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 <[email protected]> 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 <[email protected]> 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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