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Hytec Electronics Ltd. |
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SERIAL CRATE CONTROLLER SCC2401 The SCC2401 is a two-width Type L-2 Serial Crate Controller satisfying IEEE 595 (EUR 6100). It interfaces the Serial Highway to CAMAC at up to 5 MHz, bit or byte serial, and at the same time can support Auxiliary Controllers on the ACB. Introduction The Serial Highway connects to the SCC2401 via two 25-way ‘D-ports’, forming a single unidirectional loop starting and ending at a Serial Coupler or Driver. Up to 62 SCCs can be connected on this loop, which is two-pair (data/clock) cable for bit-serial, and nine-pair for byte-serial operation. The signal std. is RS422. The SCC2401 contains a clock restoration circuit which will correct back to a nominal 1:1 mark:space a clock signal which may be degraded to 3:1 or 1:3. The following Table 1 shows the commands obeyed by the SCC2401 : Table 2 gives the bit allocation of the status register. 
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Operation
Each controller connected to the Serial Highway is given a different crate address (01-62 on front panel dials). The message to the SCC is a series of eight-bit bytes (in bit or byte form). Bits 1-6 are text; bit 7 is the delimiter bit and is made ‘1’ for the last byte in a message and for the WAIT bytes between messages. Bit 8 is the (odd) transverse parity bit.
In a command message byte 1, the first or Header Byte, contains the crate address bits (SC1- SC232); codes 00 and 63 are illegal. Bytes 2 and 4 contain A, F and N as shown below with the reply and demand messages:
The normal ‘Start-up’ sequence is to write ‘400 to the Status Reg., setting bit 9 (Enable DMDs) and resetting bit 13 to put the SCC on-line (it powers up off-line) by doing N (30A (0) F (17) to the crate address in question. This also resets Status bit 12 to zero, and ‘un-bypasses’ the SCC. Subsequent commands may then be directly sent to modules in that crate.
The pin-out of the ‘D’ port is given in Table 4: a 25-pin fixed plug on the left is the input, and a DBC-25 S on the right is the output connector.
Although for byte-serial operation a full serial highway of eleven pairs and three singles is normally used, linked thus:
The simpler format below may be used in the bit-serial case:
Where the little square represent ‘turn-round’ connectors in which Bus 1 is linked to Bit serial data and Bus 2 Bit/Byte Clock. This arrangement uses Bus 1 to return the data stream and Bus 2 to return the clock, and can be used for any number of crates on the highway.
Demands
At the back of the SCC there is a 52-way double-density fixed cannon plug, the SGL-Encoder connector. This is used as the equivalent of the LAM-Grader connector of a Type A Crate controller and is also the ACB connector. The even-numbered pins from 2 to 46 bring out the L lines L1 to L23 from the Dataway, L24 is generated in the SCC (Status bit 10) and is on pin 48;pin 50 is the OR of these, or ‘L-Sum’, and pin 52 is 0 volts. The pin-out of the rest is given in Table 5.
DMI indicates the presence of one or more demands (after any masking in the SGL-Encoder attached) when it goes to ‘1’, initiating the generation of a demand message by the SCC if Enabled.
SLP controls Status bit 16, and may be generated in a complex SGL-Encoder by masking L signals, but is more commonly obtained by simply linking to L-Sum.
When DBSY = 1, the SGL-Encoder should staticize SGLE 1-5 while the demand message is being transmitted.
The ERPT signal from the SGL-Encoder indicates the presence of a Hung demand, i.e. a demand that has persisted longer than some present time-out period. When ERPT goes to ‘1’ the SCC generates the demand message 11111.
BCK is derived by the SCC from the incoming byte stream, for use by the SGL-Encoder. STIM is used to control the SCCs internal timer.
TIMO is the output of that timer.
CBY means SCC doing command reply.
N1 to N16 allows an Aux Controller normal ACB Dataway access.
ACL is generated on the ACB when the SCC receives a Header byte addressed to it, and is kept there until execution of the command is completed or abandoned. Its effect on any Aux Controllers which may be connected to the ACB is to keep them from using it, or to stop using it as quickly as they can if they had already started.
It should be added here that a perfectly acceptable SGL-Encoder may be constructed out of a free 52-way socket strapped so that pin 50 (L-SUM) joins pin 23 (STIM) and pin 19 (TIMO) joins pin 21 (DMI). This will have the effect of the SCC generating a zero SGL (Serial Graded LAM) whichever L is set. If, in addition, up to 5 specific Ls are linked to the 5 Graded L pins, identifiable SGLs will be sent for those 5 with all other Ls being represented by SGL zero. In each case the arrangement will cause Hung demands to be sent as 11111. The SCC internal timer allows the time-out period to be screwdriver set over the range 1mS to 10S. It is possible for the STIM signal to go to ‘0’ if the module generating the only L present is addressed. To overcome this, a staticising circuit is provided at the Repeat Timer input to store STIM whilst dataway BUSY is asserted. There is a delay of 200nS from the end of BUSY before the staticiser is switched off, to allow L signals to be re-established.
Range and Speed
These matters are governed by type of cable, bit or byte serial, clock rate and use of U-ports or only D-ports; and of course the software driving the Serial Highway can be quite critical in determining the speed obtainable. The range of RS422 at 5MHz using 7/0.12mm twisted pair is about 250 meters, or twice that at 2MHz: but 1/0.9mm twisted pair would make those figures 550 and 1100 meters respectively. Clearly if one uses thicker cable, or runs slower, one can achieve far greater ranges, the ultimate being to go via Modems or convert the bi-phase modulated line to microwave etc.
The pros and cons of bit-or byte-serial are complex. To begin with, since in bit-serial each byte is sent in a 10-bit frame of start bit, 8 bits text and Stop bit, it is tempting to think it will be ten times slower. However, transition through the SCC introduces a one-clock delay: one bit out of 130 bits in bit-serial, one byte out of 13 bytes in byte-serial, for each crate on the highway (typical figures). So for 10 crates on a highway running at 1 MHz, each speed in ‘conservative’ mode (waiting for one command to go round the SH before sending the next) with no allowance for software delays or demands is 30K/sec in byte, and 6.7K in bit-mode, only 4.4 times slower. When software allowances are made, they get even closer.
Highway Drivers
Any Serial Highway Driver that meets the specification will drive these Serial Crate Controllers, but it is worth mentioning here that Hytec make three such drivers: SHD992 – two width standard CAMAC module; VSHD2992 – single width 6m VME driver; and SH-CPR-1- Serial Coupler, for use in Hytec System Crate.
Power Requirements
2.2A at +6V only.
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