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Data Communications
DATA CHECKING. There are two types of data checking used with the multipoint protocol.
The first is a check of each character as it is received and is called a vertical redundancy check
(VRC) or parity. This check is only used for ASCII characters. The second is a check of an entire
block ofdata and is called a block check. Two types ofblock checking are available. The first is a
Longitudinal Redundancy Check (LRC). The second is a more complex method called a Cyclic
Redundancy Check (CRC). Note that a CRC is a more thorough form of data checking than a
LRC and that both the HP 1000 and 3000 computers use the CRC.
Character Checking. The vertical redundancy check is also known as a parity check. When an
ASCII character is transmitted by the computer or the terminal, the high order (eighth bit of
each character is set to a cC1" or a cCO" to make the number of CC 1" bits in the character either even
(EVEN parity) or odd (ODD parity). There is also a variety ofVRC in which the parity bit is
always set to a "0" or a "I". The parity must be the same for both the computer and the ter-
minal. For example, if even parity is used the high order bit of each character would be set to
cause the number of "I" bits in the character to be even.
Character checking is not done when ASCII8 codes are used or when operating in transparency
mode.
The types of\lRC available for use in a multipoint configuration are as follows:
EVEN, ODD,
O's (parity bit always zero)
l's (parity bit always one)
NONE (no parity used - 8-bit data only)
Block Checking. Each block includes a Block Check Character (BCC). The BCC is in addition to
the parity bit associated with each character (VRC). The BCC can be either a one-character
(LRC) or two-character (CRC) check sum. To select which type of block checking you want
performed you use the cCBCC" field of the datacomm configuration menu described earlier in
this section.
The LRC character is a 7-bit check sum obtained by exclusive cCOR"ing the low order 7 bits of
each character included in the text block. A parity bit (VRC) is then added to this character
when it is transmitted. ForASCII8 all 8 bits are cCOR'ed" together and no parity bit is generated
for the LRC character.
The CRe is a 16-bit (two-character) check sum calculated using a formula that is compatible
with that used by the IBM Bisynchronous communications protocol. VRC parity is never added
to these characters.
TRANSMISSION CODE (ASCII7/ASCII8). The terminal can be set to use ASCII 7-bit or
ASCII 8-bit data code. The uASCII8" designation specifies that 8-bit codes (with no VRC
conversion) be used. All terminals on the same communication line must use the same
transmission code. You use the CCCode" field of the datacomm configuration menu to select the
desired type of transmission code.
7-36
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