DART  Project


Upstream Terminator Module




--PRELIMINARY--




Wednesday, May 10, 1995
Revision 1.0


John T. Anderson
John Chramowicz
Electronic Systems Engineering Department
Computing Division 
Fermi National Accelerator Laboratory




Document # ESE-DART-95051



1.  GENERAL INTRODUCTION	3
1.1 SYSTEM INTRODUCTION	4
1.2 DESCRIPTION OF COMPONENT & HOW IT FITS INTO THE SYSTEM	7
1.3 LIST OF COMPONENT REQUIREMENTS	7
2.  THEORY OF OPERATION	8
2.1 BASIC FEATURES & OPERATION	8
2.2 DIAGNOSTIC FEATURES	10
3.  INTERFACE SPECIFICATIONS	11
3.1. FRONT AND REAR PANEL I/O, TEST & MONITORING	11
3.2.1.1. Connector Pin Configurations	11
3.2.1.2. Signal Descriptions	12
4. ELECTRICAL & MECHANICAL SPECIFICATIONS	13
4.1 PACKAGING & PHYSICAL SIZE	13
4.2 PC BOARD CONSTRUCTION	13
4.3 POWER REQUIREMENTS	13
4.4 COOLING REQUIREMENTS	13
5. SAFETY FEATURES & QUALITY ASSURANCE PROCEDURES	14
5.1 MODULE FUSING & TRANSIENT SUPPRESSION	14
6. EXAMPLE OF COMPONENT OPERATION WITHIN THE SYSTEM	15


1.  General Introduction

The Upstream Termination Network (UTN) is designed to be used in the DART Data 
Acquisition System. Within the Dart System, Data Sources and Destinations are connected to each 
other by DART Data Links which carry data from the Data Sources to the Data Destinations.
Terminology is necessary to provide a common frame of reference. In this document, the term 
“Upstream” refers to that end of a cable which is farthest away from the destination. “Downstream” 
refers to the closest point on the cable to the eventual destination. Both terms are stated in reference to 
the flow of the data and the associated data strobe, and are used in this way to describe the Data Link.
The UTN module is to be placed at the beginning of a Data Link Cable on the “Upstream” side 
of a Data Source .  

	The UTN is packaged in a 1U high, 19-inch wide rack-mounted box with an internal DC power 
supply.  Power is supplied through a standard 110 VAC line cord. Status indicators, a NIM Trigger 
input and a port for connection to a logic state analyzer via a diagnostic card are on the front panel . 
DART cable connectors, fuse access and connection for the power cord are located at the rear. Status 
indicators (LED’s) on the front give visual indication of data transfer and module power.  The Logic 
state analyzer connection allows diagnosis of data transfer errors and checking of signal quality without 
having to break the data cable to insert the test equipment.
	






















	Placement of the UTN in the DART System can be shown as follows:

 
	


1.1	System Introduction

	DART consists of a set of Data Source modules including the Fastbus Smart Crate Controller 
(FSCC), Crate Trigger Interface/Readout Controller (CTI/RC) and Damn Yankee Controller (DYC).  
These Data Sources drive differential RS-485 data over a cable to a Data Destination module, the 
Access Dynamics DC2 (and it’s daughter board, the DM-115).  A series of Data Sources may be linked 
together on the same cable (a “stream’) to in turn drive the Data Destination, and this cable may 
connect to multiple Data Destinations, each one of which is resident in a unique VME chassis.

	Commonly, data flow is defined as progressing “downstream” (from data source towards data 
destination) or “upstream” (from destination back towards source).  These terms will be found 
throughout this and other DART system documents.

	The data flow is carried on a parallel differential cable using RS-485 signal levels. The internal 
data path is carried on two parallel ribbon cables, one of 34 pins and a second of 50 pins.  The 
organization of the cables is as follows:

	1) Low 16 data bits plus control signals on the 50-pin connector.
	2) High 16 data bits on the 34-pin connector.

