IBM 2821 Control Unit

[1] Six models of the IBM 2821 Control Unit were available, as follows:[2] The 2821 is fully buffered, that is it has buffers large enough to hold an entire card to punch, an entire card read, or an entire print line for each attached device.

This frees the channel from having to meet the peripheral devices' tight timing requirements.

[2]: 8  A two-channel switch is available, allowing the control unit to be connected to two channels, on one or two computer systems, controlled by manual switches or by programming.

[2]: 65–66 The circuitry in the 2821 is largely older-technology IBM SMS, unlike much of the System/360 line, but later revisions were partially implemented in SLT or MST.

[5] As logic became cheaper, the functions of the 2821 were increasingly moved inboard to the central processor[6] or outboard to the individual device.