DX20 Programmed Device Adapter
The DX20 Programmed Device Adapter was an adapter which could interface between an IBM I/O channel-style bus and a MASSBUS. It could act as either a channel controller or a control unit on the channel bus. When operated as a control unit, in theory it could have been used to attach a DEC machine to a IBM System/370 (appearing as a device to the /370); it was mainly intended to allow use of OEM-produced peripherals on DEC machines.
The original intended configuration was with a TX02 Magnetic Tape Controller, along with one or more (up to eight maximum) TU70, TU71 OR TU72 magnetic tape drives; the ensemble was designated a 'TU70 Magnetic Tape Subsystem' (or TU71 or TU72 as appropriate). DEC software referred to a DX20 in such an assembly as a 'DX20A'.
The DX20 was later used in the RTP20 Master Subsystem, where it was used to control an RP20 8000 Controller, which ran one or more (up to four maximum) RP20 disk drive units. Each RP20 unit contained two independent RP20 spindles. (A DX20 in such a subsystem was referred to as a 'DX20B'.)
The DX20 was principally intended for use on a KL10 system; it plugged into an RH20 MASSBUS controller on the KL10. (The DX20 will not function with an RH11 MASSBUS controller, so cannot be used on an KS10.) There is a report that after the PDP-10 was terminated by DEC, the DX20 was used on the VAX to support Tu7x tape drives there.
The DX20 is built around a microprocessor (in the old sense of the word; i.e. a block of logic which executes microcode. The microcode (2K of 16-bit μwords) is held in a 'writeable control store', and was loaded into the DX20 by the host processor after power-on.
The DX20 was composed of four major sub-systems - "microcontroller, [MASSBUS] computer interface, high speed data path, and a device [channel] interface" - held mostly on five hex boards (with a sixth optional), and two quad boards:
- M8602 microprocessor
- M8601? (control store memory)
- M8603 MASSBUS data (quad)
- M8604 MASSBUS control
- 3xM5903 MASSBUS transceiver (dual width)
- M6505 data storage (quad)
- M6506 data formatter
- M8607 channel bus
- M8608 channel extension (optional, for bus 1; only bus 0 is standard)
- G891 power fail and select (single width)
They plugged into a custom system unit-sized backplane, mounted in a separate cabinet (either an H950 or an H9502) along with the power supply. The manual states that the boards were wire-wrapped, which if accurate, indicates that the DX20 was produced in small numbers; perhaps that only refers to early production units, though.