	The control signals shown on  front panel LED’s are defined as:

1) DSTROBE (Data Strobe) is pulsed up and down once per data word sent from Data Source 
to Data Destination.
2) EOR (End of Record) is pulsed up and down once per block of data words sent from one or 
more Data Sources to the Data Destination, and is used to delineate ‘events’ or ‘blocks’ of 
data.
3) WAIT is asserted by the Data Destination to signal the Data Sources that transmission of data 
should be halted.  The WAIT signal propagates backwards along the cable with respect to 
Data, DSTROBE and EOR.
4) TSTROBE (Trigger Strobe) is pulsed once for each trigger received from the Trigger 
System. The Trig Strb RS-485 Led is pulsed as a result of a trigger input from the rear panel 
Trigger cable connection and the Trig Strb NIM Led is pulsed if the front panel Trigger Strobe 
input is used.
























A DART system and it’s relationship to other systems within an experiment may be shown graphically 
as follows:


 



	In response to triggers generated from the Trigger System, the Data Sources take data from the 
front end Converters and pass it down a cable to the Data Destination. If required, Repeater modules 
are used to insure that the data arrives intact at the Data Destination. The Event Builder takes the data 
from the Data Destinations and uses it to build and store complete events. In order to maintain signal 
integrity, termination networks are required at both the UPSTREAM and DOWNSTREAM ends of the 
data link. A UTN is placed before the first Data Source and a DTN (see DTN module documentation) 
after the last Data Destination on the cable. 

	Multiple Data Sources are allowed on a single cable.  Data Sources use a Permit token signal to 
pass control of the data cable amongst themselves.  For this Permit scheme to function, one Data 
Source must be the ‘first’ and one other must be defined as the ‘last’, with all others defined as ‘middle’.  
The ‘first’ will be the first to send data, and the ‘last’ carries the responsibility of sending the End of 
Record signal to the Data Destination. ‘Middle’ and ‘last’ Data Sources wait to send data until the 
receipt of a Permit token.

1.2	Description Of Component & How It Fits Into The System

The UTN is used to control signal reflections due to impedance mismatches caused by either the 
total cable length, the total number of loads and or the distribution of the loads along the cable.
The UTN has the following basic characteristics:
1) Ability to provide termination at the Upstream side of a cable to contain reflections.
2) Allows for better signal quality on longer cable lengths.
3) Allows more flexibility in placing of loads on cables.
4) Simple, low-cost construction.


1.3	List Of Component Requirements

The UTN must be able to provide the following:
1. The ability to receive and properly terminate the 32-bit DART data.
2. The ability to receive and properly terminate the DART control signals; WAIT, EOR and 
DSTROBE.
3. The Ability to receive and properly terminate the TRIGGER ID and TRIGGER STROBE 
signals.
4. An ability to provide easy connection of a logic state analyzer to assist in debugging data 
transmission errors in the field without the need to break the data cable.
5. Provide for a NIM TRIGGER STROBE input on the front panel to pulse a front panel led.
6. Provide visual indication of DART control signal status via front panel LED’s.




2.  THEORY OF OPERATION

The UTN provides AC termination of all Data signals, the Data Strobe and the EOR signal, plus 
DC bias/termination of the WAIT and TRIGGER STROBE signals. The AC terminations are used to 
minimize reflections on the cable. The DC bias networks are designed such that all signals float to their 
“inactive” state (receiver output = TTL high for Data Strobe, EOR and WAIT signals and receiver 
output = TTL low for data signals).
All unused signal lines coming into the UTN from the DART connectors have the option of 
being grounded on the UTN.

2.1	Basic Features & Operation

		a) Termination

	All RS-485 Data lines along with the Data Strobe and EOR signal lines will be AC terminated 
with a 61 ohm resistor from each differential leg to ground through a 0.001uF capacitor as shown 
below:


 












		
		b) Termination and Biasing


	 The RS-485 Trigger lines will be terminated using a 240 ohm resistor across the differential 
input, with 120 ohm bias resistors on each leg.  The ‘plus’ input is biased to 3.3. volts and the ‘minus’ 
input is biased to ground, yielding a net matching impedance of 120 ohms. A sketch of this termination 
and bias technique is shown below :




