UDMA explained

The Ultra DMA (Ultra Direct Memory Access, UDMA) modes are the fastest methods used to transfer data through the ATA hard disk interface, usually between a 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