IBM 3174 C 6.4 Microcode Disks?

Grant Taylor cctalk at
Sun Feb 17 21:45:36 CST 2019

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.

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 

I think there's a significant overlap in those two terms and their 
associated definitions.

Please correct me as you see fit.

> 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.


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.

> 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.

> 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:

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.)

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".

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?

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.

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

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?

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.

> 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.

> 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


Grant. . . .
unix || die

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