Patent GB-1288195-A (GB)
IPC: H04L12/423
Filed: 1971-04-19
Abstract: 1288195 Data processors; input/output INTERNATIONAL BUSINESS MACHINES CORP 19 April 1971 [16 March 1970] 23738/71 Heading H4P [Also in Division G4] Serial data is transmitted between a central station and a number of remote stations connected in a loop in a format in which data and control signals are arranged in a number of cyclically recurring "frames" each of which includes a first group of synchronizing bits, a second group of control bits including a first part indicative of one of several functions to be carried out by a remote station, and a second part identifying the frame and a number of bit positions within the frame which are to be used by the remote station to carry out the required function, and a third group of bits identifying the remote station. The format is illustrated in Fig. 2. Each data frame contains a number of synchronizing bits followed by three control bits only one of which may be significant at any one time. The control bits indicate respectively that data is to be received by a remote station, that data is to be transmitted by a remote station, and that error diagnosing functions are to be performed. The next five bits identify the frame and a following time slot within the frame during which the specified transmission is to fake place. The next eight bits are reserved for the address of the remote station which is performing the specified function in the specified time slot. The next sixteen bits repeat the control and address information and the frame is completed by a slot for containing a number of eight bit data bytes. The system may be as shown in Fig. 1 with the central and local controllers (which are described in some detail) being operative to extract data from and place data on to the transmission loop. The central controller contains an addressable memory into which data, control, and address bits are written. The contents of the memory are selectively read out by a gating system for transmission to, e.g. a computer, or selected ones of a number of input/output units connected to respective local controllers. The data is handled in parallel in the central controller. On receipt of a frame each local controller scans the three control bits. If C1 is a one, indicating that the frame is available for a transmission to the central controller, and the local controller has data for transmission, the local controller accepts the address identifying the frame contained in bits A1-A5 and utilizes this address to communicate with the central controller by placing its data in the data positions in that frame. In order to prevent a further controller from utilizing the same frame it changes bit C1 to zero. The controller inserts its address in bit positions T1-T8, its data in positions Data 1-n, and transmits the frame along the loop to the next controller. The controller continues to use the same frame for transmission until it has transmitted all its data whereupon it releases the frame for use by another controller by including an end of message code with its final data transmission. The remaining controllers on receiving the frame will scan the control bits which indicate that the frame in use, and retransmit the frame unchanged. If control bit C2 of a frame is a one, indicating that the frame contains data from the central controller for one of the local controllers, each receiving controller scans bits T1-T8 to determine whether the data contained in that frame is to be accepted. If this is the case the controller will store the address bits A1-A5 in the second control slot address and continue to accept data from the designated locations in the designated frame until it detects an end of message code. If bit C3 is a one then the remote station identified by bits T1-T8 will perform error diagnosing functions.