1877S Multihit Time-to-Digital Converter with Data Supression


The FASTBUS Model 1877S is a 96-channel, high resolution, multihit TDC designed for high rate experiments. This TDC provides precise time measurements for spectrometers and hodoscopes consisting of multiwire propor tional, drift or time projection chambers. With a least count of 500 psec and a 16-bit range, the Model 1877S can measure time intervals up to 32 µsec duration. The instrument operates in Common Start or Common Stop modes. Common Stop Mode eliminates the need for expensive delay cables normally required for each input channel of a fixed target experiment.

The 1877S provides the high performance, high density, versatility, and automated test features required for large scale experiments. By using the FASTBUS standard, the LeCroy 1800 family of data acquisition modules is one of the most sophisticated data acquisition systems today.


Input Circuits

The individual channel inputs are located on the front panel. They accept differential ECL signals and are termi nated in 110 W. The COMMON input (Common Start or Stop) also accepts ECL signals and has a removable 110 ohm termination. By removing this termination, the COMMON signal(s) can be cascaded, as long as the termination in the last module of a block of modules is left in place. This scheme eliminates the extra propagation delays caused by embedded repeaters and leaves only the very slight delays due to the cables cascading the individual modules (approximately 50 psec/cm).

Conversion Technique

At the heart of the 1877S lies the MTD133B Monolithic Time Digitizer, an 8-channel, 16-bit dynamic range TDC circuit with 500 psec least count. Arrival times can be recorded for either the rising, falling or both edges. The double pulse resolution is 20 nsec.

The MTD133 uses a high-speed clock, a continuously counting scaler, and a three state interpolator. When a pulse arrives, the contents of the scaler and interpolator are stored in memory. Up to 16 words of data can be stored in the LIFO (Last In First Out memory) for each channel. During readout each of the stored signal times is automatically subtracted from the COMMON hit time. Therefore, the output is the actual time difference with virtually no pedestal. The difference value is further compared to a programmable upper threshold and discarded if it is greater than this threshold. This threshold is programmable from 8 nsec to 32 µsec with a resolution of 8 nsec.

In this way the MTD133 accommodates many different, difficult measurement scenarios. For users who wish to detect only pulses coming within the drift time of the chamber, the programmable threshold produces a program mable full scale. In addition, the user can program the depth of the LIFO from 1 to 16 hits, thus creating a guaran teed maximum conversion time.

Model 1877S Block Diagram


Control registers configure the TDC to report either the time of the rising edges, falling edges or both, facilitating pulse width measurements. Test functions allow verification of each channel's behavior. Full scale time out in the COMMON START mode can either be programmed or supplied externally. Compatibility with the Model 1810 Calibration and Timing Module is maintained for COMMON START/STOP, fast clear, fast clear window, and test pulse distribution.

Multiple Event Buffering

The module contains an eight event buffer. This digital memory buffer provides two primary advantages. First, dead time in the experiment is reduced because data readout can be done during the acquisition of subsequent events. Second, the event data can be stored temporarily while the trigger decision to read or discard the event is made. Events in the buffer are discarded with a FASTBUS command to skip the event. This skip command causes an internal pointer to increment, positioning the next event at the top of the readout queue. As each event is recorded, a modulo eight event tag number is appended to it in order to allow the coherence across multiple modules to be verified.


The 1877S complies with the FASTBUS Standard (ANSI/IEEE-960). FASTBUS functions allow remote control and operation of the 1877S. Data from the 1877S is automatically zero suppressed. Readout is in a FIFO-like manner, consisting of a header word followed by a variable number of data words. Channel identification informa tion is appended to each data word.

The modules may be read out via a LeCroy Model 1821 FASTBUS Segment Manager/Interface (SM/I) at data transfer rates up to 10 Mwords/sec. In addition, the Model 1877S is compatible with the LIFT (LeCroy Interactive FASTBUS Toolkit) software package.

Multi-Range ADC Compatibility

The 1877S provides the features necessary for interface and readout of single and multi-range charge-to-time converting front ends. The 1877S then provides the time encoding function and FASTBUS readout of the charge signal. The charge-to-time converters always generate an output pulse, even at pedestal so that non-zero time data is recorded by the 1877S on every channel. By being able to encode both input edge polarities, widths of pulses can be encoded.

In a mode similar to the LeCroy Model 1881M, a sparse data readout scheme can be used to prevent unwanted data from being buffered. The 1877S allows the user to program 96 constants, one per channel, which can be compared to the measurement values. Data exceeding these individual thresholds is buffered reducing both the dead time and quantity of data that must be transferred over FASTBUS to the host computer. See Figure 1.

Figure 1

When the 1877S is used in a multi-range ADC application (e.g. MQT300), the TDC is programmed to operate in Common Start mode. The Common Start timeout (as appropriate for the desired full scale and resolution) can be selected by configuration at the TDCs CRSs.

A two bit hit count field has been provided in the 1877S data word format. This indicates (modulo 4) how many hits were recorded in the channel's LIFO. By using the FASTBUS controlled readout depth parameter it is pos sible to readout only 1 word per channel, while also knowing the total number of hits on that channel. This feature provides a second level of data reduction from the multi-range MQT300 so that only 1 data word per channel is necessary.



