UDMA explained

The Ultra DMA (Ultra Direct Memory Access, UDMA) modes were the fastest method used to transfer data through the ATA hard disk interface, usually between the computer and an ATA device. UDMA succeeded Single/Multiword DMA as the interface of choice between ATA devices and the computer. There are eight different UDMA modes, ranging from 0 to 6 for ATA (0 to 7 for CompactFlash), each with its own timing.

Modes faster than UDMA mode 2 require an 80-conductor cable to reduce data settling times, lower impedance and reduce crosstalk.^[1]

Number	Also called	Maximum transfer rate (MB/s)	Minimum cycle time	Defining standard
Ultra DMA	0		16.7		ATA-4
	1		25.0	80 ns	ATA-4
	2	Ultra ATA/33	33.3	60 ns	ATA-4
	3^[2]		44.4	45 ns	ATA-5
	4	Ultra ATA/66	66.7	30 ns	ATA-5
	5	Ultra ATA/100	100	20 ns	ATA-6
	6	Ultra ATA/133	133	15 ns	ATA-7
	7	Ultra ATA/167	167	12 ns	CompactFlash 6.0^[3]

See also

• PIO—The first interface type used between devices (mainly hard disks) and the computer.
• Parallel ATA
• Serial ATA

References

1. https://archive.org/stream/SCSISpecificationDocumentsATAATAPI/ATA_ATAPI/AT%20Attachment%20with%20Packet%20Interface%20-%207%20Volume%202%20-%20Parallel%20Transport%20Protocols%20and%20Physical%20Interconnect%20%28ATA_ATAPI-7%20V2%29%20Revision%204b#page/n189/mode/2up AT Attachment with Packet Interface - 7 Volume 2 - Parallel Transport Protocols and Physical Interconnect (ATA/ATAPI-7 V2) E.2.1.1 Cabling p172
2. 80-conductor cable required
3. http://compactflash.org/2010/cf-6-0-introduces-industry-leading-performance-and-feature-enhancements/ CompactFlash 6.0 Introduction