Allen Bradley PLC-5 Remote Chassis

I have attached a picture of PLC-5 comparison chart in which no of I/O remote chassis are given for different PLCs. But again the terms chassis and rack created a confusion.

for PLC-5/15:-
Remote Chassis
(I/O Racks)

12
(3 I/O Racks)

What I am able to understand is, it can support 12 remote chassis
but what are 3 I/O racks? is it group of slots? because sometimes rack is often taken for a group of I/O slot i.e 8 input and 8 output.
Can anybody explain this.

I am rusty with the '5....but

I think it should read 12 logical racks, and a total of 3 I/O racks including the local chassis.

Weird chart, why did they have them listed in that order?

I rarely worked with any PLC-5 below the 5/40 Enhanced, so someone with more experience needs to come along and correct or confirm this.

On a 5/15 you can have 4 racks
Rack 0 is the resident rack with the PLC5 processor in it and 3 other racks are 1,2,3.

Now if you have a 16 slot chassis with single slot addressing, with cards that have 16 inputs or outputs, then the first 8 are slots, 0 to 7 are the resident rack for the PLC5 processor and the next 8 slots are rack 1 in slots 10 to 17.
A second chassis becomes racks 2 and 3

The next possibility is two slot addressing, where the same 16 slots are then only rack 0, with cards that only have 8 inputs or outputs, this is the resident rack for the PLC5, then the next 16 slot chassis is rack 1, then next chassis rack 2 the onto rack 3

There are other options or a mix but the two I describe are the commonest

As for the number 12 I don't see the significance of it.


the second chassis then is racks 2 and 3

Here is a chart that makes more sense.

Note the difference between "Rack" a logical term. And "Device" (chassis/adapter) a physical term.

http://www.plctalk.net/qanda/showpost.php?p=505829&postcount=3

According to Mickey Chart,
PLC-5/15:-
Local Racks 4 (1 Resident). It means a Chassis of 4 Slots.
Remote Racks 3. It means a Chassis of 3 Slots.
But what is mean by (up to 12 physical devices)
Is it a multiplication of local and remote racks i.e

PLC-5/15 4X3 = 12 (4 local and 3 remote racks)

PLC-5/25 4X7 = 28 (4 local and 7 remote racks)

The key bit of information here...

When A-B says chassis, they are talking about hardware. Chassis come in various sizes 4, 8, 12, 16 physical card slots.

When A-B says I/O rack, they are talking about logical racks consisting of 8 I/O groups. The limitation here is the PLCs I/O table.

Depending on the cards installed and the "slot addressing" used, a 16 slot chassis can be 1, 2 or 4 logical racks.

A 16-slot chassis with 16 8-point cards and 2-slot addressing will be one logical rack.

A 16-slot chassis with 16 16-point cards and 1-slot addressing will be two logical racks.

A 16-slot chassis with 16 32-point cards and 1/2-slot addressing will be four logical racks.

There are other combinations but I don't want to confuse the issue more.