Could anyone advise me on the following problem.
I have a machine, with allen bradley analog output card 1746-no4v, this is being used to control an Smc air regulator itv2030-312bs3-q, the problem is as follows, the smc is 3 wire + - and analog in , the Allen Bradley module has external psu and 2 wire analog out, the smc has been connected as follows + and - to supply and analog in to analog + of Allen Bradley the analog out common from Allen Bradley is not connected.
It works partially but is not correct, can I connect the supply common to the analog common on the AB module or is there an easy answer? I think I might have to purchase a 4 - 20 ma version of the smc unit.

Connect the analog common from the 1746-NO4V to the V- on the regulator. This will give the regulator a zero voltage to which it can compare the analog input voltage.

Eddie, thankyou for your reply,in regulator I assume you mean the 24volt power supply common-?

The SMC User Manual for this regulator (I had to look it up) shows the Power common and the Analog Input common to be the same terminal.

Connect the Analog Common from the 1746-NO4V channel to the SMC Regulator's DC Common terminal (blue wire).

On the 1746-NO4V module, the Analog Common is pin 1, 3, 5, or 7 depending on which of the four channels you are using. See page 3-13 of the User Manual, Publication 1746-UM005.

It's not really correct to say that a 24 VDC power supply has a "common". It has a positive and negative terminal. You establish the "common" or "signal ground" by which one you reference.

I don't know if they still do it but at one time, British cars used a "negative" voltage system. The common was the positive terminal.

It is also pretty standard to see op amps require both a positive and negative rail in electronics, leading to +/-12VDC or +/-15 VDC power supplies for instrumentation everywhere outside of industrial equipment. I don't know what the standard is anymore especially in the era of low voltage CPU's for speed reasons but at one time a standard PC power supply provided 5 VDC (which was the regulated output) as well as +/-12 VDC which typically all the rotating discs and fans. In that case, the power supply may be a 24 VDC power supply but it will have a V+, a V-, AND a common because it is supplying +/-12 VDC.

It is also typical to see a separate earthing ground, shield, and signal common because they all have different purposes. If you have AC equipment, the earthing ground is typically very noisy and will cause lots of problems in your analog circuitry if you connect the two ground planes except at the customary single point where the system bonding jumper goes. Tying the shield to ground on shielded signal wiring at both ends creates a loop antenna which tends to couple and amplify any nearby magnetic fields into your clean analog circuits, especially with voltage inputs.

Both points very well taken. A positive-ground Volkswagen once caused me a lot of head-scratching.

I may have taken some liberties with the nomenclature because of the original poster's confusion.


Bulk Power Supply +24VDC -> SMC Regulator "Power Supply", Pin 1, Brown Wire
A-B Analog Output (+) -> SMC Regulator "Input Signal", Pin 2, White Wire

A-B Analog Output (-) -> SMC Regulator "GND (COMMON)", Pin 3, Blue Wire
Bulk Power Supply 0 VDC -> SMC Regulator "GND (COMMON)", Pin 3, Blue Wire

thankyou, I will try later when I get to work

yes that cured the problem