RL8-A disk controller

The RL8-A disk controller for the OMNIBUS PDP-8s was for the RL01 and RL02 disk drives. It could be set under program control to either 8-bit or 12-bit mode; in the latter, a sector contained 170 words, with 8 bits left unused. (These drives protect the data, and sector headers, with separate CRCs.)

The RL8-A was a hex board, the M8433. It had a 40-pin Berg connector which receives a special inter-drive cable (round in cross-section, with special locking connectors). The cable runs to the first drive; up to 4 drives can be connected to a single controller, using a bus running from one drive to the next, with a terminator in the last drive of the string.

It can be configured as device 60/61 (first controller) or 62/63 (second controller); its normal data break priority is 0, but it can be jumpered to use 1 instead.

Controller capabilities

The controller and drive combination can execute 8 different commands:

• Maintenance
• Reset
• Get Status
• Seek
• Read Header
• Write Data
• Read Data
• Read Data Without Header Check

Device instructions

The RL8-A is controlled with 14 instructions:

Code	Mnemonic	Function
6600	RLDC	Clear controller, all register
6601	RLSD	Skip on function done, clear if set
6602	RLMA	Load memory address register
6603	RLCA	Load command register A
6604	RLCB	Load command register B
6605	RLSA	Load sector address register
6607	RLWC	Load word count register
6610	RRER	Read error register
6611	RRWC	Read word count register
6612	RRCA	Read command register A
6613	RRCB	Read command register B
6614	RRSA	Read sector address register
6615	RRSI	Read silo word
6617	RLSE	Skip on composite error, clear if set

Device registers

The RL8-A controller includes 7 internal registers:

Register	Abbreviation
Command Register A	CA
Command Register B	CB
Memory Address Register	MA
Sector Address Register	SA
Word Count Register	WC
Error Register	ER
Silo Buffer Register	SI

Command Register A

During a seek:

DIR	HS	Unused	CYL DIFF
00	01	02	03	04	05	06	07	08	09	10	11

DIR - Direction
HS - Head Select
CYL DIFF - Cylinder Difference (distance to move)

During a data read/write:

Unused	HS	Unused	CYL
00	01	02	03	04	05	06	07	08	09	10	11

HS - Head Select
CYL - Cylinder

Command Register B

Unused	MAIN	MODE	IE	DRVSEL		EMA			FUNC
00	01	02	03	04	05	06	07	08	09	10	11

MAIN - Maintenance
MODE - 12- OR 8-bit mode
IE - Interrupt Enable
DRVSEL - Drive Select
EMA - Extended Memory Address

Function Code	Function
0	Maintenance
1	Reset
2	Get Status
3	Seek
4	Read Header
5	Write Data
6	Read Data
7	Read Data, No Header Check

Error Register

DCRC/HCRC	OPI	DLT/HNF	Unused							DE	DRDY
00	01	02	03	04	05	06	07	08	09	10	11

DCRC/HCRC - Data CRC/Header CRC
OPI - Operation Incomplete
DLT/HNF - Data Late / Header Not Found
DE - Drive Error ('Get Status' will provide details)
DRDY - Drive  Ready

Silo Buffer Register

During a 'Read Header' command - word 0:

Unused				CA0	HS	SECTOR
00	01	02	03	04	05	06	07	08	09	10	11

During a 'Read Header' command - word 1:

Unused				CYLINDER CA8-CA1
00	01	02	03	04	05	06	07	08	09	10	11

During a 'Get Status' command - word 0:

Unused				DT	HS	CO	HO	BH	STATE
00	01	02	03	04	05	06	07	08	09	10	11

DT - Drive Type
HS - Head Select
CO - Cover Open
HO - Heads Out
BH - Brush Home

State Code	State
0	Load Cartridge
1	Spin Up
2	Brush Cycle
3	Load Heads
4	Seek
5	Lock On
6	Unload Heads
7	Spin Down

During a 'Get Status' command - word 1:

Unused				WDE	CHE	WL	STO	SPE	WGE	VC	DSE
00	01	02	03	04	05	06	07	08	09	10	11

WDE - Write Data Error 
CHE - Current Head Error
WL - Write Lock
STO - Seek Time Out Error
SPE - Spin Error
WGE - Write Gate Error
VC - Volume Check
DSE - Drive Select Error

Bootstraps

Address	Data	Comments
21	7600	Clear accumulator
22	6600	Clear controller
23	7332	Generate constant
24	6605	Load SA with 20
25	1021	Load constant into accumulator
26	6607	Load WC
27	2327	Generate constant
30	6604	Load CMD B
31	6601	Skip on done
32	5031	Jump backward if not

Start at 21; the loaded operating system will overlay the bootstrap.