.

There are a couple others that do as well, but the vast

It is quite enough that PC commport can't do that. Which means the difficulties with debugging of the protocols. So, 9-bit UART modes should never be used.

In the past few

Most if not all of UARTs support for parity/noparity bit. By toggling parity/noparity, you can simulate the 9-bit operation. Although it is a very inconvenient way to do it.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

If you really want to use 9-bit mode (it does provide a way to design a very nice low-overhead protocol), there are PCI serial boards that have 9-bit support.

True. Among other inconveninces, you have to disable the FIFOs which is painful at high baud rates.

On MC68360/MPC860/MPC8260 etc. the SCC UARTs can be programmed to automatically implement the address/data functionality using the 9th bit. The SMC UARTs on these chips support 5-14 bit characters, so you can easily roll your own address/data protocol using for instance the

Paul

Even the NS8250 -family of UARTs (like those on PC) can send and receive 9 bit frames by using the badly documented 'stick parity' control bit.

However, none of the normal drivers understand the mode, AFAIK.