distortion testing of my an/fgc-25 teletypewriter set

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Distortion Testing of My AN/FGC-25 Teletypewriter Set

by Random73

June 29, 2015

I do not have an oscilloscope, so I wrote a program for my

MC6800-based microcomputer to capture samples of the 60ma

current loop during signaling by the keyboard or the tape reader,

in order to measure any bias or end distortion that might be

present. I have never before had a capability to make any such

measurements on this teletype set, or any other set.

The program waits for a mark-to-space transition and then

samples the loop until the sample buffer is full (7935 samples).

Version 3 of the program simultaneously samples the output of

the 1 millisecond timer on the TTY interface card. After filling

the sample buffer the program dumps the buffer to the MIKBUG

console terminal, for porting into Microsoft Excel 2010.

1. Version 1 Testing June 17, 2015, Results

With version 1 I used MIKBUG Punch command to save the sample

buffer after a capture. Today (June 27) I reloaded the buffer

from the portions that I had saved on the 17th, and then ran

just the dump portion of version 3 so that I could plot the data

in Excel.

Keyboard Capture of June 17, Letter "R";

Note the "glitch" shortly after each space-to-mark transition.

The space-to-mark transition starting the STOP bit occured at

sample #1146. The elapsed time since start of the START bit

should be 6*22 = 132 ms. The sampling period was therefore

approximately 132/1146 = 115.2us. The sampling rate was

1/.0001152 = about 8.7 kilosamples per second.

2

Keyboard Mark Interval Durations:

Sample 396 through 567 172 samples = 19.81 milliseconds


Keyboard Space Interval Durations:



These estimates suggest that there was some slight "space bias

distortion" occurring in the keyboard transmitter, about

(1.27/22)*100 = 5.8%.

Tape Reader Capture of June 17, Letters "YRY";

June 17, 2015 Tape Reader "YRY" Signal Capture

This tape reader snapshot captured 3 complete signaling

intervals, or 3*163 = 489ms. Samples 2 through 4206, or 4205

samples, should correspond to 489ms. 489/4205 = 116.29us for the

sampling period, or 8.60ksps.

There were no observed glitches after space-to-mark transitions.

Tape Reader Space Interval Durations:



Tape Reader Mark Interval Durations:



These estimates suggest that there was significant "space bias

distortion" occurring in the tape reader, (5.79/22)*100= 26.3%.

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2. Version 2 Testing June 25, 2015, Results

In version 2 I increased the sampling rate. My target was 10

kilosamples per second. Here is the keyboard waveform after I

adjusted the keying contact, to reduce the spacing bias;

Keyboard Capture June 25, "RYR", manually typed;

I forgot to use the REPEAT key, so the stop bit intervals are

longer than 31ms. Reacall that the keyboard data of June 17

showed glitches. No keyboard "glitches" occurred in this capture.

Samples 2-1251 cover 6 bit periods, should span 132ms;

132/1250 = 105.60us per sample, or 9.47ksps.

Keyboard Mark Interval Durations:



Keyboard Space Interval Durations:



The bias distortion is now 6% marking bias (worst case), so I

over-adjusted the keying contact.

4

Tape Reader Capture June 25- "YRYR"

Note that space-to-mark glitches started happening on the 2nd

character and continued thereafter. Recall that the tape reader

data on June 17 did not show any glitches.

I have not yet tried to reduce the tape reader's spacing bias so

there is no need yet to tabulate mark and space intervals again

for tape reader.

3. Version 3 Testing June 26, 2015 Results

In version 3 I adjusted the sampling rate again, and I added

sampling of the 1 millisecond timer output from the TTY

interface card. The sampling rate estimate for version 3 is

102.7us per sample, or 9.74ksps. My target was 10ksps. Version 3

also upscales the current loop sample values from 1 to 4, so

that they are more clearly distinguishable from the timer

samples when both are plotted on the same Excel chart.

I adjusted the keyboard keying contact again, in an attempt to

reduce the marking bias. Here are the measurements after that

adjustment.

Keyboard June 26- "YYY" using REPEAT key, Version 3

The first and third mark pulses did not glitch. This glitch

thing is a real mystery, especially since it is happening

intermittantly on both the keyboard and the tape reader.

5

I counted the 1ms timer periods in the data base to manually

construct reference marks for the theoretical bit transition

instants. I set the reference mark values to 5 so they would

stand out;

Keyboard June 26, 1st character compared to reference markers

The keyboard timing/bias performance looks great!

Tape Reader June 26, 1st character compared to reference markers

The tape reader's "spacing bias" is now clearly evident. There

is also some slght "marking end" distortion.

I am more concerned about the glitching phenomenon. It is

happening on both the keyboard and the tape reader, frequently.

I have studied figures 60 and 115 in TM11-2246 and I understand

how this serial keying mechanism is supposed to work. I thought

I had cleaned the keying contacts well enough, but maybe not.

My next planned action is to remove and thoroughly inspect and

clean the keying contacts (again) for both the keyboard and tape

reader, this time using burnishing tool (I will have to get one)

as instructed in the manual.

Any suggestions or advice would be most appreciated.

