Four-of-eight code
The four-of-eight code was a 64 character set plus six control characters, encoded in eight bits. Like other kinds of 6b/8b encoding, exactly four bits of each character were one bits — there are 70 such possible bit combinations.Data codes
This is the four-of-eight code. The first column of graphics is for the 7701, where no graphic is shown the code is unassigned. Plus zero and minus zero had distinct values for many machines, and are shown as +0 and -0. The second column of graphics is for the 1130 SCA, graphics are shown only if different from the 7701, mostly characters from theControl codes
These are the synchronous transmit-receive control codes. Control characters were reserved; there was no provision for transparent operation.Longitudinal Redundancy Check
The ''Longitudinal Redundancy Check'' byte was the only character that could have other than four one bits. This byte contains a zero in each bit position where the data record contained an even number of one bits, and one where an odd number. "Idle characters and control sequences are not included in the LRS.Line control
When no data is being transmitted the line is maintained in ''synchronize mode'' — a terminal transmits idle characters for approximately 1.3 seconds followed by ''end-of-idle'' sequence (CL Idle
). It then listens for the same sequence for approximately three seconds. For four wire operation, when data is being transmitted the other wire pair is maintained in synchronize mode. At least one idle character is sent before each transmission.
Ownership of the line is established by the transmitting station sending an ''inquiry sequence'' (TL INQ
). The receiving terminal then responds with ''acknowledge'' (CL ACK2
). Following this initiation sequence the transmitting terminal sends SOR1 TL
followed by the first block of data. The data is terminated by an ''end-of-transmittal-record'' (TL LRS
). The receiving terminal replies with CL ACK1
, or CL ERR
if the longitudinal redundancy check indicates a transmission error. This process continues with even/odd SOR
and ACK
, until the sending terminal has transmitted all the data. It sends the ''end-of-transmission'' sequence (CL EOT
). The receiving terminal responds CL EOT
and re-enters synchronize mode.
References
{{Reflist Link protocols IBM computer hardware