Re: Need a simple protocol

On 20 Jul, in article
<1153413323.594141.31890@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
dave.bryan@xxxxxxxxx "Daveb" wrote:

I'm involved in an application which has a up to 10 multidrop devices
at the other end of a 4km length of cable. We need to implement a
communications protocol which will allow a master to reliably
communicate with the slave devices. Due to the distance, the data rates
will be kept to a few kbit/sec but its mainly device control & status
rather than lots of data acquisition that is required.

Rather than implementing a custom protocol I'm keen to use something
that has been tried and tested in the field. The "MODBUS over Serial
Line" protocol looks a good example of this. I wondered if anyone has
experience of Modbus in this kind of application and can point out any
potential pitfalls. Any other suggestions for a suitable protocol
welcomed!

Thanks to all who replied to my original post.

The main problem of Modbus binary serial encapsulation is the
difficulty of marking the frame boundaries.
.
.
IMHO, you'd be better off using a packet protocol resembling
the PPP over HDLC-style encapsulation. It's completely
binary-transparent and it can easily catch up after a
character loss / mutilation. Of course, the frame with the
dropped character is lost, anyway.

Just the kind of experience I was hoping to hear about, I'm curious why
Modbus is so popular though with this drawback. The HDLC protocol looks
interesting. I had some experience with PPP a long time ago & remember
it being quite complex, perhaps unncessarily so for this application. I
also thought it was point-to-point rather than point-to-multipoints but

Cannot comment about PPP-HDLC encapsulsation, but if you look at the
original HDLS specs they contain sync characters and address fields as
well as CRC, which was usually done in a dedicated hardware controller.
Having worked on HDLC[1] controllers that were boxes added to PDP-11s, long
before dedicated chips came out that were HDLC controller on a VLSI chip.

HDLC was the backbone of many protocols like X25 and was one of the
granddaddies of packet based protocols.

I presume more than one connection can be established over the same
link. Will need to look a bit into this option.

Again, thanks for the pointers

[1] if you see mention of SDLC this was IBM version of HDLC that was slightly
different but the difference was minor so even early VLSI chips had one
bit to determine which of HDLC and SDLC it was to use.

--
Paul Carpenter | paul@xxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

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