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APPENDIX A- A Brief History of This Set

My Kleinschmidt AN/FGC-25 Teletypewriter Set

I acquired this set in 1977 in two phases. First I bought the

printer unit and the manual, from two different sellers, at a

ham radio swap meet in Atlanta, GA. After studying the manual I

wrote to the seller, in Mobile, AL, and offered to buy the rest

of the set. I drove there one Saturday in my '75 Chevy Monza

hatchback and somehow managed to get the reperforator and the

table in my car and back to Marietta, GA.

I spent the next year or so, off and on, performing a complete

tear down, restoration, and rebuild of the set. Mr. Fred Schmidt

of Typetronics supplied all the parts I needed (he had sold the

manual to me at the swap meet). I had to replace several springs

that had rusted through, and some other corroded parts.

When I moved to California in fall of 1979 this set came with me

on the moving van. It had been mostly idle and in storage from

1983 until late spring 2014, when I began its resurrection.

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APPENDIX B- Distortion Tester Program Listing ---

PAGE 001 DISTORTI

00001 NAM DISTORTION TESTER V3

00002 OPT O,S,NOG

00003 *

00004 * JUNE 26, 2015 BY D.R.SENTZ

00005 *

00006 * THIS PROGRAM SAMPLES THE CURRENT LOOP BEGINNING AT

00007 * THE 1ST DETECTED MARK-TO-SPACE TRANSITION, WHICH

00008 * NORMALLY INDICATES THE BEGINNING OF A START BIT. I

00009 * ESTIMATE THAT THERE SHOULD BE ABOUT 220

00010 * SAMPLES PER BIT PERIOD (22MS), UNTIL THE MEMORY

00011 * BUFFER IS FULL. SAMPLES ARE STORED BEGINNING AT

00012 * THE HIGHEST ADDRESS OF THE BUFFER.

00013 * UPDATE V3: ADDED SAMPLING OF THE 1MS TIMER OUTPUT

00014 * AND BOTH SAMPLES STORED TO SAME MEMORY LOCATION.

00015 * ADDED DUMP OF SAMPLE BUFFER TO CONSOLE AFTER

00016 * BUFFER IS FULL, IN FORMAT

00017 * LOOPSAMPLE,TIMERSAMPLE

00018 * THIS FORMAT ENABLES EASY PORT TO EXCEL 2010 FOR

00019 * PLOTTING AND ANALYSIS.

00020 *

00021 1EFF EOBADR EQU $1EFF

00022 8008 PIA EQU $8008 PIA ON MEK6800D1 CPU BOARD

00023 A048 RESTRT EQU $A048

00024 E07E PDATA1 EQU $E07E STRING OUTPUT SUBROUTINE

00025 E1D1 OUTEEE EQU $E1D1 CHARACTER OUTPUT SUBROUTINE

00026 E0E3 MIKBUG EQU $E0E3 MIKBUG REENTRY POINT

00027 *

00028 1F00 ORG $1F00

00029 1F00 CE 8008 LDX #PIA

00030 *

00031 * CONFIGURE PIA REGISTERS A AND B

00032 *

00033 1F03 4F CLR A INIT PIA REG. B

00034 1F04 A7 01 STA A 1,X POINT 8008 TO DDRA

00035 1F06 A7 03 STA A 3,X POINT 800A TO DDRB

00036 1F08 86 FE LDA A #$FE REG.B B7-B1=OUTPUTS, BO=INPUT

00037 1F0A A7 02 STA A 2,X PUT TO DATA DIR. REG.B

00038 1F0C 86 7F LDA A #$7F REG.A B7=INPUT, B6-B0=OUTPUTS

00039 1F0E A7 00 STA A 0,X PUT TO DATA DIR. REG.A

00040 1F10 86 04 LDA A #4 POINT TO I/O REG.

00041 1F12 A7 01 STA A 1,X 8009 IS PIA CONTROL REG.A

00042 1F14 A7 03 STA A 3,X 800B IS PIA CONTROL REG.B

00043 1F16 86 01 LDA A #1 NOW OUTPUT A MARK SIGNAL TO

00044 1F18 A7 00 STA A 0,X CLOSE THE TTY CURRENT LOOP

00045 *

00046 * WAIT FOR MARK-TO-SPACE TRANSITION ON LINE.

00047 *

00048 1F1A A6 02 WAIT LDA A 2,X SAMPLE THE LINE STATE 5 C

00049 1F1C 26 FC BNE WAIT IF MARK, WAIT FOR SPACE 4 C

00050 1F1E CE 1EFF LDX #EOBADR ELSE INIT BUFFER POINTER 4 C

00051 *

00052 1F21 F6 8008 NEXT LDA B PIA SAMPLE THE TIMER STATE 4 C

00053 1F24 C4 80 AND B #$80 MASK ALL BUT INPUT BIT 2 C

00054 1F26 1B ABA MERGE; A=A+B 2 C

8

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PAGE 002 DISTORTI

00055 1F27 A7 00 STA A 0,X SAVE SAMPLES IN ONE LOC. 6 C

00056 1F29 09 DEX 4 CYC

00057 1F2A 27 0C BEQ DONE 4 CYC

00058 * VARY THE ARG. BELOW TO ADJUST SAMPLE PERIOD.

