Data Link Layer
A message begins with five normal 1 bits (A driven low for 500 ns, then B driven low for 500 ns) for bit synchronization, followed by a special frame sync pattern, three bit times long, that violates the usual Manchester encoding rules. A is driven low for 1500 ns, then B is driven low for 1500 ns. This is like a 1 bit sent at 1/3 normal speed (although the premphasis pulses remain 250 ns long).
This pattern is followed by up to 256 16-bit data frames. Each data frame consists of a start bit of 1, an 8-bit data field, a 3-bit station address, and an even parity bit (which includes the start bit, so it equivalent to odd parity over the data and address fields only). This is then followed by three or more fill bits of 0. Unusually for an IBM protocol, the bits within each frame are sent lsbit-first.
All messages are sent between the controller (master) and one slave device. The first frame in a message from the controller contains the device's address, from 0 to 6. The address field of following frames can be any value from 0 to 6, although is usually set to the device's address as well. The final frame in a message includes an address of 7 (all ones) as an end-of-message (EOM) indicator. A single-frame message does not have an EOM indicator.
When a command calls for a response, the device is expected to respond in 30 to 80 μs. A device's response also consists of up to 256 frames, and includes its address in all frames but the last. In this case, a single-frame response includes the EOM address, and the controller assumes it comes from the device it most recently addressed.
Generally, the first frame in a message is a command byte, and following frames are associated data.
Other articles related to "link, data link layer, layer, link layer, data":
... The uplink will typically be a port (or link aggregation group) connected to a router, firewall, server, provider network, or similar central resource ... Allowing direct data link layer communication between customer nodes would expose the local network to various security attacks, such as ARP spoofing, as well as increasing ... Ports can be isolated from each other at the data link layer (for security, performance, or other reasons), while belonging to the same IP subnet ...
... Link, a single sausage, originally part of a chain. ...
... work of the TCP/IP (Internet Protocol Suite) model, OSI's data link layer, in addition to other components, is contained in TCP/IP's lowest layer, the link layer ... The Internet Protocol's link layer only concerns itself with hardware issues to the point of obtaining hardware addresses for locating hosts on a physical network link and transmitting data frames onto ... Thus, the link layer is broader in scope and encompasses all methods that affect the local link, which is the group of connections that are limited in scope to other nodes on the local access network ...
... A data link layer technology, such as media access control (MAC) protocols on local area networks, has an address that has no significance beyond a single medium ... Besides Ethernet bridging based on MAC layer addresses, other data-link-layer technologies using forwarding tables include frame relay and Asynchronous Transfer Mode (ATM) switches ... ATM has both link-local addresses and addresses that have end-to-end significance in the ATM domain ...
Famous quotes containing the words layer, data and/or link:
“After a few months acquaintance with European coffee, ones mind weakens, and his faith with it, and he begins to wonder if the rich beverage of home, with its clotted layer of yellow cream on top of it, is not a mere dream after all, and a thing which never existed.”
—Mark Twain [Samuel Langhorne Clemens] (18351910)
“Mental health data from the 1950s on middle-aged women showed them to be a particularly distressed group, vulnerable to depression and feelings of uselessness. This isnt surprising. If society tells you that your main role is to be attractive to men and you are getting crows feet, and to be a mother to children and yours are leaving home, no wonder you are distressed.”
—Grace Baruch (20th century)
“I know that there are many persons to whom it seems derogatory to link a body of philosophic ideas to the social life and culture of their epoch. They seem to accept a dogma of immaculate conception of philosophical systems.”
—John Dewey (18591952)