MIM Module

=======================================================

Clement Engineering, Inc.



SALES:

  P.O. Box 1086

  Severna Park, MD  21146

  Tel: (410) 268-6736

  FAX: (410) 268-4612



OFFICE:

  1261 Dogwood Road

  Arnold, MD  21012

  Tel:  (410) 518-6591

  wclement@the-hermes.net



NOTE: Firmware for MIM module v1.06 and MIC Encoder

v1.04 are combined into one microcontroller.  Additional

hardware to interface the MIM to one's microphone is

required for MIC Encoder operation .



New features in MIM module v1.06 and MIC Encoder v1.04:

(1) Selection of ANY NMEA sentence for transmission (and

      wildcards for multiple sentences).

(2) Special "RAW mode" message formats for position

      reporting intervals less than 3 seconds.

(3) Message intervals selectable down to 1 second.



I.  Description.  The MIM module is an APRS-compatible,

packet radio telemetry unit.  When attached to a

suitable transmitter, the unit sends GPS position

reports, telemetry data, a beacon text message and CW ID

at user-selectable time intervals.  It is capable of

sending these messages via a digipeater path with as

many as eight hops.  The unit accepts Global Positioning

System (GPS) data in NMEA 0183 format for position

reporting and can telemeter five A/D inputs (8-bit

resolution) as well as eight bits of digital input. 

Through the use of configuration software "MIC104.EXE,"

the user is able to customize call signs and other

parameters.  An on-board EEPROM stores configuration

information indefinitely and "MIC104.EXE" can be run as

often as needed to change settings.  An on-board voltage

regulator accepts power from an unregulated 6.5-40 VDC

source (or it may be run from a regulated 5 VDC source). 

The regulated voltage is also available for other

applications, as less than 15 mA is required for the MIM

module but the regulator can supply 100 mA at +5 VDC. 

The MIM module measures 1" by 1.7" and is fabricated

using surface mount technology (except for the

microcontroller which is in a socket for easy firmware

upgrades).  All user connections are on the top of the

board (where the microcontroller resides) for easy

access even if the MIM module is mounted in a housing.



======================================================

Table 1.  Device Pinout

Pad Name       Function

=======        ========

V+             Unregulated supply (6.5-40 VDC)

GND            Ground for power and A/D and Digital

                  reference

+5             +5 VDC regulator output (or +% VDC input)

A0-A4          Analog inputs 0-4

D0-D7          Digital inputs 0-7

RXD            RS232 received data

TXD            RS232 transmitted data

HOLD OFF       RXA hold off pin

PTT            Push-To-Talk OUTPUT (open-collector NPN)

TXA            Transmit Audio

MIC GND        Ground for TXA

======================================================



II.  Technical Specification.  Around the periphery of

the board, solder pads are conveniently located to allow

connection to power and data.  Table 1 gives pad names

and the corresponding function.

     Two power options are available.  Unregulated DC

voltage (6.5-40 VDC) may be supplied to V+ with GND

being the ground pin.  An on-board voltage regulator

provides a clean +5 VDC power supply to all components

on the MIM module and may be accessed through the pin

labeled "+5" for use in other portions of the user's

circuit.  The MIM module uses approximately 15 mA,

leaving 85 mA at +5 VDC available for other

applications.  Components that produce noise on the

power line should be buffered in some fashion. 

Alternately, the user may supply +5 VDC to the module

through the +5 pin.  The single ground pin, GND, is

common to all portions of the MIM circuit except the

transmit audio TXA which has its own ground MIC GND. 

This includes RS232 communications, power, and analog

and digital inputs.  The pad labeled MIC GND can be

connected to the board's ground GND by solder jumpering

pad J2.

     The MIM module provides 5 analog-to-digital

telemetry inputs, labeled A0 through A4.  The reference

voltages for these inputs are the + 5 VDC supply voltage

(+5) and GND, which are digitized to 255 and 0

respectively (i.e., 8-bit resolution).  Analog channels

are sampled immediately prior to transmission and the

A/D converter is powered down when not in use to save

power.  Source impedance for any device connected to the

A/D pins should be below 10 K .  Unused channels should

be tied to GND to reduce power consumption and eliminate

crosstalk with adjacent channels.

     Eight TTL digital input telemetry bits are provided

(labeled D0 through D7).  When left unconnected, these

bits are pulled high and are read as logic 1.  The

analog and digital inputs lines may be disabled using

"MIC104.EXE".

