Indicators
As well as System/360 being an engineering icon, a fascinating feature to today's eyes is the line’s extensive use of indicator lights on the front panels. [Although the casual term “front panel” is almost always used today, the IBM documentation refers to it as the “system control panel”.] These indicator lights are used to display the contents of the many registers, triggers, and latches that make up the internal logic of the CPU and associated circuitry.
The extent of front panel indicator usage varied across the model line, and the Model 65 can be considered a 'medium' example in this regard. Some higher-performance models had far more individual indicators, but not necessarily proportionally more functionality – looks could be deceptive. For example, a Model 91 (pictured below) has a front panel crammed with indicators and is about three times the size of a Model 65 panel. However, each Model 91 indicator is dedicated to displaying one single bit of information. The Model 65 uses six-position roller switches which allow a single indicator to selectively display any of six different internal bits of information.
The roller indicator section of the Model 65 front panel has six rows of 36 indicators. The S/360 word length is 32 bits or four bytes, with each byte having a parity bit (odd parity). As the parity bits are also displayed 36 indicators are needed for each word. Each row can be switched to show, for example, six different registers. So the total number of bits displayable is 6 x 6 x 36 = 1,296 bits. In a practical sense the Model 65’s bit indications, though still slightly fewer in number, are not too far behind the Model 91’s in their utility.
It is true to say the roller displays can only show one-sixth of the bits at any one time, selected by the roller position. However, in practice this is not a handicap. The indicators are primarily intended for use by the Customer Engineer while the machine is in the CPU-stopped state. This could be due to “single cycle”, “instruction step”, “stop on address compare”, or “stop on error” operation. While stopped the internal logic bits are 'frozen' and the rollers can be rotated from position to position and continue to display accurate data.
While the CPU is running the indicators just glow in varying intensities and are only really useful for impressing visitors and the finance director! This extract from the Model 65 Customer Engineer Handbook aludes to their actual intended purpose:
“Don't limit yourself – Some people see a red-light machine check and jump right into ALDs [logic circuit diagrams]. Look around first. There are over 1500 indicators [slight exaggeration!] on the system control panel. USE THEM! What instruction is in E-reg? Which register and adder gates are on? What is happening in core and channel at this time? Don't use a scope when the indicators can tell you as much.”
The rest of this page will take us along the path from the front panel indicators to the internal logic that is being displayed. Along the way we will be exposed to the Automated Logic Diagrams (ALDs) which are the circuit diagrams and schematics of the machine. There are two levels of ALD. The first, higher, level shows the relationship between logic blocks and also to their external interfaces, such as cables, indicators, and switches. The lower level is the Card ALD, which show individual electronic components such as transistors, diodes, resistors, etc. These card ALDs usually represent the circuitry for several of the logic blocks on the higher-level ALDs.
Travelling this path serves a number of purposes: it will be useful for anyone with a S/360 front panel who wishes to animate the original indicator lamps and needs to know the driver circuit parameters; for maintaining the machine, it is a worked example of tracing a signal through the logic; and it is of general interest to anyone wishing to know more about S/360 hardware. The following relates specifically to the Model 65, but other models are similar.
So, starting with basics, at the very end of the path there is the indicator lamp itself. All the small round lamps on the front panel are of the incandescent type and of the same design – a one-piece tubular plastic body incorporating a lens. The colour of the plastic varies according to the required lens colour. The plastic base of the tube is a separate piece which incorporates the two connector pins. The colour of this base signifies the (DC) operating voltage of the lamp.
There are two pin lengths – short or long, to match two different styles of housing and mounting arrangement. All the Model 65 front panel indicator lamps are of the short pin and 3-volt (black base) type. There are 233 lamps in total, mainly clear lens.
The indicator lamp we will trace back is the 32nd indicator on the second roller. In the logic this is "IND ROLLER 2 BIT 31" (the first indicator being "IND ROLLER 2 BIT 00" – IBM always counts from zero, not one). All indicators and switches on the front panel can be identified according to their physical location, using grid reference coordinates. IND ROLLER 2 BIT 30 is located at E45. Note that the roller is rotated to the second position and is displaying Check (error) Register 1, hence the red background. Specifically, ROS bits 2-42 parity error.
The logic driving each indicator lamp in a row needs to know which one of the six roller positions is selected so it can display the correct register – in our case Check Register 1.
A six-position rotary switch placed between the roller knob and the roller itself accomplishes this by sending a signal (a ground or an open circuit) into the logic, as we shall see later.
The indicator lamps are all fed from a common positive 3-volt supply, which is the black wire in the picture. If the indicator lamp is to be illuminated a ground is supplied from the logic via a yellow wire – one for each indicator, so 216 wires for the roller lamps alone. The lamps are a simple push-fit from the front of the panel into their plastic housing. Replacing blown lamps was usually done by the operator, as required.
The signals to the indicators and from the switches are connected to the logic boards in the main frame by an umbilical bunch of yellow flat ribbon cables. There is also an umbilical of black wires for the voltage supplies.
The other ends of the flat cables are soldered to single-width SLT (Solid Logic Technology) cards such that each wire connects to its own signal pin. These are then known as paddle cards.
The paddle cards are plugged into the various SLT boards implementing the indicator driver logic.
The front panels were assembled in the factory separately from the main frame of the computer and then mounted on to the frame and the umbilicals connected.
Turning now to the logic diagram for the indicator driver of our lamp. It is one of many shown on Automated Logic Diagram (ALD) page PL021. Click on the image to see the whole page.
[ALDs are one of the outputs from the computer program used by the S/360 engineers during the design and manufacturing process.]
