The Model 51 or 51C provides the required termination of the CAMAC Branch Highway. As such it must reside at the extreme end of the Highway. It can be placed in any unused CAMAC station, as it makes no use of the Dataway except to provide power. The Model 51 contains a female Hughes connector while the 51C contains a captive branch highway cable and male connector. Although references are made to the 51C only, they apply equally well to the 51.
The visual display feature of the Model 51C allows the user to analyze indivi- dual Branch Highway operations, facilitating the debugging of CAMAC software and hardware. To properly utilize this feature the user must be familiar with CAMAC Branch Highway protocal, as described in IEEE Std. 596-1976.
The Model 51C operates in either of two modes, selected by the single front panel switch.
LATCH MODE
This is the normal mode of operation, in which the Model 51C captures the last CAMAC Branch operation. The signal on each Branch Highway line, during the cycle, is displayed on an individual LED. Appropriate Halts may be required in the Soft- ware, as the display will be overwritten each time a Branch Cycle is performed.
Certain Indicators operate in a special way, namely BTA, BD, BZ. These signals are not latched at capture time, but represent the real-time static state of the lines at the time the indicator is viewed. However, since these signals may be of short duration, they are "stretched" to give a minimum visible indication lasting approx- imately 1 second whenever they are asserted.
All other indicators are treated in the standard fashion, and display the logic signal captured during the cycle. Specifically, capture occurs at the trailing edge of the BTA signal.
It should be noted that all CAMAC Branch Highway signals are asserted low, with the exception of the seven BTB lines, which are asserted high by an addressed crate controller at capture time. Between CAMAC cycles an on-line Crate Controller asserts BTR (low), whereas an off-line or absent Crate Controller causes BTB to be high. The Model 51C LEDs are illuminated by any signal (including BTB) that is asserted (low) at capture time. Therefore, after a CAMAC cycle, the BTB light corresponding to an addressed crate controller will not be illuminated, nor will the lights corresponding to non-present or off-line controllers. The light will be on, however, for any on- line controllers that are not addressed.
TRACK MODE
In this mode, the Model 51C displays the actual logic signal on each Branch Highway line at any point in time. An asserted (low) level causes the corresponding light to be illuminated. The primary purpose of this mode is to detect hardware malfunctions on the Branch Highway, either in the Driver, Cable, or a Crate Controller. Between Branch Highway cycles all lights should be off, with the possible exception BD, which will be on if a Demand is pending, and one or more BTB lights. In accordance with the BTB protocal described above, the BTB light should be on for each on- line crate Controller, and otherwise off. Any deviation from this pattern when the Branch Highway is inactive (and the 51C in "Static" mode) should be interpreted as a mal- function.
CLEAR
This momentary position of the select switch clears any indicators that have been set in "Latch" mode. As such, it is useful in determining if a Branch Highway cycle has occurred.
SERVICE
Servicing of the Model 51C is straightforward, using the documents appended to this manual.
The instrument consists of two circuit board assemblies, A 5303 and A 5304. In addition a small assembly (A 5305) is provided to mount the display LEDS. A5303 (right hand facing instrument) contains only the termination resistors for the Branch Highway. These are networks consisting of 150 Ohm to +6v and 330 (Ito ground.
A 5304 provides the logic necessary to capture Branch Highway signals and drive the LED displays. Schematic 51C-C-001 pertains to the circuitry on this assembly. In servicing this assembly,one precaution should be observed. The CAMAC Specifications concerning logic voltage levels on the Branch Highway specify levels somewhat higher than those used in ordinary TTL circuits. For maximum noise immunity, Branch Highway receiving devices should have a switching threshold of approximately 1.8 volts. This is achieved in the Model 51C by operating all TTL circuits between the CAMAC positive supply voltage (+6v), and a return that is approximately 0.7 volts above CAMAC ground. To this end, a diode is placed between logic return and CAMAC system ground. This is permissible because the Model 51C interacts with no other devices and does not utilize the Dataway. This diode could be short circuited,however, if a "ground" from a test instrument were connected to the logic return voltage. Test instrument grounds should be connected to CAMAC ground, and the above fact noted in interpreting readings from such equipment.