We are looking at Digi Xbee modules for basic terminal connectivity. I noticed that there is Modbus iDigi Dia activity. Has there been any activity with porting traditional wired HART using iDigi Dia ?
The present physical layer of HART is Bell 202 FSK running on 4-20 mA Analog Control at a blinding speed
of 1.2kbps. Despite the slow speed, there are tens of millions of devices that use wired HART.
HART devices communicate using a Device Description (DD)
file which details low-level HART frame formatting instructions for each variable in the device.
The HART foundation is introducing a new wireless HART specification without including a wireless platform for legacy systems. The previously mentioned tens of millions of HART devices do not have a readily available upgrade path.
If iDigi Dia had a HART Device Description parser, the only physical barrier to using Xbee modules for a legacy wired HART gateway would be a HART FSK modem and support for the ultra-slow 1.2kbps baud rate.
Moreover, traditional 4-20 mA control applications could be retrofitted with a wireless upgrade for data logging and asset management by implementing a independent iDigi Dia HART Master.
One could use a serial Modbus-to-wired HART device. Digi doesn’t have the modem circuit in the AIO adapter. One could obviously create your own XBee to 4-20mA circuit with the HART modem in place, moving the HART registers via serial-encap.
As is Digi has no concrete plans to make wireless-HART as it is a small niche market with DUST proprietary firmware.
Yah, hard to imagine it as a niche if that ALL you hear about, yet the volumes for wireless-HART are tiny compared to Zigbee and other wider standards.
My point was that Wireless HART is tiny. Wired HART is huge. Support for the HART signal using XBee modules would help both extend legacy systems and offer new data collection and asset management options.
Unfortunately, XBee modules do not support 1200 baud. Programmable live HART modem support would be needed.
Very small technical issues.
My vision is an XBee link from an existing HART master to multiple HART Slaves. HART supports a “Multidrop” mode with up to 64 devices on a link.
A HART modem can talk to a HART slave even without 4-20mA power. The modem tones are independent of the loop current. Existing HART devices could become instant XBee wireless devices.
It would be an interesting use - setting zero (0) in the Zigbee version of the XBee gives you 1200 baud.
However I would probably pay the $1 extra to put a small PIC/micro in between the XBee and the FSK modem anyway. That way you can handle the HART packets - as well as direct Modbus/RTU-via-mesh requests.
I don’t know if any adhoc form of HART-in-TCP exists, but if you had both HART and Modbus access, then one could use Modbus/TCP to the edge of the mesh and receive Modbus responses from your HART field nodes.
I guess I am in the wrong Forum. IDigi Dia is all about creating new applications. I was thinking of a Legacy DCS-to-XBee application.
In reality, I would like a XBee HART Master adapter and an XBee HART Slave adapter-- wireless black boxes, not a PC application. The XBee HART Master Adapter would sit on the DeltaV/Ovation/NI Lookout or Siemens Step7 HART interface. The XBee HART Slave adapters would be connected to each slave device. Each devices polling address would be set so they would not conflict.
Some plants would like a remote configuration capability, rather than full DCS support. Adding XBee HART Slave adapters to each device with a XBee HART master adapter attached to a HART communicator such as a Rosemount 275/Emerson 375 or Meriam MFC 4100 would add such a capability without having to replace the plant equipment to new (and untested) wireless HART devices.
You are correct: It wouldn’t take much more than a PIC to implement it. Another “spare time” project…