Inputs: 96 ECL differential line receivers. Input impedance 110 ohm ±10%. Minimum pulse width 10 nsec FWHM (must be > 1 time bin width). Input swing 400 mV, differential.

Least Significant Bit: 500 psec.

Total R.M.S. Error: 400 psec (Note: The R.M.S. of a gaussian distribution is equal to sigma).

Time Out: Differential ECL input to mark the end of acquisition in COMMON START mode.

Full Scale: 0 to 32.768 µsec, ±0.0025%; programmable via CSR18 in steps of 8 nsec.

Pedestal: 14 to 20 counts.

Double Edge Resolution: The 1877S can measure two edges separated by as little as 10 nsec. No two pulse edges should be closer than 10 nsec. This implies a lower limit on the double pulse resolution of 20 nsec.

Common Start/Stop: From the Model 1810 CAT via TR6 line or from front-panel differential ECL input. CSR selected.

Fast Clear Window Input: From the Model 1810 CAT via TR5 line.

Fast Clear Window (FCW): Starts at end of Time Range (Common Start) or at Common Stop. Can be pro grammed 1024 nsec to 512 µsec. During this period, the user can apply a FAST CLEAR to discard the event just captured.

Zero Suppression: Automatic for channels that have no hit.

Long Term Stability: < 100 ppm/year.

Temperature Coefficient: < 10 ppm/°C.

Differential Non-Linearity: Maximum ±0.2 LSB.

Integral Non-Linearity: < 25 ppm full scale.

Fast Clear: Differential ECL input via a 2-pin front-panel connector (removable termination resistors) or via backplane TR0 line. Minimum pulse width 40 nsec.) When applied during the FCW, clears data in the current event and readies module for acceptance of a new event. Fast clear settling time is < 250 nsec. Fast Clear is leading edge sensitive and must be performed during FCW.

Time Out: Differential ECL input via a 2-pin front-panel connector. Minimum width 50 nsec. In Common START mode, terminates measurement in progress and starts conversion.

Busy Output: Differential ECL output via a 2-pin front-panel connector. Indicates the module is converting hit information. The unit is unavailable for data capture.

Conversion Time: 1.2 µsec + 50 nsec per hit within the programmed full scale; 1.75 µsec minimum.

On-Board Tester: The tester generates square wave pulses (50% duty cycle). The pulse trains can have 1, 2, 4 or 8 cycles with half periods of 125, 250, 1000 nsec or 2000 nsec.

Multiple Event Buffer: The digital data memory is logically organized as a circular buffer, large enough to store the results of up to eight events.


Front-Panel Indicators: Slave: Indicates module is being addressed. COMMON: Indicates whether Common Start/Stop was hit.

Power Requirements: 4.0 A at 5 V; 7.7 A at -5.2 V; 3.1 A at -2 V; 0.1 A at +15 V; 0.1 A at -15 V.

Packaging: Single-width FASTBUS module (ANSI/IEEE-960-1989).


Module Identification Code: Read Only, 103D h.

Implemented Addressing Modes: Logical (16 bits), Geographical, Broadcast.

AS-AK Handshake Time: 125 nsec typical, 150 nsec maximum.

DS-DK Handshake Time: 65 nsec typical, 75 nsec maximum (Block Transfer).

Implemented Broadcast Functions:

Code Significance Comments

(01)h* General Broadcast Select The TDC modules are selected and respond to subsequent
data cycles.

(05)h Class N Broadcast The TDC modules of Class N (programmed via CSR7) respond to subsequent data cycles.

(09)h Sparse Data Scan TDC modules containing one or more buffered events assert (SDS) their "T pin" on the following read data cycle.

(19)h Device Available Scan TDC modules respond by asserting "T pin" if no events are (DAS) buffered.

(0D)h All Device Scan All TDC modules assert their T pin on the following read
data cycle.

(CD)h TDC DS TDC modules assert T pin if current event contains a non-zero
number of data words.

* An h subscript denotes a hexadecimal number, i.e., base 16.

Module Control Space Registers

CSR 0h Module ID and control/status

CSR 1h Mode and control

CSR 3h Logical address

CSR 5h Block transfer word count register

CSR 7h Broadcast class

CSR 10h Buffer memory page pointers

CSR 12h Full scale time measurement

CSR C0000000h Sparsification constants, one per channel
- CSR C000005Fh

Slave Status Responses to Data Cycles

SS Significance

0 Valid action.

2 End of data.

3 Error: Error in token pass during multi-module data scan.

7 Error. Invalid secondary address loaded into internal address register.

DSR0 Output Word Bit Definitions


When logical addressing is enabled use top configuration.
When logical addressing is disabled use bottom configuration.


D = Data
S = Edge: Rising Edge = 0, Falling Edge = 1 (on "+" inputs)
A = Channel number 0 - 95
G = Geographic address
L = Logic address
P = Parity
W = Word Count
H = MTD133 Hit Count

Copyright© March 1997. LeCroy is a registered trademark of LeCroy Corporation. All rights reserved. Information in this publicaction supersedes all earlier versions.