     The MIM module interfaces to one's transmitter

through the connections labeled TXA, MIC GND, PTT, and

HOLD OFF.  Prior to transmitting, HOLD OFF is sampled. 

If logic low (<0.8 VDC) is sensed, transmission will be

held off.  This is  important for collision avoidance on

the channel.  Once the communications channel is clear,

the Push-To-Talk signal PTT is activated (low or high,

as selected in software) to key the transmitter and the

information packet is sent out Transmit Audio TXA.  The

radio's microphone ground should be connected to MIC

GND.  When transmission is complete, PTT is deactivated. 

The single-turn potentiometer near jumper J2 controls

the signal level on TXA.  Clockwise rotation gives

maximum amplitude (there is no stop, so if the deadband

is encountered, simply turn the potentiometer another

1/2 turn).

     Alternately, the device may be jumpered for

Handy-Talky (HT) operation.  In this mode, the TXA pin

provides both DC transmitter keying and the audio

signal, superimposed.  With jumper J1 installed, HT mode

is enabled.  With jumper J1 removed, both PTT and TXA

must be connected to the radio.  NOTE: Due to the

presence of an open-collector NPN transistor on the PTT

output (for power and isolation considerations when

connecting to the radio's PTT), the PTT signal is

inverted between the microcontroller and the pad labeled

PTT.  The configuration software therefore selects the

PTT active state based on the PTT pad.  In HT mode,

however, the superimposed audio and PTT uses the PTT

signal directly from the microcontroller.  Hence, in HT

mode, the user must select the PTT active level opposite

that which is desired.

   Serial communications with the MIM module are

required for configuring the device with the user's call

sign and other information and also for receiving data

from the GPS receiver, if attached.  Pads labeled TXD

and RXD are the RS232 transmit and receive connections,

respectively.  The signal levels are relative to GND. 

On-board transistors provide signal inversion and

over-voltage protection of the MIM module from the

higher RS232 voltage levels received via RXD.  The 5

volt signal sent out the TXD is satisfactory for RS232

communications over the short distances typically used

for configuring the module.  Only the RXD connection is

used when connecting to a GPS receiver, since the MIM

module performs no configuration on the user's GPS

receiver.



III.  System Operation.  Upon power-up, the MIM module

verifies that its EEPROM contains a valid configuration. 

If not, a packet is transmitted stating "CONFIG FAILED"

to alert the operator of its failed status (every 15

seconds).  The remedy for this is to first cycle the

power to ensure that the failure was not simply due to a

poor power-up condition and, it the system still fails,

to run the configuration software "MIC104.EXE".  This

software allows the module to be configured as either a

MIM module (default) or a MIC encoder (although

additional hardware is necessary for MIC encoder

operation).  The default settings are:



   --  Call sign "NOCALL-0"

   --  MIM module operation

   --  PTT active low

   --  Transmission delay = 213 milliseconds

   --  Base transmission period = 1 second

   --  GPS position reporting interval = 60 seconds (if

         GPS attached)

   --  A/D and digital telemetry interval = 60 seconds

   --  Beacon Text OFF

   --  CW ID OFF



Selection of MIM module operation (vice MIC Encoder) is

made by the command "mode mim" within the software

"MIC104.EXE".  Once in this mode, its operation is as

described below.

   There are four telemetry message types that the MIM

module can send -- A/D and digital telemetry data, GPS

position report, beacon text, and CW ID.  The first

three are digital packet-radio signals which are

compatible with standard 1200 baud TNC units.  The last

is a 20 word-per-minute Morse code identification.  All

message types are individually enabled or disabled by

the configuration software "MIC104.EXE".

   Upon power-up the MIM module enters telemetry mode,

sending messages at intervals designated by the user and

stored in the configuration EEPROM.  The base time

period (in seconds) is set by the command "per <n>"

where "n" is in the range 1 to  254, inclusive.  Each of

the four message types (data, position, beacon text, and

CW ID)  is sent at an interval which is a multiple of

the base time period and each type has its own interval. 

For example, with a base time period of 15 seconds

(command "per 15"), the telemetry data message can be

set to occur every 15 seconds using the command "tele

1", the position reporting period can be set to 5

minutes using the command "pos 20", the beacon text

every 1 hour with the command "b 240", and the CW ID

every 10 minutes with "cw 40".  Setting an interval to

zero turns that feature off (e.g., "cw 0" turns off the

CW ID feature).

