[GreenKeys] newbee AGAIN RE: HAL ST800a

[weather]

I posted a question earlier today about HAL modem ST800a, to which someone else added an additional question.  While their added-on question got response, my question seems to have "fallen through the cracks."  I had asked about the Hal modem(s) on ebay and their application/compatability to running the old 15s,19s, & 28's including supplying loop power and ability to adjust to the 60ma requirements and the possibility of the unit running multiple machines simultaneously. What I see tells me this is exactly the application (plus much more) this unit was built for.  However, my knowledge is SO lacking that information from experienced/knowledgable sources would be welcomed. 
Also the other poster asked about difference(s) between the ST800 and ST800a.  I have not seen any response to that, although since I get the digest version it may come later.  THANK YOU!