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RS-485 implementation on Lantronix XPort-485

Hi,

just browsed the manuals for Lantronix' XPort-485

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and I'm a little bit confused:

From RS-485 spec I understood, that 485 is a multidrop bus, which

features a ninth bit to distinguish between "Hey here comes an address" and "Whoever feels responsible for the previous address, here's data". How is this ninth bit pushed on the bus by Lantronix Xport-485 ? What I see is 7/8 bit, stop and parity. But where is the "This is an address"-bit? Unfortunately, they don't have a forum on their site, therefore I a'm asking the whole planet....

Kind regards

Frank

Reply to
Frank
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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

Reply to
The Real Andy

RS485 is a physical layer specification, half duplex..

9 bit communications is a function of some "enhanced" UART variants
Reply to
News

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

Reply to
Meindert Sprang

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

Reply to
James Beck

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?

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Reply to
Grant Edwards

[...]

You are correct.

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

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Reply to
Grant Edwards

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and I'm a little bit

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

Reply to
Frank

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

J
Reply to
no-reply-john

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.

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Reply to
Grant Edwards

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

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Reply to
Vladimir Vassilevsky

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.

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Reply to
Grant Edwards

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

9th bit.

Paul

Reply to
Paul Keinanen

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.

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Reply to
Tauno Voipio

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