RS 485 port device driver for 68681 on OS9

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Thread Starter

Pravin Fatnani

Hello,

Pl. excuse me for directing this question to the list.I am looking for some help in writing RS 485 device drivers for Motorola 68681 serial interface controller chip on OS9.

We have the driver for RS 232 port and the same needs to be modified for RS 485. This driver seems too complex without proper comments for help.

Would like to know if someone has this kind of (implementing 485 port using 68681) driver for OS9. Would appreciate if some one can direct to an active mailing list on OS9.

Thanks.

Pravin
 
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A.K.Malai Samy

Hello,
I know that the following firm is making the device driver for 68681. Southern Automac Systems, Chennai-600 096, FAX; 044- 496 1029
 
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Alex Pavloff

Sure. Grab two converters from B&B or Your Favorite Supplier Co, stick them on either end, and wire them up. I've seen it done before, and it works.

Alex Pavloff
Software Engineer
Eason Technology
 
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Putting my pedant mode on maximum, RS422 is strictly speaking a point to point protocol (i.e. equivalent to RS232 but with a differential pair), so designed for a two station full duplex connection. Multidrop on RS422 is possible providing the stations tri-state their outputs, RS485 style, when not transmitting (i.e. place them into a high impedance state so they don't
interfere with other transmissions). Note also that typically in a master slave arrangement, the master doesn't tristate it's output as it has it's
own dedicated transmit channel.

All this is well worth knowing. Although most so-called RS422 devices (including our own), are in fact the superset hybrid allowing multi-drop,
you do occasionally come across the odd piece of hardware whose designer has stubbornly followed the letter of RS422, so the device won't co-exist with others.

Standards, standards, where would we be without them...?

Tim Linnell (Eurotherm Controls)
 
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Michal Casterline

232 was designed to connect Data Terminal Equpment or DTE (in modern times a PC) to Data Communications Equipment or DCE (in modern times a modem). You can check the EIA-232 standard, but I think the design limitation is 50'. I would never run it outside of a control panel, though people get away with it every day.

EIA-422 (commonly known as RS422 or just 422) was the first standard that the EIA came out with for serial communications with a differential pair of
signals enabling a distance of 4000' if proper terminations and grounding practices were observed. It allowed for only 16 nodes and had no half-duplex capability. So, it was normally used for full-duplex applications. There would be one pair for the master to transmit on and another pair for all slaves to respond on.

Then came EIA-485. Electrically, it allows 32 nodes. Additionally, it is tri-state. That is, you can use a hand-shake line to determine transmit and receive modes enabling half-duplex.

It is now no cheaper to make a 422 chip than a 485 chip. Any contemporary chip you buy is electrically 485 (32 nodes) and has a handshake line.

The result is that on the street, 422 means full duplex despite the fact that it is technically 485 full duplex.

Now, consider your application where all you need to do is boost the distance between two 232 devices.

If a device advertises itself as a 232-422 converter, you'll be able to use it without any problem. One pair will take care of your 232 transmit line, and the other pair will take care of the 232 receive line. The fact that it's 485 electrically is unimportant.

A 232-to-485 converter could potentially accomplish the same thing, but be aware that the assumption that it will be used in a half-duplex application means that it will either require a handshake line (RTS for instance) or utilize a timer that is bit-rate (Baud for the sake of this argument) dependent.

My advice? If you are the least bit uncomfortable with the details, find yourself a pair of 232-to-422 adapters and don't worry about the details.

If you're a knucklehead like me, find a good 485 converter that is very flexible and can be used in a variety of applications. This knowledge might
pull you out of your next conundrum.

Michal Casterline
Ingersoll Cinetic Automation
 
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Mauricio Orellana

You won't have problem doing this way. I think it is the cheapest way to do it.
good luck
Mauricio Orellana
Ingdesi SA.
 
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Curt Wuollet

A pair of short haul modems from BlackBox or Global would handle this without any conversion and simple wiring. Most have some degree of surge suppression and robustness. Or even regular cheap old modems if you can inject talk power.

Regards

cww
 
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Bob Peterson

> 232 was designed to connect Data Terminal Equpment or DTE (in modern times a
> PC) to Data Communications Equipment or DCE (in modern times a modem). You
> can check the EIA-232 standard, but I think the design limitation is 50'. I
> would never run it outside of a control panel, though people get away with
> it every day.

Actually, the design limitation is a certain amount of cable capacitance. The 50' rule of thumb comes from the amount of capacitance in commonly used shielded cables. Low capacitance cables are available that can greatly increase the distance you can run RS232. You can also run it a LOT farther by reducing the baud rate. A friend of mine once worked for an aerospace
company. They did some extensive testing on RS232 error rates, and found that by reducing the baud rate, they could reliably extend the distance to quite some distance. I do not recall the exact numbers, but the figure of 1000' @ 2400 baud sticks in my mind.

RS485/RS422 has the advantage of inherent nosie immunity, that RS232 does not. This is an even better reason to use it.

Bob Peterson
 
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