00059 1F2C C6 06 LDA B #6 SETS SAMPLING RATE 2 C

00060 *

00061 *WAIT LOOP BELOW IS 10 CPU CYCLES. I WANT ABOUT

00062 *220 SAMPLES PER BIT (100US SPACING)

00063 *

00064 1F2E 5A WLOOP DEC B 2 CYC

00065 1F2F 27 02 BEQ CONT 4 CYC

00066 1F31 20 FB BRA WLOOP 4 CYC

00067 *

00068 1F33 B6 800A CONT LDA A PIA+2 GET NEXT LOOP SAMPLE 4 C

00069 1F36 20 E9 BRA NEXT AND KEEP GOING 4 C

00070 *

00071 * DUMP THE SAMPLE BUFFER TO THE CONSOLE USING

00072 * BETWEEN EACH SAMPLE. COPY/PASTE THE

00073 * DUMP FROM TERA TERM WINDOW TO EXCEL, THEN

00074 * SELECT THE DATA AND INSERT 2D LINE CHART

00075 * TO PLOT THE SAMPLE DATA.

00076 *

00077 1F38 CE 1EFF DONE LDX #EOBADR INIT BUFFER POINTER

00078 1F3B A6 00 OUTLP LDA A 0,X GET SAMPLES

00079 1F3D 36 PSH A

00080 1F3E 84 01 AND A #1 MASK ALL BUT LSAMP

00081 1F40 48 ASL A

00082 1F41 48 ASL A QUADRUPLE IT TO 4

00083 1F42 8B 30 ADD A #$30 CONVERT TO ASCII

00084 1F44 B7 1F6B STA A LSAMP

00085 1F47 32 PUL A

00086 1F48 84 80 AND A #$80 MASK ALL BUT TSAMP

00087 1F4A 49 ROL A

00088 1F4B 49 ROL A PUT IT IN LSB

00089 1F4C 8B 30 ADD A #$30 CONVERT TO ASCII

00090 1F4E B7 1F6D STA A TSAMP

00091 1F51 FF 1F6F STX XTEMP

00092 1F54 CE 1F69 LDX #OSTRNG

00093 1F57 BD E07E JSR PDATA1

00094 1F5A FE 1F6F LDX XTEMP

00095 1F5D 09 DEX DECREMENT POINTER

00096 1F5E 26 DB BNE OUTLP IF >0 THEN KEEP DUMPING

00097 1F60 CE 1F00 LDX #$1F00 SET UP PROG RESTART ADDRESS

00098 1F63 FF A048 STX RESTRT

00099 1F66 7E E0E3 JMP MIKBUG AND EXIT

00100 *

00101 * DEFINE OUTPUT PRINT STRING

00102 *

00103 1F69 0D OSTRNG FCB $0D,$0A

00104 1F6B 0001 LSAMP RMB 1

00105 1F6C 2C FCC /,/

00106 1F6D 0001 TSAMP RMB 1

00107 1F6E 04 FCB $04 EOT

00108 1F6F 0002 XTEMP RMB 2 NOT THE SAME AS MIKBUG XTEMP

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---

PAGE 003 DISTORTI

00109 *

00110 A048 ORG $A048 INITIALIZE PROG. COUNTER

00111 A048 1F00 FDB $1F00

00112 END

TOTAL ERRORS 00000

DISTORTION TESTER V3 JUNE 26 2015

S00B0000444953544F52544982

S11E1F00CE80084FA701A70386FEA702867FA7008604A701A7038601A700A642

S11E1F1B0226FCCE1EFFF68008C4801BA70009270CC6065A270220FBB6800A2E

S11E1F3620E9CE1EFFA60036840148488B30B71F6B32848049498B30B71F6DE5

S11D1F51FF1F6FCE1F69BDE07EFE1F6F0926DBCE1F00FFA0487EE0E30D0AB2

S1041F6C2C44

S1041F6E046A

S105A0481F00F3

S9

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APPENDIX C- 60mA Current Loop Interface for Microcomputer

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Appendix D- Demonstration Videos on Internet

Episode 1 http://www.zippcast.com/video/a73d8b3f9842714ee97

An introduction and power up, March 2015

Episode 2 http://www.zippcast.com/video/a722081e609549512fa

Explanation of blown fuse at power up, and repair

Substitute circuit for 1/16 amp slo-blo fuse

Demonstration of tape reader and printer working

Episode 3 http://www.zippcast.com/video/bee7e30fdfdb47e24da

Keyboard "simultaneous mode" and reperforator

Keyboard contact bail fix

Keyboard "sequential mode" and printer

Episode 4 http://www.zippcast.com/video/31acf5aa666f99256a9

Sending ASCII text file from Macintosh 512K to

microcomputer, and printing the file on the AN/FGC-25

Demo http://www.zippcast.com/video/8a0f40e3806bff93b64

Sending Baudot-coded text file from teletype paper

tape reader to Windows 7 PC running terminal emulator

"Tera Term" (via the MC6800 microcomputer).

distortion testing of my an/fgc-25 teletypewriter set

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