IBM 3174 C 6.4 Microcode Disks?

Dave Wade dave.g4ugm at
Mon Feb 18 02:20:08 CST 2019

I will apologise at the top as this is slightly rambling...

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Grant Taylor via
> cctalk
> Sent: 18 February 2019 03:46
> To: cctalk at
> Subject: Re: IBM 3174 C 6.4 Microcode Disks?
> On 2/17/19 2:23 AM, Dave Wade wrote:
> > I would say the 3174 is acting more as a Terminal Server rather than a
> > gateway.  So 3270 CO-AX terminals which are directly attached to the
> > 3174 can connect via TN3270 to another host. Often this is Hercules
> > where Hercules provides a a TN3270 server which is presented to the
> > host as a channel attached 3174...
> I think we should clarify what "Terminal Server" and "Gateway" mean in this
> context.
> I was using "Gateway" to mean going between two different types of
> networks, SNA / 3270 on one side and TCP/IP / Telnet on the other side.

So the 3174 does not do this. 3270 terminals don't talk SNA/3270 to the 3174
as defined in the IBM 3270 data streams. They are usually pretty dumb and from 
what I can gather all keystrokes go to the 3174 just as for an ASCII terminal. 
It only becomes a 3270 protocol when it exits the 3174.

You get dumb terminals, either ASCII or 3270 Screens in, and at the other 
side you connect 3270 over SNA/SDLC, SNA/Token Ring, BiSync, X.25 or whatever.  

> I'm not sure how to define "terminal server".  I would think that a terminal
> server does what is needed to connect a terminal and drive a terminal.

That’s exactly what a 3174 does. IBM calls it a Terminal Controller.

> I think there's a significant overlap in those two terms and their associated
> definitions.
> Please correct me as you see fit.

I think there is, but to me a gateway has LAN protocols on both sides.

> > A 3174 can have either a Token Ring or Ethernet interface but not both.
> > (it can also have hdlc/sdlc/bi-sync and bus+tag)
> The reading that I did last night agrees with that.  But I don't see how that's
> germane to this discussion.
> > The 3174 never acts as a telnet server but.........
> >
> > A channel attached 3174 can be used to pass TCPIP into the mainframe
> > and there are products that run on the mainframe that act as
> > Telnet/TN3270 servers.  So in effect it acts as a channel attached
> > network interface for the mainframe.
> Okay.
> Maybe I'm supplementing a poor understanding of what a 3174 is with what I
> /think/ a Cisco router with a Channel Interface Processor is.  Namely that the
> Cisco terminates the TCP/IP connection and generates a new 3270 based
> terminal connection into the mainframe.  Note how there is no TCP/IP
> passing through the Cisco + CIP into the mainframe.

The 3174 NEVER accepts any sort of incoming connections. Just physical terminals.
When used to connect network traffic to a mainframe the 3174 does not terminate
the TCPIP connection., it passes the frames across to the channel. I may be wrong 
its been a long time since I did this and I don't want to go delving into the VTAM documentation. 

> > Some 3174s have RS232 ports, so locally attached serial terminals can
> > connect to the mainframe. This can be via Channels or via remote
> > network connectivity, SNA token ring, X.25, SNA over Ethernet.
> It's my understanding that 3174s that connect to channels / ESCON / FICON
> (via adaptation) are "Local" 3174s.  They can have network (Token-Ring /
> Ethernet / RS-232 / other) interfaces that are used to talk to other "Remote"
> 3174s.  The "Remote" 3174s then connect and drive local 3270 terminals
> which communicate across the network.

Its kind of odd. RS232 (so X.25/SDLC/HDLC/Bi-Sync) connections can only
be used to connect to a Mainframe, not another 3174. 

The Token Ring or Ethernet interface can be used to connect traffic to the mainframe 
But from what I remember the 3174 isn't too involved at this level it is acting as a network router/bridge.

Just to confuse things this is an IBM manual where IBM does use it as a "gateway"... 

so using the Token Ring interface on a remote 3174 to connect SNA traffic to the host via SDLC....
... again no TCPIP, working at the frame level, and the host end cannot be a 3174...

