ModbusRTU over US internal telephone system


Thread Starter

James R Collins

My hardware has Modbus Capabilities and my client wishes to access this data over his telephone system. He intends to run software on a remote computer which is connected to the internal telephone system via a modem. A second modem will be required at the remote end and has to be interfaced with my hardware. Can anyone advise on which hardware will enable this remote access.
There are two ways to do it. You can treat it just like any other pots system as long as the internal PBX isn't completely digital. You can also use a leased-line type modem from data-linc or telenetics and just run two twisted-pairs from your equipment to his. This would be the best way since there is no dialing up and you don't need to waste 2 extensions. You can use the same cables as your phone system, just jump them together bypassing the PBX.
A good question - but one to be wary of - dial-up lines can become a huge "support liability". Make sure your liability stops after demo'ing the system and training the user.

First, I'd suggest you buy good external modems - expect to pay many hundred $$ each.

Have ALL the same brand & model so you don't have double or triple work to support. Especially avoid a "we'll buy ours - 'they' can buy their
own" situation. Buy 2 the same, test them sitting next to each other and carefully document every setting or problem you have.

If there is only 1 remote phone number involved you could even get a modem which can be internally programmed to dial a fixed number &
connect when activated. Because such modems often offer "security features" such as password or dial-back you probably can use this to justify the added expense to your customer (ie: no unauthorized person will be able to easily dial into the remote site).

Be warned that some modems CANNOT support an 11-bit async char - such as 9600,8,e,1. They only support a 10-but async char so parity can only be
used with 7,e,2 - but even then I've seen older commercial modems which cannot handle 8,n,1 (binary data with 8-data bits and no parity). For
example, I had to work with one which claimed binary support, but a particular 2-byte binary sequence (^A + something else) would flip it
unconditionally back to command-line mode. So either make sure your modems fully support 11-bit async chars, or plan to use Modbus/ASCII which is all data converted to 7-bit printable hexadecimal chars.

Finally, I'd make sure there is no better alternative based on TCP/IP and the customer's "other" systems. You may find that (after talking to the dreaded "IT" department) that they already have a way to access the remote site by direct TCP/IP. Avoiding the dialing issue this way will likely cost more up front, but cost less each month in infra-structure and especially support.

Best Regards

Lynn August Linse, Senior IA Application Engineer
15353 Barranca Parkway, Lantronix Inc, Irvine CA 92618
[email protected]
Tel: (949)279-3969 Fax: (949)453-7152
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