RX11, RX211 — RX01/RX02 floppy subsystem

The RX01 and RX02 are DEC’s 8-inch floppy subsystems: a box holding two drives and the microcontroller that runs them, joined to the bus by a small interface card. The RX11 (RXV11 on the QBUS) drives an RX01, moving every byte of a sector through its data register. The RX211 (RXV21 on the QBUS) drives an RX02 in single or double density and moves sectors by DMA. QUniLator emulates both subsystems, each as three parts:

	RX01 subsystem	RX02 subsystem
Interface card	rx	ry
Drive box, with its microcontroller	rxbox	rybox
Drives	rx0, rx1	ry0, ry1

The box’s type reads RX0102uCPU. The ry card’s type reads RX211 on a UniBone and RXV21 on a QBone.

UNIBUS · UniBone

The cards are an RX11 and an RX211: base address 777170, vector 264, bus request level 5. The RX11 arbitrates in backplane slot 16, the RX211 in slot 17. The RX211 addresses 18 bits of memory.

QBUS · QBone

The cards are an RXV11 and an RXV21: base address 17777170 (177170 as a program names it), vector 264, bus request level 4, both in backplane slot 16. The RXV21 addresses 18 bits of memory, or 22 with mxv22 on.

Both cards answer the same address, as the real ones do at their factory settings, so a machine carries one of them. Enabling the second while the first is on the bus is refused and leaves it off. To carry both, give one of them its own base address and vector, and on a QBone its own slot.

What the machine sees

Registers	RXCS at the base address, RXDB at base + 2 (RX2CS and RX2DB on the RX211)
Functions	Fill Buffer, Empty Buffer, Write Sector, Read Sector, Read Status, Write Sector with Deleted Data and Read Error Code; the RX211 adds Set Media Density
Transfers	RX11: the sector buffer passes byte by byte through RXDB. RX211: Fill and Empty Buffer move it by DMA, with word count and bus address loaded through RX2DB
Drives	Two per box, units 0 and 1. 77 tracks of 26 sectors: 128-byte sectors and 256,256 bytes a disk in single density, 256-byte sectors and 512,512 bytes in the RX02’s double density
Timing	The disk turns at 360 rpm, and a sector read or written waits for it to pass. The head steps at 5 ms a track and settles for 25 ms after a seek
Power-up	Both heads go to track 0, and drive 0 reads track 1, sector 1

A drive is ready once its image is open. An image the board cannot write turns on the drive’s WRITE PROTECT lamp, and the drive then drops every write without reporting an error to the machine.

A sector written with the deleted-data mark keeps the mark while the image stays in the drive. The image file has no room for it, so attaching an image starts with no marks set.

Using it

1. Add the interface card, rx or ry, on the Configurations screen, and its box, rxbox or rybox.
2. Enable the card first, then the box. Enabling the box switches it on, and the drives in it with it; their own enabled follows the box.
3. Set a drive’s image to a floppy image. An RX01 image is 256,256 bytes; an RX02 drive takes its density from the size of the image it is given.

A floppy image normally holds the sectors as they lie on the disk, track by track from track 0. A drive also serves the two other forms floppy images come in: one that leaves out track 0 (imagetrack0), and one that holds an RT-11 volume’s blocks in order (layout). Left at auto, the drive tells an RT-11 volume in block order from one in sector order by itself.

2.11BSD on a QBone

2.11BSD drives a stock RXV21 through 18-bit addresses, so on a machine with more than 256 KB a transfer to a buffer above that misses the memory it is meant for. Turn on mxv22 on ry in the configuration that boots it: the driver then finds a 22-bit controller when it attaches the RXV21, and RX02 floppies read and write.

What has been verified

• 2.11BSD on the QBone reads and writes RX02 floppies through the RXV21 with mxv22 on, and what it writes is in the image afterwards.
• The RX01 diagnostic ZRXAF0 (CZRXA) runs clean passes on the QBone’s RXV11 with a real KDJ11, and on a UniBone’s RX11 with the emulated PDP-11/34.

