[Info-vax] retro VMS hardware

[Michael Moroney]

John Wallace <johnwallace4 at yahoo.co.uk> writes:

>On Sep 26, 9:56=A0am, H Vlems <hvl... at freenet.de> wrote:
>>
>> ISTR there are two H4000 models, the first model required a jig and
>> and a drill to get it installed. The other had the same clamp as the
>> H4005.
>> The first model was not fit for use with H4005's and thinwire kit,
>> there's a heartbeat issue that preventsproper operation in mixed
>> configurations.
>> Mixed meaning combining 10base5 and 10base2 gear, like fitting a DESPR
>> to a 10base5 backbone. There is a hardware fix for the newer H4000
>> (which makes it an H4000-AB IIRC). It involves removing one small
>> capacitor. The H4005 has this fixed too.
>> Hans

>Rings a bell here too. My vague recollection is that the original
>stuff was Ethernet whereas the later stuff was 802.3, and the relevant
>difference in this context is the "carrier detect check
>(failure)" (aka heartbeat) circuit. Very vague though.

The issue was of "heartbeat", which uses the collision signal.
I believe the problem was with repeaters in general, not specific
to mixed configurations.  Repeaters had problems with heartbeat,
propagating them as collisions IIRC.  There were also similar
issues with DELNIs, which are old Ethernet devices.