[Simh] VAX PSI

[Paul Koning]

I thought 2780 uses BISYNC, which is a different protocol.  X.25 is HDLC.  Yes, both need modem eliminators -- like an async null modem but an active device that also supplies the synchronous bit clock.

X.25 (and I think HDLC) is indeed asymmetrical.  Whether that can be gotten around is not entirely clear to me.  I do know X.25 has the concept of "call collision", a protocol failure caused by a protocol design error.  It uses a two-way connection handshake, which was well known at the time to be wrong, and both DDCMP and NSP as well as other protocols such as TCP have always used a three-way handshake for that rason.

	paul

> On Mar 19, 2020, at 7:48 PM, John Welsh <johnwelsh at optusnet.com.au> wrote:
> 
> Hi Dave,
> I have been involved in a project using VAX-2780-3780PE and have been able to connect two VAX’s together using a Black Box Synchronous Modem Eliminator ( SME ).
> Product code: ME204A-F. 
> No VAX PSI involved.
> Needed Decnet Plus and Wan drivers.
> 
> Was able to test using a utility that comes with the Protocol Emulator - XJTEST which can use either VAX as a loopback.
> In my case it was a DSV11 to a DSW42 using the SME and the V.24 cables that come with the adapters.
> There is also another utility called REMOTE that can transfer simple files from VAX to VAX.
> I don’t think there is a driver for SIMH that can emulate a sync device. ( yet ).
> 
> Regards,
> John Welsh.
> 
> Sent from my  iPhone
> 
>> On 20 Mar 2020, at 03:38, dave porter <dave_list_addr at verizon.net> wrote:
>> 
>> I know almost nothing about VAX PSI, but I have a vague idea
>> that even though X.25 is asymmetric and intended for DTE to DCE communication (i.e., your computer to the phone company's premises), it may be possible to coerce it
>> into talking from one VAX to another (via null modem
>> cable connecting two devices such as DUP-11s).
>> 
>> I worked on RSX-11M PSI and that's how we tested in
>> the lab.  I can no longer remember how that was done.
>> Undocumented option somewhere, no doubt.
>> 
>> 
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