What was 8-bit XT-IDE or XTA?

In the early days of DOS, hard disks employed one of two interfaces - the IBM PC/XT Fixed Disk interface (often referred to as the "ST-506" interface, named after the 5-megabyte Seagate drive of that model designation) and the AT Fixed Disk Interface, and required a separate hard disk controller card to be installed into a free expansion slot. This controller would then act as intermediary between the drive's electronics and the motherboard to get or put data from/to the drive.

We know today that this has been replaced with the modern 16-bit ATA / IDE interface. However, before the modern IDE that we know so well, there was XTA.

For XT-class systems that have only an 8-bit data bus, XTA, or "XT Attachment", was essentially the 8-bit version of what would later become ATA (AT Attachment). Hard disk manufacturers created XTA hard disks for use with this interface, with capacities ranging from 20 MB up to 40 MB. Two examples of this were the Seagate ST351A/X and Western Digital 93028X. The former can actually be made to work either in XTA mode or the newer ATA mode via an onboard jumper.


A 20MB XTA hard disk as part of a "hard card"

These still required an interface card, but some PC manufacturers embedded this, giving their motherboards an XTA socket, so you could connect an XTA drive to it. Amstrad did this on some of their later XT clones as well as some PCWs, as did Philips. Tandy's 1000TL and RL/RLX range used XTA as did IBM with their Model 25/286 and Model 30/286. Commodore made use of the XTA interface on their PC20-II, PC20-III and even the Amiga 500's hard disk.

Common XTA interface cards include the Western Digital WDXT-140 and 150. Supposedly XTA is also fully compatible with WD-1002AX controllers.

The Interface

The 40-pin interface for XTA is very similar to ATA (IDE), except that it's designed for 8-bit systems, so only 8 data lines and 2 address lines are used. Some XTA hard disks can work in "XT mode" (XTA) and "AT mode" (ATA), switchable via a jumper. Seagate's STnnn A/X drives are all flexible in this way.

XTA Interface Pinouts

Pin Signal Pin Signal
1 -RESET 2 Ground
3 Data Bit 7 4 Ground
5 Data Bit 6 6 Ground
7 Data Bit 5 8 Ground
9 Data Bit 4 10 Ground
11 Data Bit 3 12 Ground
13 Data Bit 2 14 Ground
15 Data Bit 1 16 Ground
17 Data Bit 0 18 Ground
19 Ground 20 Key (Missing pin)
21 AEN 22 Ground
23 -IOW 24 Ground
25 -IOR 26 Ground
27 -DACK 3 28 Ground
29 DRQ 3 30 Ground
31 IRQ 5 32 Ground
33 Address Bit 1 34 Ground
35 Address Bit 0 36 Ground
37 -CS1FX 38 Ground
39 -Drive Active 40 Ground

ATA Interface Pinouts

Pin Signal Pin Signal
1 -RESET 2 Ground
3 Data Bit 7 4 Data Bit 8
5 Data Bit 6 6 Data Bit 9
7 Data Bit 5 8 Data Bit 10
9 Data Bit 4 10 Data Bit 11
11 Data Bit 3 12 Data Bit 12
13 Data Bit 2 14 Data Bit 13
15 Data Bit 1 16 Data Bit 14
17 Data Bit 0 18 Data Bit 15
19 Ground 20 Key (Missing pin)
21 DRQ 3 22 Ground
23 -IOW 24 Ground
25 -IOR 26 Ground
27 I/O CH RDY 28 SPSYNC:CSEL
29 -DACK 3 30 Ground
31 IRQ14 32 -IOCS16
33 Address Bit 1 34 -PDIAG
35 Address Bit 0 36 Address Bit 2
37 -CS1FX 38 -CS3FX
39 -DA/SP 40 Ground
41 +5 Vdc (Logic) 42 +5 Vdc (Motor)
43 Ground 44 -Type (0=ATA)

XTA uses IRQ5 and I/O Port 320h, whereas ATA uses IRQ14 and I/O Port 170h.

Note that XTA is different to the modern project known as XT-IDE, which aims to provide full modern IDE hard disk support to old 8-bit PCs .

XTA Hard Disks


A 1/3rd-height 20 MB XTA hard disk

Here is a list of known XTA hard disks:

Make Model Capacity Cyl     WPC    Hd   LZ   Access  SPT     Total
Western Digital WD-93028-X 21.4 615     615     4       616      70     17      41820
Western Digital WD-93038-X 32.1 615     615     6       616      70     17      62730
Western Digital WD-93048-X 42.5 977     977     5       978      70     17      83045
Western Digital WD-93024-X 21.4 615     615     4       616      39     17      41820
Western Digital WD-93034-X 32.1 615     615     6       616      39     17      62790 
Western Digital WD-93044-X 42.5 977     977     5       978      39     17      83045
Western Digital WD-95028-X 21.4 615     615     4       616      70     17      41820
Western Digital WD-95038-X 32.1 615     615     6       616      70     17      62730
Western Digital WD-95048-X 42.5 977     977     5       978      70     17      83045
Western Digital WD-95024-X 21.4 615     615     4       616      39     17      41820
Western Digital WD-95034-X 32.1 615     615     6       616      39     17      62730
Western Digital WD-95044-X 42.5 977     977     5       978      39     17      83045
Miniscribe 8225XT 20 805     805     2       806      68     26      41860
Miniscribe 8450XT 40 805     805     4       806      68     26      83720
Seagate ST-325X 20 615                4                            17
Seagate ST-325A/X 20 615                4                            17
Seagate ST-351A/X 40 820                6                            17
Seagate ST-352A/X 40 980                5                            17
Conner      
Epson      
Prairietek      

The Seagate hard disks listed above, like later 16-bit IDE ones, are auto-parking (so no need for a Write PreCompensation cylinder or Landing Zone figures), and are low-level formatted at the factory. Their performance was for the most part similar to the best MFM or RLLs drives of the day with an average seek time of 28ms.

The Western Digital hard disks listed here came with a low-level formatting utility, and DO require the heads to be parked before being moved. Their performance was typically 40ms average track seek time.

Data transfer rates were of course much slower with XTA when compared with 16-bit ATA due to being limited to 8-bit transfer speeds.

You would be hard-pushed to find these in working order today, but fortunately modern IDE hard drives can work well in really old PCs, including the original PC and XT through the use of an XT-IDE card. These come is several types, with some supporting just CompactFlash cards as hard disks, and others offering a modern IDE connector.