Retronetworking

First, do you need that much isolation ?
Most RS422 transceiver can tolerate a few tens of Volts of potential difference by themselves. For the datacenter, out of an abundance of caution, I went with several kV isolation ..

Second, might be worth considering just using isolated RS422 transceivers. Back when I made the board, there were all out of stock, so I didn't have much choice, but situation has improved a bit since then.

Thanks for your quick feedback!

On Fri, Feb 03, 2023 at 06:19:42PM +0000, tnt wrote:

First, do you need that much isolation ?

good question. Given that I expect anyone using such a device (myself
included) will interface some arcane retrocomputing hardware which is
impossible to replace, I would rather prefer to be on the safe side.
Yes, he isolation adds something like maybe 15 EUR of BOM cost, but then
I doubt there will ever be more than a few hand-assembled boards. And
most retrocomputing folks would probably rather invest in a few more EUR
for proper galvanic isolation to be on the safe side.

I could check if ITU says anything about the isolation requirements of
X.21. But even then, you don't know if some random equipment has a

Second, might be worth considering just using isolated RS422 transceivers. Back when I made the board, there were all out of stock, so I didn't have much choice, but situation has improved a bit since then.

My past experience with such transceivers (about a decade ago) indicated
that those were extremely expensive - and given that we'd need 5
bi-directional transceivers on a single board, it might actually be more
expensive even if available. But I'll have a look.

Another situation comes to mind: You might operate your Teletex or whatever retro equipment using an isolation transformer on the AC line input (for safety purpose, or during repairs) and in that case you cannot afford to leak a GND (with potential PE reference) back into the device via the X.21 adapter.

So the more I think about it, the more it makes sense to have isolation for at least 230V AC / 400V DC plus some margin.

Thanks for your feedback.

I've now pushed my draft schematics to https://gitea.osmocom.org/electronics/osmo-small-hardware/src/branch/laforge/x21

• have DTE/DCE/sniffer mode completely configurable by software (via the GPIO expanders)
• have proper isolation

Regarding your idea with two connectors and jumpers: also interesting. I don't have the time right
now to think of all related consequences.

laforge Make sure to check the ecn.md from the e1 hw repo, that contains some erratas that were manually reworked on the boards.

On 8 February 2023 10:22:34 CET, tnt <redmine@osmocom.org> wrote:

Issue #5463 has been updated by tnt.
laforge Make sure to check the ecn.md from the e1 hw repo, that contains some erratas that were manually reworked on the boards.

Yes, I'm aware, thanks.

vogelchr so who and how do we proceed here? should we have a call/chat at some point?

There was the question whether male/female are used to indicate DTE/DEC on X.21 for the 15pin connector.

• male connector towards the DCE
• female connector towards the DTE

• Oddly enough, my Olympia Disque (DTE) has a female connector towards the DCE.
• An Alfatron A600 is a device that can switch X.21 between DTE and DCE and it uses female
• Both of my ISDN X.21 TA (DCE) have female connectors
• My Siemens DAG (DCE) has female connectors

So the majority is clear: a DCE has female, and a DTE has male.

I just confirmed the A/B polarity vs. X.21 logical levels using the Wandel_Goltermann_DTX-1:

Data lines (R/T)¶

Control lines (I/C)¶