Yes, with a 4-20 mA current signal, you will have less loss than with a 0-10 V voltage signal. However, if the distance becomes really long (more than 30m), it would be wise to consider a solution by IOLink for a greater precision of the signal measurement.
What unit is m in "30m"? 30 meters or 30 miles? Neither computes as a long distance communication solution according to the summary statements on IO-Link on the ISA site:
"The IO-Link organization describes its goal as bridging the last meter to intelligent sensors and actuators."
"IO-Link is a serial point-to-point communication connection between two devices, typically at transmission rates of 4.8 kbaud and 38.4 kbaud."
"The connection between the IO-Link master and device is accomplished with an unscreened cable (maximum length 20 meters) that includes power for devices."
I've seen current loop used for serial transmission (RS-232 on one side of a pair of current loop modems) at 9600 baud for 2.5 miles point to point. I've seen 4-20mA run 5 miles point to point.
While I/O-Link will provide the advantage of device power on the signal wires and digital precision (over analog), I-O Link can not be characterized as a "long distance" communications solution with a limitation of 20 meters.
Well noted!
Thank you for these precisions.
I have some questions:
- Would you still recommend the 4 20 mA current loop if there are several signals to follow?
- Are there any disadvantages with the 4 20mA current loop?
Thank you!
> Would you still recommend the 4 20 mA current loop if there are several signals to follow?
Absolutely. The learning curve and capital investment to work with digital protocols is not worth it when 4-20mA/HART is so widely established and accepted. Case in point, a local plant that jumped on the Foundation Fieldbus (FF) bandwagon in the late 1990's is ripping it out and replacing all the field instruments with 4-20mA/HART. All the guys they trained in FF 20 years ago no longer work there or are in different positions and they can't find people who are skilled in FF to keep the network up and running.
> Are there any disadvantages with the 4 20mA current loop?
Yes, all the promotions for digital protocols will pooh-pooh 4-20mA because it is a "single variable" communications network. Yup, and that's the beauty of 4-20mA - it's not a complex digital protocol with 482 rules to learn before you do anything. but that's the advantage of 4-20mA. Ordinary people who can work with a DVM can keep a 4-20mA/HART system running.
The one drawback to 4-20mA is its power consumption. 4-20mA is not a good choice for field instruments when battery powered wireless communications is involved. PLC's or DCS's analog output cards are a major DC power consumer. PLC's or DCS's running large numbers of analog output cards need to be sure that the power supply is adequate for total load.
