AB Backplane Head "ache"

Ive just come across a problem that Ive seen before. Upgrading a
PLC2-30 with four racks. All the ASB modules were set up ahead of time. Rack one, two and three are A4B 16 Slot. The fourth is an 8 Slot. I Pulled the comm modules and set the back-plane switches, installed the PLC5/40 and the ASB modules. Powered everything up logged in and did autoconfig. Rack one is the PLC resident rack configured as Rack one rather than the PLC5 default. (for addressing on the wires). Ive done this before without any problems, but this time racks one and four were OK, but racks two and three could'nt be configured as two slot addressing. I need all the racks set at two slot addressing or I've got a lot of rewiring to do. The modules are 8 point input and output, so I could replace them with 16 point modules, but rewiring 19 modules??? Ouch! One other time this happened and we ended up changing the rack. It solved the problem but what is with these switches?
Has anyone seen this before and what did you do to fix it? What can these racks be used for?

Special note:
These racks have plastic "locking" ID Tabs, not the metal rail on the top, above the modules. Rack two backplane switch is black in colour, and doesnt look like a repair job. I have never seen a black one before. Rack three is red, but the numbers on the switch are arranged backwards. The backplane calls position one on the top, on the switch one is the bottom. Open is labeled on the backplane as Left and closed is right. Because the switch is backwards, "open" is on the right. I used the backplane switch numbers and the label on the switch for the state, right being open. This didnt help.
I spent two hours changing these settings on both racks. I had to quit to put the PLC2-30 back in and test. Playing with these settings wears the switch to the point they no longer work.

My next step is to buy two new racks and preconfigure both the ASB modules and the racks. This means pulling all the modules at change over. Is there something easier I could do? Are these old racks now junk? No one wants to send them out to be repaired.

I love this job!!!
Any info would be gladly received.

I may not have read all of this correctly but I assume the two chassis giving trouble are older Series A. If so I suspect your most effective course is to ditch the old ones and use two new Series B chassis.

You may be able to get it all to work with the old Series A's, but frankly it will always be a timebomb for someone down the road.

Rack

No, I checked this before I began. The two bad racks are Identical to The first one, which works fine. Using the older series would give me a configuration error, I think. Not sure on that tho. Looks like I have to dump them.
Thanks for your reply, Im doing some searching on this now.
Bruce

The part number of the chassis will tell you its Series letter for certain.

Series A racks were "1771-A2", "1771-A4", etc.

Series B racks are "1771-A2B", "1771-A4B", etc.

Your description of the DIP switch being backwards really throws me. I've never heard of such a thing either.

Tomorrow I have to inventory a bunch of old PLC-5 and PLC-2 hardware.... I'll eyeball the DIP switches while I'm hoisting them onto shelves.

If I remember correctly there was an issue with those switches. If I remember correctly A-B purchaced some switches that had a slightly higher profile than the old red ones. The new switches were too high, however, and would get mashed when inserting certain modules. If I remember correctly A-B did a massive recall on these things.

Does anyone remember this? Should have been around 1990-1991.

Regarding racks 2 and 3 and 2-slot addressing, if the racks are old A4 chassis then they ONLY have 2-slot addressing. Could rack 1 be set up for 1 slot instead of 2 slot addressing? This would make the local racks 1 and 2. Worth looking at.

Good Luck,

(8{)} ( .)

update

The racks are A4B and are old. They are in a very cold area of the mill and the switches are probably shot. The Different type of switch was a higher profile. Havent heard of a recall tho. Probably too late for us. I ordered two new racks today. These old ones are destined to the burner.....(evil grin)

Bruce