View source for 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 [[Digital Equipment Corporation|DEC]] machine to a [[IBM System/370]] (appearing as a device to the /370); it was mainly intended to allow use of [[Original Equipment Manufacturer|OEM]]-produced [[peripheral]]s 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 drive]]s; 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.

==Implementation==

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 [[DEC card form factor|hex]] boards (with a sixth optional), and two [[DEC card form factor|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 [[H960 rack|H950]] or an [[H9502]]) along with the [[power supply]]. The manual states that the boards were [[wire-wrap]]ped, which if accurate, indicates that the DX20 was produced in small numbers; perhaps that only refers to early production units, though.

==See also==

* [[DX10 Data Channel]]
* [[DX11-B System 360/370 Channel to PDP-11 Unibus Interface]]

==External links==

* [http://www.bitsavers.org/pdf/dec/pdp10/periph/EK-0DX20-TM-PRE_DX20_Programmed_Device_Adapter.pdf DX20 Programmed Device Adapter Technical Manual] (EK-0DX20-TM-PRE)
* [http://pdp-10.trailing-edge.com/BB-H311C-RM/01/swskit-documentation/rp20-notes.mem.html TOPS-20 RP20 Software Functional Specification]
* [https://www.inwap.com/pdp10/images/rp20.jpg RP20] - RTP20 group

[[Category: MASSBUS]]
[[Category: PDP-10 Machine Interfaces]]