Twinax balun, etc. questions

[doshea]

I've got one of these 5250-to-USB converters, some clone 5250 adapters and a compatible terminal. I've got twinax pigtails and cables which work, but also have some RJ-45 baluns and I was wondering if I could use a pair of baluns and an RJ-45 cable between them instead of a twinax cable like this:

           terminal                5250 adapter                5250 adapter
              |                         |                           |
nothing -- pigtail -- twinax cable -- pigtail -- twinax cable -- pigtail -- balun -- RJ-45 cable -- balun -- converter

or can the baluns can only be used with a hub?

https://www.twindata.com/technical/whatisabalun.htm says this amongst other things:

• Balun pin-out must match at both ends.
• Never daisy-chain baluns.
• Never use in a series.

The "at both ends" part makes me think I can have them at both ends of an RJ-45 run.

The manual I got with the adapters says "Never connect two baluns to one Y-Cable", but can I have two RJ-45 cables in one twinax "network" (not sure what the correct term is there) so long as they're not both attached to the same pigtail, or is that "using in a series"? i.e. could I do the following?

           terminal                                   5250 adapter               5250 adapter
              |                                           |                          |
nothing -- pigtail -- balun -- RJ-45 cable -- balun -- pigtail -- twinax cable -- pigtail -- balun -- RJ-45 cable -- balun -- converter

Regarding pin-outs, all the baluns I have say they use pins 4 and 5, and the above says they have to match at both ends, so I assume I can use those with a straight-through RJ-45 cable?

I realise the page I linked to seems to confirm some of these things, but I'm not sure if what I'm doing is what "normal" users of these things do!

I also have some twinax inline surge protectors. Would there be any drawbacks to using them? There shouldn't be too much risk of a surge in my setup but I do like the idea of doing a little something extra to try to keep my terminal alive for a bit longer

 

[bladamson]

Yeah, my assumption is that it's straight through. I am assuming they're just looking for a 1:1 current balun and there isn't any voltage matching going on there, since I assume the impedance is the same on both coax ends. Presumably all they are doing is using the transformer to convert the unbalanced signal on the coax (data on center, shield grounded) into an balanced signal on one pair in the cat5 (rather like a differential pair, in that any noise that gets in there affects both wires in the pair equally, but since the signal on one wire is exactly opposite the other, the noise gets cancelled out on the other end (in a perfect world lol)).

That is all supposition, so it might be wise for you to get a second opinion from someone who actually knows what they are talking about (unlike me).

If you are just setting these machines up as an educational exercise and there isn't a long run of twinlead going on here to try to save money vs coax, I would personally just delete the transformers and the twinlead and use coax. 5 or 6 feet isn't going to break the bank and it will probably be less prone to noise and weird stuff than the twin-lead. I like balanced line for RF stuff because it's way less lossy than coax at high SWR, but for an application like this coax is probably a far superior choice.

 

[doshea]

Thanks! But it's twinax, not coax, so from here at least it seems pretty expensive to buy, to the extent that I can even find it (the fact that there seems to be some standard for 10GB+ Ethernet transmission over some very different type of twinax makes it hard to search).

 

[Al Kossow]

IBM never really documented the 5250 twinax protocol the way they did for 3270.
National wrote some informative app notes you can find under http://bitsavers.org/components/national/_appNotes/
for their DP8244 biphase communications processor, in particular AP-516 and AP-517

so for point-point you would need a way to terminate the balanced transmission line on the computer and pcmcia adapter ends.
that's that the pigtails do. the unused twinax female is self-terminating. baluns have no terminators in them.

5250 is bi-phase encoded on two wires. Later versions of the adapters had RJ style connectors instead of the twinax to go to twisted-pair hubs
AP-516 shows the wiring for transmitters and receivers.

Twinax Baluns, Twinax Cables, and Twinax Wiring Products

RJ45 TO TWINAX BALUN 2 PINs active TWISTED PAIR | eBay

RJ45 TO TWINAX BALUN. TWISTED PAIR. We can send match sets of manufacture and active pins. 2 PINs active. Mixed Manufactures.

www.ebay.com

and the hub

Startek 299AT-1P Active Twinax Hub - 7 Port for RJ11 Connections | eBay

Click here to see description.

www.ebay.com

You will also need to know what the bits on the wire mean

5250 Data Stream Programming - MC Press Online

Getting complete control over your 5250 display by Craig Pelkie

www.mcpressonline.com

you can find the data stream reference here:

http://bitsavers.org/pdf/ibm/5250_5251/SA21-9247-6_IBM_5250_Information_Display_System_Functions_Reference_Manual_198703.pdf

 

[Al Kossow]

this is what a twinax twisted pair active hub looks like inside

 

[Al Kossow]

nothing -- pigtail -- twinax cable -- pigtail -- twinax cable -- pigtail -- balun -- RJ-45 cable -- balun -- converter

so you need to terminate the right-hand end with a terminator. the 'nothing' at the left end is actually a terminator

 

[doshea]

this is what a twinax twisted pair active hub looks like inside

Thanks, I'm afraid it doesn't mean all that much to me but from some Googling I suppose it's just converting to/from differential signals and doing some buffering or something?

nothing -- pigtail -- twinax cable -- pigtail -- twinax cable -- pigtail -- balun -- RJ-45 cable -- balun -- converter

so you need to terminate the right-hand end with a terminator. the 'nothing' at the left end is actually a terminator

Oops, I forgot about termination and the terminators that are in the pigtails! The converter has internal termination resistors which are enabled by default (there are jumpers to disable them) so I think what I have should work in that regard.

Thanks for the info (and all the other info I found on bitsavers)!