The logic block identifies the lamp part number and its location on the front panel, and that it is driven by the signal "-IND ROLLER 2 BIT 31" from ALD page KT151 line BJ4.
How this signal is generated is shown in the next section below.
This part of ALD page KT151 shows how the driver signal is generated. Click on the image to see the whole page, and here for an explanation of the logic blocks.
We will come back to the OR gate inputs, but for now assume they are all logic 1 (high, +3V) so its ouptput will be low* (0V). This signal is inverted (indicated by the wedge*) by the AR block – the low on the input causes a high on the output. The output is thus inactive, and no ground is supplied to the lamp (via SLT card pin D13) so it remains off.
The SLT driver card is of type (part number) 5804197 and is physically located at E-E4J6 in the main frame, and contains the circuit type (code number) U55AD, which is a 15 mA driver into a 1 kΩ load.
*A wedge symbol on the output of an ALD logic block (outputs are always shown on the right hand side of the block) indicates the output is active low (0V). This leads to a "-" sign in front of the signal name.
Above left is a picture of an actual SLT card type 5804197. There are 18 square silver-coloured SLT modules mounted on the card. Each SLT module contains a few transistor and diode chips mounted on a ceramic substrate, along with silk-screened resistors and printed connections. These form a small logic circuit, or a part of one. The module part number is the six-digit number printed on the module cap, which always begins with "361" (for SLT – there are other types). Note the varying orientation of the SLT modules. This is not due to random printing, but because the automated layout process selects the optimum orientation to match the module pins to the required connections.
The blue components are resistor networks, used where the required power dissipation is greater than possible with the resistors internal to the SLT modules. Above right is the component layout sheet of the SLT card ALD set. A closer view of the relevant part of the sheet is described further down the page.
Above left is one (of three) of the card ALD circuit schematic sheets for the 5804197 SLT card. On the right is a larger view of one of the circuits shown on the schematic. We know from ALD page KT151 above that our signal -IND ROLLER 2 BIT 31 is generated by a U55AD circuit and exits the SLT card on pin D13. Finding that pin (highlighted in yellow above) identifies which circuit it is on the schematic. Highlighting all the individual components of the circuit that have a common component code of "Z2" in blue allows the extent of that component (the SLT module) to be outlined in green. This, as we shall see below, is a 361480 module, and the module pin numbers are shown in the green squares. The module's physical location on the SLT card is "13", as highlighted in orange.
Note that circuit code U55AD uses two additional 1 kΩ resistors external to the SLT module. These are part of component A3 (one of the blue resistor networks) at physical location 12 on the SLT card.
Also note that U55AD consists of two identical driver circuits, one exiting on SLT card pin D11 and the other on pin D13. The inputs to the driver circuits are on pins 1 and 4 of the SLT module respectively. It is useful to remember that card pins are always three alphanumerics beginning with "B" or "D", and module pins are always just numerics.
This is an enlarged view of the relevant part of the component layout sheet. We can see that there are three different types of SLT modules used on the card: Z1, Z2, and Z3. There are three of the Z2 type of interest to us, part number 361480. We know from the schematic that ours is at location 13, so we can identify it as highlighted in blue.
Component locations are numbered from top to bottom in columns starting on the right of the card. Note the associated A3 resistor network is at location 12 immediately above our Z3 module.
Components have 0.125" pin spacing and are mounted on the card in a matching grid of holes. The numbers "93 93" in the top left of our module indicate where it is positioned on an imaginary 0.025" grid, referenced from the bottom left of the card.
Here are edited extracts from the SLT handbooks which describe some of the characteristics of module 361480. It can be seen that the module may be used in High-Speed (5-10 ns) or Medium-Speed (30 ns) classes of SLT circuit. There is also a Slow-Speed (700 ns) class, but this module is not suitable for use at that speed.
The only difference in characteristics between high and medium speeds is the maximum length of wire from the module to the indicator lamp. As we have seen, flat cables are used between the module and the front panel indicators. The maximum length of this cable is 30' for high speed and 50' for medium speed.
The S/360 Model 65 uses the 30 ns medium speed class of SLT exclusively.
Now we return briefly to the indicator roller switch logic. ALD page PK001 (click on the image for the whole page) shows the roller switches, with the second rotary switch at front panel location E52 highlighted. With the roller in position 2 a ground is removed from the normally closed output signal "+ROLLER SWITCH 2 POS 2" which floats high and feeds into ALD page KT621. It is AND'ed with "-BLOCK IND SWITCHES" which is also high (as the switches are not being blocked) to generate a logic 1 into the OR gate at logic block location 3L. The OR gate logic 0 output is inverted to a logic 1 by the inverter (logic symbol "N") at logic block location 5M and fed to ALD page KT151 as "+SCAN OUT WD 12R+ROLL 2 POS 2".
This signal is interpreted as "scan out the right-hand half of word 12" or "roller 2 is at position 2". We are not concerned with the scan out function at this time so we can read it as just "roller 2 is at position 2".
The +ROLL 2 POS 2 signal feeds into ALD page KT151 where we saw (higher up the page) the indicator driver signals being developed. There it is AND'ed, bit for bit, with the various signals that are to be displayed on Roller 2 Position 2 – which are the bits in Check Register 1. The output of one of these AND gates (the one for bit position 31 in our example) is one of the inputs to the OR gates we saw earlier on KT151. Only one of the OR gate inputs will be 'active', as selected by the roller switch position, and will be logic low (indicator lamp on) if the corresponding Check Register 1 bit (ROS 2-42 parity error) is set, or logic high (indicator lamp off) if it is reset.
Camiel Vanderhoeven's animation of his Model 65 front panel:
Find illustrations of the front panels of all S/360 models on this page.
