OT..RS-232 shielding

Skidood

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Oct 2016
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Hi group, I have (2) 6-foot lengths of cable, one is the HMI serial comm cable between a PLC and an Automation Direct EA9 HMI ....the other cable goes between a 2nd PLC serial port and a printer. The wiring is not routed with high voltage lines and the highest voltage in the panel is 110V. How important is shielding in this case? The cabling is mostly hand-made-up and I have to install 2 DB9 connectors basically just as couplers within each cable and bringing the shields thru each connector will be a bit of a pain.
Any advice? Thank you.
 
RS232 is only 5 VDC signal you should always use a shielded cable on it.
The shield should be terminated only at one place, usually at the source end.
The cable run should be as short as possible without splices.
normally RS232 is limited to 25 ft max cable between devices I have extended that to 50 ft with slower baud rates
 
RS232 is only 5 VDC signal you should always use a shielded cable on it.
The shield should be terminated only at one place, usually at the source end.
The cable run should be as short as possible without splices.
normally RS232 is limited to 25 ft max cable between devices I have extended that to 50 ft with slower baud rates

RS-232 is a +/- 12V signal by standard. Some people make "RS-232" devices with only a -5/+5 Vdc signal, but mostly they will tell you it is RS-232 "compatible", as the mark/space voltages are < -3V, and > +3V.

I've even also seen a so-called "RS-232" interface that used TTL voltage levels of 0 to 5V dc. Strictly speaking, neither the -5/+5, or the 0/+5 signal levels can be called RS-232.

Yes, cable shield connected at one end only to a good ground point. Do not connect the "Common Ground" pin of the DB-9 (or DB-25) connector to the shield. Most of the Google images of RS-232 wiring do not even show the shield, as it is NOT part of the RS-232 signal wiring - think of it as a "conduit".
 
Before we switched to Ethernet for everything, 20 years ago, and we has RS232 between the PLC and the door-mounted HMI, we just used plain standard 2m null-modem cables with no shielding. Never had any issues with it. Some times there were VFDs and big motor starters in the panel, but it didnt make any difference.

Anything longer than 2 m, we would use 232/422 converters and shielding.
 
Before we switched to Ethernet for everything, 20 years ago, and we has RS232 between the PLC and the door-mounted HMI, we just used plain standard 2m null-modem cables with no shielding. Never had any issues with it. Some times there were VFDs and big motor starters in the panel, but it didnt make any difference.

Anything longer than 2 m, we would use 232/422 converters and shielding.

A properly shielded RS-232 cable, swinging a decent voltage below -3V and above +3V, routed away from VFD cables and other sources of interference, up to 50 feet is a "good" connection. The higher the voltage swing, the better the noise immunity. Due to cable resistance, you can have moise impulses at one end, and much less at the other.

Admittedly RS-422 will give you a greater noise immunity, as it is a bi-directional current signal, and any noise it picks up will be on all conductors, so the interface transceivers reject any noise much better.
 
Sure, shielded is better, but is it necessary.
I never had issues with unshielded RS232 over short connections.

Admittedly RS-422 will give you a greater noise immunity, as it is a bi-directional current signal
For smugness points, I have to point out that the big difference between RS232 and RS422 is that only the latter uses differential signals.
It is still voltage, not current. You must be thinking about current loop aka TTY.
 
I have had the 'opportunity' to work on some automation machines with IDEC PLCs & HMIs with serial comm's but only 3 screw terminals on each. The RS-232 was run as 3 separate wires in the bundle to the operator panel. These were the older IDECs as the newer ones have serial cable ports.

I ended up as a rule rewiring the older ones with a shielded cable.
 
I've worked with RS-232 for over 20 years now and I've only come across one instance where noise was a problem and that involved the RS232 cable between the radio modem and PLC was laying on top of 3000 VDC battery charging cables and capacitors (at SeaTac Airport). Shielding isn't bad but in most cases not required. The limitation on distance has nothing to do with noise but rather the ability for the driver to move the current through the lines. The longer you go the harder it gets (I.E. the lower the received signal).
 
Grounding is the real kicker with RS-232. If your devices are not well grounded, you will have problems. The reason being is RS-232 is a ground referenced signal. If you get grounding differentials between devices, well... I've seen some things actually catch on fire from it. Communications cabling isn't designed to handle current. :)

As others have stated, shielding is good and sometimes necessary. Personally I wouldn't even think of putting in RS-232 without shielding. It's just cheap insurance.
 
Sure, shielded is better, but is it necessary.
I never had issues with unshielded RS232 over short connections.

For smugness points, I have to point out that the big difference between RS232 and RS422 is that only the latter uses differential signals.
It is still voltage, not current. You must be thinking about current loop aka TTY.

You can have your smugness points, I stand corrected. Perhaps I was thinking of RS-485, but that also is a voltage, not current.

It's been many years since I did any serial interfaces, so long ago that one of the ones I put on a job was for an RS-232 dot-matrix printer located about a quarter of a a mile away from the PLC cabinet. We used RS-232 to current loop converters at both ends and it worked first time, and was probably still working before the site closed down.
 
Just know this: At some point you or your replacement will have problems with one of these analog inputs. If non-shielded wire is used then that will be first thing someone will bring up and will recommend replacing with shielded cable to eliminate any chance of noise causing the problem.

Why not avoid this problem in before it happens.
 
Just know this: At some point you or your replacement will have problems with one of these analog inputs. If non-shielded wire is used then that will be first thing someone will bring up and will recommend replacing with shielded cable to eliminate any chance of noise causing the problem.

Why not avoid this problem in before it happens.

RS232 isn't an analog signal which is why shielding typically isn't needed. It takes very little "noise" to disrupt an analog signal (depending on the resolution) because the noise can look like the signal itself but with digital it takes a lot of noise especially when you're dealing with the upper end of the spec (12VDC or higher).
 
RS232 isn't an analog signal which is why shielding typically isn't needed. It takes very little "noise" to disrupt an analog signal (depending on the resolution) because the noise can look like the signal itself but with digital it takes a lot of noise especially when you're dealing with the upper end of the spec (12VDC or higher).

Oops! I put the above response in the wrong thread. There has been another recent thread about analog wiring. Sorry for any confusion.
 

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