 > > It depends on what is at the other end. If you are going TCPIP/TN3270
> > then normally you use a router. However if you are going SNA over LAN
> > into the mainframe, from what I remember SNA LAN protocols are
> > non-soutable so you need a bridge/gateway.
> I think we just fell off the end of the continental shelf into the ocean.  -  But
> IMHO this is fun.  This is how we (I) learn new things.  :-D
> After having skimmed the PDF that Kevin linked to, I've mostly settled on the
> following:

That really muddies the waters because it uses the term "3270" connection in two senses. 
It uses it to refer to the co-ax type connection from a work station (CUT or DFT) with 
with 3270 over Channel/SNA as defined in the 3270 data streams manual and these 
really are different protocols.

> The 3174s speak TCP/IP on the downstream (grey) (Token Ring / Ethernet)
> LAN / side.  (I'm ignoring the protocols across the other circuits that the 3174
> supports between 3174s.)

That’s where you are going wrong. The protocols that the 3174 supports between other 3174s are IBM SNA protocols.
The "other 3174s" do not need to be 3174s and can be any SNA device.

Where does it say that?. In particular on page 39 it says..

IEEE 802.2
• PU 2/LU 2
• PU 2.1/LU 6.2 (in migration mode)

In the sort of use Kevin is talking about for connecting to Mainframe channels there is generally no TCPIP on the 3174.
In effect it looks like a Mainframe NIC...

> There are two frame types that are supported on Token Ring, 802.2 Logical
> Link Control and 802.2 LLC with SubNetwork Access Protocol.
> TCP/IP on Token Ring traditionally uses SNAP.
> Conversely, there are four frame types that are supported on Ethernet; LLC,
> SNAP, Ethernet v2, and 802.3 "Raw".  TCP/IP on Ethernet traditionally used
> Ethernet v2.
> So, we have a discrepancy between the frame types that traditionally carry
> TCP/IP on Token Ring and Ethernet.  The easiest way to deal with this is to
> use a router that uses the proper frame type for the underlying network.
> But that's /routing/, and not bridging.  Hence why my qualm / uncertanty was
> "routing" vs "bridging".

But the 3174 generally doesn't use TCPIP on the ring...

> I think it may be possible to have a piece of software / hardware actually
> /bridge/ the IP packet or TCP datagram between SNAP and Ethernet v2 (and
> vice versa).  But I've /rarely/ heard of that being done.  Especially when you
> have MTU differences between Token Ring and Ethernet that are difficult to
> deal with transparently.
> I concede that data is being connected between the two networks much like
> a bridge allows cars to connect between islands.
> So, back to  your statement.  "TCPIP/TN3270 then normally you use a router."
> Where does the TCP/IP connection that is the TN3270 traffic terminate?  Is it
> on the router?  Or is it on the mainframe?

The TN3270 traffic originates from the 3174 and terminates on the Mainframe.
TN3270 (and normal Telnet) traffic NEVER terminates on the 3174...

> Please elaborate on what you mean by "SNA over LAN".  What protocols
> would that be seen as on the LAN client?  NetBIOS?  Something other than
> TN3270 over TCP/IP?  Are you referring to DLC?  It's my understanding that
> DLC is a point-to-point protocol and not usable on a LAN.

IBM describes it as LU6.2.....

> I ask because I did not see any reference to anything other than TCP/IP in
> conjunction with the 3174 LAN interface.

See above....

> I agree that NetBIOS is not routable.
> > If you run Hercules on a PC with a Token Ring card then you don't need
> > anything.
> How is the 3174 going to connect a 3270 terminal to Hercules running on a PC
> via Token Ring if not TN3270?

That’s how it connects, but this is not the normal operating mode of a 3174. 

> Or is that the difference in terms that I was commenting about above.
> Also, I would expect a 3174 with Ethernet to similarly be able to connect a
> 3270 terminal to Hercules running on a PC via Ethernet.

Yes, but a 3270 terminal does not talk 3270 protocol to the 3174....

> > Do a search on "CISCO SNA Token Ring Bridging". Lots of papers on there.
> Yep.  As with all things mainframe related, it's complex and there are a lot of
> options.  The trick is finding the options that will work in the various
> constraints.

Yes and the waters get muddied because the 3174 has had extra features added 
along the way that allow it to be used in odd ways.... 

> > Last time I fired this lot up I used the CISCO as an IP router and
> > then used NAT In the CISCO hide the token ring from main network.
> NAT on the 2500 series Cisco is decidedly routing and not bridging.
> > Now I have a 3174 with An Ethernet card and a P390 with a bigger
> > selction of Oss I have lots of options...
> :-)
> > ... but I am currently distracted by a pile of VAXen
> ~chuckle~
> --
> Grant. . . .
> unix || die


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