Coexistence of MODBUS & HDLC


Thread Starter

Alireza T.

My project is design and implementation of an industrial automation communication link between a master (a pc) and 40 slaves (8051 microcontroller boards), (via 20 rs485 ports of master). We chose STAR topology and the link baud rate must be 2MBps (Asynchronous). we must include CRC checking. our frame length must be restricted to 10 bytes (we have 4 data bytes).

at the first stage i chose MODBUS protocol for doing that, but i found out that its baud rate is restricted to 1MBps, although the medium used is RS485 & as you know better, its baud may be a value up to 2MBps, i don't know which parameters of MODBUS restrict baud rate, and if we can choose a higher baud rate of 2mBps.

beside this, as we know MODBUS is an application layer protocol and it utilizes HDLC as it's layer 2 protocol. so i'm confused that by considering this, how MODBUS (an asynchronous protocol) is joint to HDLC (a synchronous one) and i don't know if i can use only one of these two for my project or not.

at last, i don't know how to program the link between layer 7 (MODBUS) and layer 2 (HDLC) if i have to do so. finally i do thank you alot in advance for your help and i'll be so grateful if you send me some source code that will help me.

Ganesh Okade

Dear Alireza,
On the face of it, your statement "MODBUS supports baud rate of upto 1 MBps only" is wrong. MODBUS is an application layer protocol and does not specify which physical media is to be used, which means that it also does not speak about baud rates or for that matter restrict it. Actual baud rate supported is governed by the media that you use and for RS485 it can go upto 12 Mbps theoreotically.
If you have any more questions on this please let me know.

Ganesh Okade
Sunlux Technologies Ltd.

You seem to have confused between MODBUS - serial, asynchronous, open (profusely documented) and MODBUS PLUS - HDLC, proprietary (hardware and software).
The first has no baudrate limit. The second supports only one baudrate (of the proprietary hardware).

Meir Saggie

Lynn at Alist

Looks like you are confused between "modbus" as a generic terms, and ModbusPlus, which is Modicon's unpublished peer-to-peer token passing network with transformer-isolation running at 1mbps. I do believe it uses a (possibly modified) HDLC layer as part of the token bus.

Modbus as an app layer can run on async serial or TCP/IP or I guess anything else you desire. It has no direct relationship to HDLC.

But as for ModbusPlus, you'll have to talk to Modicon and sign a bunch of contracts to get any answers to your questions ;^)

Best Regards
- Lynn A Linse, Digi.Com