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

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