Listed below are many of the I/O interfaces produced for use in the HP 21xx processors from 1966 to the late 1970s. The objective here is to list everything that would legitimately plug into the I/O bus interface slots of the processors, but I cannot claim it to be comprehensive.

Identification

Identifying an interface or board can be somewhat perplexing. Most of them are referred to by a number of the form "12nnn" or "13nnn", however:

• Applicable 12nnn or 13nnn numbers actually identify an interface kit which includes the interface board (in HP terminology: PCA - Printed-Circuit Assembly), cable, software, manual and in some cases the external device.
  
  
• Some early interface boards do not have the associated interface kit number on them, rather the board is identified by a number of the form "02116-nnnn".
  
  
• 12nnn or 13nnn numbers do not necessarily indicate an I/O interface. Other HP products use numbers from those ranges.
  
  
• Not all I/O interfaces have a number in the 12nnn or 13nnn ranges, i.e. the 59310.
  
  
• In some cases the same board type may be used by several interface kits. Such boards may or may not be configured differently via jumpers or have slightly different component populations for different interface kits and may have two identifying numbers on them. One number specifies the board and the configuration, the other specifies the board foil pattern.

Logic Technology & Compatibility

As the original CPUs were based on CTµL, the I/O bus utilised the logic levels of that logic family. Even after the CPUs switched from CTµL to TTL the I/O bus continued to be based around the original CTµL logic levels for compatibility with existing I/O interfaces. The logic levels for the bus are near to, but not quite directly compatible with, those of TTL.

A few of the earliest interface boards were constructed purely from CTµL, however most interface boards contain a mixture of TTL and CTµL. In most cases the CTµL is present merely to interface to the processor I/O bus. Later boards avoided the old CTµL ICs by using more-recent driver ICs to talk to the bus.

As near as I can figure interface boards are forwards and backwards compatible across processors. Some very late boards completely avoid the use of -2V on the I/O Bus and it is conceivable that these boards may not be backwards compatible to earlier processors.

Multiple-board Interfaces

In the case of interfaces which use two channels even though the circuitry fits on one board, two board slots are effectively used since the second slot is useless as the channel assigned to it has already been utilised. In such cases an interrupt priority jumper board is installed in the second slot.

Each I/O slot actually has two select codes assigned to it, in an overlapping sequence, for the benefit of interfaces such as these.

Key to Refs:

• 67: List in 1967 2116A I/O manual from bitsavers.
• 68: List in 1968 HP catalog.
• 69: List in 1969 HP catalog.
• 75: List in 1975 HP catalog.
• 77: List in 1977 HP catalog.
• MBnn: Scanned HP interface manual from Bitsavers HP2100 interfaces directory. nn is earliest year mentioned in manual.
• LJnn: List of interface manuals on Jeff Moffat's HP 2100 Archives site. nn is earliest year mentioned in list.
• LJ2100: 2100A processor manual on Jeff Moffat's HP 2100 Archives site.
• PB: A physical board in my hands.