What the host does: terminal driving

In 1979 Diebold released its 910 Direct Connect software, and NCR followed with NDC, NCR Direct Connect, when it launched the 50xx series in 1983. Between them they settled where an ATM's intelligence lives: at the host, not in the box. The screens a customer sees, the states the machine walks through, the tables of which cards it accepts and what it charges, are all pushed down from the bank's central computer in a downline load. The machine executes. An internal NCR training deck states the consequence in one sentence: if the network goes down for any reason, the ATMs are unavailable.

That is why the host had to be a different kind of computer. Tandem shipped its first NonStop system to Citibank in May 1976, the first commercial fault-tolerant machine, and in 1982 Applied Communications of Omaha put BASE24 on it. The pairing became the industry default, and under the name ACI Worldwide BASE24 is still being sold in 2025. Jim Gray's 1985 Tandem report on why computers stop used ATM networks as its worked example, and found that administration and software, not hardware, caused most of the outages.

The thing on the other end of the line goes by several names, and new people lose weeks to the vocabulary. Processor, switch, ATM driver, host, network: at a bank they can be four systems or one vendor contract. The terminal handler talks the machine's protocol, NDC or DDC, and manages its states. The switch decides where a transaction goes: to the bank's own core for an on-us card, or out to a debit network, STAR, NYCE or PULSE, for everyone else. Almost every regional switch founded between 1976 and 1985 ended up inside FIS or Fiserv; SHAZAM, founded by the Iowa Bankers Association in 1976, is one of the few still independent.

Terminal driving is also the monitoring feed. The host knows a cassette is low, a printer is out of paper or a card has been captured because the machine reports its status on the same link it uses to ask for money. Stand-in processing, where the switch approves a withdrawal under preset limits while the issuer is dark, is what keeps a fleet dispensing through an outage. XFS, first released on 24 May 1993, is the other half of the software story and belongs to a different track: it lets one application on the machine drive any maker's hardware, but it says nothing about the wire. This track is about the wire.