I've built a line-sniffer for my mid-80s System/36 (5362) in an attempt
to fully understand the TwinAx electrical and protocol layers. The long
term goal is to build an affordable open-source terminal emulator
providing either a TN5250 bridge to emulated TwinAx terminal sessions or
a more simple telnet/ASCII/vt10x translation on a series of TCP ports by
a WiFi ESP32. offers a lot of similar products but they
are closed-source and far outside of the price range of most retro
computing enthusiasts. There doesn't appear to be a lot of people
running older System/32, S/34, S/38 and gen-1 AS/400s. But the keyboard
pirates are slowly destroying older terminals and I want the surviving
machines to be easy to interface to so they can be as usable as

I've worked through all the electrical and framing issues and annotated
a few dumps of the S/36 IPL with both an InfoWindow II and 3179
attached. The output of which can be found here:

The message structure and limited command descriptions came from this
document starting on PDF page 122 / printed page number 4-9:

However it is incomplete. Not all commands are enumerated, the modifier
address bits are not explained (addresses a LU inside the terminal), and
none of the context or arguments for the commands are elaborated on. A
simple example of what I am referring to is, for example, the
explanation of the argument bits for the SET MODE command, etc.

Can anyone point me to a document or resource that fully explains the
TwinAx 5251 model 12 display protocol? I've been searching for days now
and no luck. And I really can't make anymore progress until I'm
unblocked by documentation. I simply don't have enough time to
trial-and-error reverse engineer bit meanings.


Alan Hightower

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