 
















		c) Termination and Biasing of Control Signals

	
	The WAIT and Trigger Strobe lines will be terminated using a 360 ohm resistor across the 
differential input, with 91 ohm bias resistors on each leg.  The ‘plus’ input is biased to 3.3. volts and the 
‘minus’ input is biased to ground, yielding a net matching impedance of 120 ohms. A sketch of this 
termination and bias technique is shown below :





 





	










   2.2	Diagnostic Features

An internal, recessed header is provided that yields access to the actual signal lines connected to 
the UTN.  A small diagnostic card, with internal series resistors, plugs in to the recessed header through 
a slot at the front of the UTN.  This has the dual advantage that either oscilloscope or logic analyzer 
connections may be made, and that the excess capacitive load is only present when diagnosis is in 
progress.  A card guide mounted to the UTN circuit board insures correct mating of the diagnostic card, 
and a cover plate guards the diagnostic connector when not in use.  A 120-pin DIN connector is used 
which insures correct orientation.  The diagnostic connector provides access to all DART data signals 
plus the Trigger signals.  Different plug-in diagnostic cards may be produced if necessary to support 
either logic analyzer or oscilloscope viewing of the signals on the DART cable.
3.	 INTERFACE SPECIFICATIONS

3.1.	Front and Rear Panel I/O, Test & Monitoring


1. One 50-pin, one 34-pin and one 10-pin ribbon cable headers for carrying data, control and 
trigger information, respectively, located on the rear panel.
2. LED's indicating state of DataStrobe, EOR, WAIT , TriggerStrobe and power, located on 
the front panel.  Signal monitoring LED’s are pulse stretched for easy viewing of narrow 
signals.
3. LEMO connector for NIM-level Trigger Strobe input located on the front panel.
4. Internal (hidden behind panel) 120-pin DIN Diagnostic Connector.			
												 
These are sketched here in Figure 1:							
						

 
	
Figure 1


3.2.1.1.	Connector Pin Configurations

	DART data signals are carried on a 50-pin cable and a 34-pin cable as RS-485 differential 
signals.  All signals are sent with the ‘+’ side of the differential pair on the odd-numbered pin of the 
connector, and the matching ‘-’ side of the differential pair on the even-numbered pin directly across the 
connector.   A third 20-pin connector is used for connection of trigger signals, with the same polarity 
scheme employed.  The pinout of the 20-pin connector is designed to match the FSCC readout 
controller. 

	One of each connector exists on the UTN.  Pinouts are as shown here in 			
	Figure 2:


 
	
 




















				Figure 2


3.2.1.2.	Signal Descriptions

	DART Data and Strobe, the Trigger ID and the Trigger Strobe on ribbon cables are RS-485. 
The front panel Trigger Strobe input is a NIM signal which is input via a standard Lemo connector. 
4.	ELECTRICAL & MECHANICAL SPECIFICATIONS

4.1	Packaging & Physical Size
.
 The UTN will be a self-contained 19", 1U high AC powered box using a standard prefabricated 
rack-mount chassis.  Diagnostic information and a NIM trigger strobe input are located at the front 
panel, all other user connections are made at the rear.

4.2	PC Board Construction
Six layer FR4 with ground and power planes.

4.3	Power Requirements
Estimated draw of 0.5 Amps at 5 volts provided by internal power supply driven from 110 
VAC, for a total power draw of about 2.5 watts.

4.4	Cooling Requirements
Convection through air vents in chassis box.
5.	SAFETY FEATURES & QUALITY ASSURANCE PROCEDURES

5.1	Module Fusing & Transient Suppression

	AC line power fused at 1/2 ampere, standard 3AG fuse in panel mount holder.  Internal 5 volts 
fused at 2 amps via board-mount picofuse with single Transzorb on board. Internal 3.3 volt bias voltage
is generated by a  regulator and is also fused at 2 amps.  Multiple types of decoupling capacitors are 
mounted across entire PC board for noise rejection.
6.	EXAMPLE OF COMPONENT OPERATION WITHIN THE SYSTEM

The UTN will be used at the beginning (Upstream point) of all the data transmission cables. The 
module is designed to minimize the reflections which  result from impedance mismatches caused by 
multiple loads along the cable or clumps of devices in the middle of the cable, thus keeping the signal 
strength high.

Upstream Terminator Module	8/2/95
	






	
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