friends,
Pls. let me know why in PLC I/O cards, grouping is done either 4,8 0r 16 i/p's or o/p's. Any specific reason for this ?

A single common is used for either 4 /8/16 channels - any technicalities ??

 

The answer in one simple word is "cost".
I remember the very early days of PLCs (oh, yawn) when I/O cards were 4-channel, or maybe even single channel. Here it was quite common to have each circuit wired individually. However, as I/O density rose over the years, and there were the additional pressures of size (make it smaller) and cost (make it cheaper), it became much more common to simplify the on-board wiring and PCB layout, and the external connectors by grouping the I/O in to clusters with common return connections for each cluster.

It's common nowadays to have 32-point I/O cards. Let's say each of these channels was individually wired. We would require a minimum of 64 terminals on the connector, plus any additional terminals for powering the card etc. On the other hand if we have, say, 4 groups of 8, then our minimum terminal count comes down to 36 (i.e. 32 connections plus 4 returns) and the this is a lot cheaper to make. Plus, in a given size of connector, can you imagine how small the terminals would have to be if there were 64 of them as opposed to 36? The down-side of this is of course that when you manage to damage part of the card with an overvoltage it may take out 4 or 8 channels at a time, rather than just one.

There are also issues associated with channel-to-channel isolation when you group them - this can never be as good as when they are ungrouped unless you push the cost back up. So, you pays your money, and you makes your choice ...

Ken

 

> Pls. let me know why in PLC I/O cards, grouping is done either 4,8 0r 16 i/p's or o/p's. Any specific reason for this ? <

Word, byte, 1/2 byte

> A single common is used for either 4 /8/16 channels - any technicalities ?? <

Load

 

Also, wiring time is reduced substantially. Generally, all the inputs in an entire machine share the same common anyway, or at least you can find enough from a given common to fill a bank, and now you have saved the effort of putting in 28 (31-3) jumpers on the common, as well as the increased wiring time to deal with the 64 microscopic terminals.

 

It's a trade off between terminal count and flexibility. Pin out each seperately and it's versatile but you have to do a lot of power/common wiring. And terminal cost and space are big factors. Bus too many together and you may have "wasted" ins or outs if they need to be isolated. As far as 4 or 8, it might have something to do with word width on early machines.
I visited this issue when I designed my IO translator cards and there are a lot of subtle tradeoffs like fusing and PCB trace size. I used groups of 8 because the IC current drivers
used happened to be 8 wide and matched it with the inputs. Four would be better, but would require discrete drivers and again more terminals. Since the whole idea was to lower costs by taking advantage of advances in technology, it came down to chip widths in the end.

Regards

cww who is currently not employed in automation.

--
Free Tools!
Machine Automation Tools (LinuxPLC) Free, Truly Open & Publicly Owned Industrial Automation Software For Linux. mat.sourceforge.net.
Day Job: Heartland Engineering, Automation & ATE for Automotive Rebuilders.
Consultancy: Wide Open Technologies: Moving Business & Automation to Linux.