Interface card parameters

For the RX01

It has no parameters of its own.

It also carries the parameters under parameters every card on the bus has.

RXV11

It has no parameters of its own.

It also carries the parameters under for the RX01.

For the RX02

Settings

What a configuration stores and an operator sets.

Parameter	Short	Value	Default	What it is
mxv22		0 or 1	0	22-bit DMA (DSD MXV-22): enables Q22 addressing; needed by 2.11BSD

It also carries the parameters under parameters every card on the bus has.

In detail

mxv22

Makes the controller a 22-bit DSD MXV-22. RX2CS then reads with bit 10 (RX_Q22) set, and a command that sets that bit takes its bus address as two RX2BA writes: the low 16 bits, then bits 21:16. At 0 it is a stock 18-bit RX211/RXV21, which is what XXDP and other guests expect.

2.11BSD needs it on a QBUS machine with more than 256 KB: its driver checks the bit when it attaches the controller at boot, and takes the 22-bit path when it is set. Without it the driver addresses a buffer above 256 KB through an 18-bit address, and the transfer lands in the wrong memory. The change shows in RX2CS at once, but the guest only looks at boot, so keep it in the configuration that boots 2.11BSD.

RXV21

It has no parameters of its own.

It also carries the parameters under for the RX02.

Drive box parameters

Settings

What a configuration stores and an operator sets.

Parameter	Short	Value	Default	What it is
powerswitch	pwr	0 or 1	0	State of POWER switch

It also carries the parameters under parameters every device has.

In detail

powerswitch

The POWER switch of the drive box, one for both drives. Enabling the box closes it, and the box runs its power-up initialize: both heads go to track 0 and drive 0 reads track 1, sector 1. Opened, the controller reads DONE and ERROR together, as with a box that is switched off. Disabling the box opens it.

Drive parameters

Settings

What a configuration stores and an operator sets.

Parameter	Short	Value	Default	What it is
density	d	text	SD on an RX01, DD on an RX02	SD for RX01 & RX02 FM; DD for RX02 MFM
imagetrack0	it0	0 or 1	1	true: File image contains track 0-76 (std), else only 1..76
layout	lay	text	auto	Sector order in the image file: physical, logical or auto

Status

What the device reports as it runs. A configuration does not store these.

Parameter	Short	Value	What it is
layoutinuse	lyu	text	Sector order the drive serves the mounted image in
track	tr	number	Track # of current head position
writeprotectlamp	wpl	0 or 1	State of WRITE PROTECT lamp

It also carries the parameters under parameters every drive has.

In detail

density

The recording density of the floppy: SD is single density, 128-byte sectors and 256,256 bytes to the disk; DD is the RX02’s double density, 256-byte sectors and 512,512 bytes. An RX01 is always SD. On an RX02, attaching an image sets it from the file’s size: anything larger than a single-density disk is DD.

imagetrack0

Whether the image file starts with track 0. At 1, the usual form, the file holds tracks 0 to 76. At 0 it starts with track 1, as some images do; track 0 then reads back as zeros and takes no data. A logical image always starts at track 1, whatever this says.

layout

The order the image file holds the floppy’s sectors in. physical is the surface as it was written, track by track with sectors 1 to 26 in place. logical is the volume’s blocks in order from block 0, the form an archived RT-11 distribution comes in: the file starts at track 1, and the drive maps every access through the 2:1 interleave with six sectors of skew per track that an RT-11 handler uses. auto looks at the image as it is attached and takes the order the RT-11 home block turns up in; an image without one is taken as physical.

layoutinuse

The order the drive serves the image in, which is what auto settled on.

track

The track the head is on, 0 to 76. It moves at 5 ms a track, and the head takes 25 ms to settle after a seek.

writeprotectlamp

Lit while the image in the drive is read-only. The drive then drops every write without reporting an error to the guest.