When Tymnet decided to provide remote terminal access to computer systems, or hosts, not operated by Tymshare, which owned Tymnet, we used several kinds of connection with those computer systems. Often we would merely install a mini computer with many cables with the sort of plug found on the telephone company’s modem. This was easy to install for no host software modifications were necessary. Most such host systems assumed that a particular host port (cable & plug) was for some particular speed (baud rate) and modification of that speed required human operator intervention. Users of such hosts were accustomed to a fixed speed however. The great advantage of this scheme was that it was very easy for a customer to try out the new service with little risk or expense on his or our parts. This was indeed how the earliest internal Tymnet service was provided.
Service to Tymshare machines had progressed to something like DMA where the code in the base (a mini computer with communications code) was able to read and write the real memory of the host.
We also built a channel adaptor so that a Tymnet base could appear to an IBM mainframe as a device on an I/O channel.