   The format of the position reports depends upon the

NMEA sentence chosen.  Only GPRMC is specifically

formatted for APRS (and only when the position reporting

interval is greater than two seconds).  APRS will accept

unformatted NMEA strings, so all NMEA sentence types can

be sent in an APRS-compatible form.  A special message

format for the two most-commonly used NMEA sentences,

GPRMC and GPGGA, is sent when the position reporting

interval is two seconds or less.  This is necessary in

order to ensure that no messages are missed since the

GPS receiver outputs its NMEA sentences at 4800 baud but

the MIM module transmits AX.25 packets at 1200 baud. 

This special format appends the telemetry data to the

end of the position report, regardless of the telemetry

interval (if telemetry is enabled), so as to allow for

the most rapid message transmissions.  Additionally,

regardless of whether one or two seconds is chosen as

the position reporting interval, the MIM module sends

position reports immediately upon receipt of the desired

NMEA sentence.  For example, if you choose a position

reporting interval of two seconds and select the GPRMC

message, position reports will be sent as often as your

GPS sends GPRMC messages out -- even once per second if

that is the rate.  This is called the RAW mode.

   If no GPS fixes are received via RXD, no position

reports are sent, regardless of the position reporting

interval set with the "pos" command (except for RAW

mode).  If valid GPS data is received, it is sent when

the next position message interval occurs and has the

text "/GPS FIX" appended to show that it is current.  If

at some later time GPS data is no longer available, the

MIM will report the last fix and append the text "/LAST

FIX" to indicate an old position.  If valid GPS fixes

once again become available, they will be so indicated

by the text "/GPS FIX".  The time stamp on the GPS fix

tells when it was valid.  If configurable, GPS

communications must be set for 4800 N81 operation.  If

your GPS cannot transmit using this configuration, the

MIM module will not be able to receive GPS reports.  The

configuration  4800 N81 is the NMEA 0183 standard.

   The serial port connection (must use both TXD and

RXD) may be used at any time to configure the device

with configuration software "MIC104.EXE" using either

COM1 or COM2.  A 3-wire serial cable is all that is

needed (connections are discussed in Section IV:  System

Setup).  The program is executed from the DOS prompt

with the command "mic104 [1|2]", where 1 or 2 is

optionally chosen to select the serial port which

connects the PC to the MIM module (the brackets [] are

not typed).  If no argument is given, the program

searches both COM1 and COM2 for a response from an

active MIM module.  If a powered-up MIM module is found,

its current configuration is displayed and may be

modified.  Additionally, if the MIM module was

transmitting when the configuration software was

started, it will cease transmitting until the unit is

reset (by software or hardware).



IV.  System Setup. Two basic electrical configurations

are discussed: (1) wiring for software configuration

using "MIC104.EXE" and (2) wiring for telemetry use.



Basic wiring for the MIM module when running

configuration software "MIC104.EXE" is as follows:

===================================================

-- EITHER 6.5-40 VDC to pad V+ OR 5 VDC to pad +5.

-- Ground to pad GND.

-- RS232 serial communications to MIM module pads TXD,

RXD, and GND.  (Note: The designation TXD and RXD --

transmit and receive data, respectively -- are relative

to the MIM module.  Consequently, when connecting the

MIM module to a PC for configuration, connect the MIM

module's TXD pad to the PC's RXD pin -- DB-9 pin 2 or

DB-25 pin 3.  Similarly, connect the MIM module's RXD

pad to the PC's TXD pin -- DB-9 pin 3 or DB-25 pin 2. 

Also connect the MIM module's GND to the ground pin of

the serial connection -- DB-9 pin 5 or DB-25 pin 7).



In addition, if one wishes to see the AX.25 output of

the MIM module (to verify configuration), the TXA and

MIC GND pads may be connected directly to a TNC input

(through a 0.1 microF blocking capacitor if  TXA is also

connected to a transmitter).  The TNC output is

connected to a PC via a serial cable.  One cannot view

both the configuration software and TNC output on the

same monitor simultaneously.  Rather, after

configuration is complete, exit "MIC104.EXE" and run a

terminal emulation program (e.g., Procomm) or APRS to

see AX.25 output.  Alternatively, two PCs may be used

for simultaneous configuration and AX.25 output display.



Basic wiring for telemetry use is as follows:

================================================

-- EITHER 6.5-40 VDC to pad V+ OR 5 VDC to pad +5.

-- Ground to pad GND.

-- If position reports are desired, connect GPS data out

and ground wires to MIM module pads RXD and GND

respectively.  No communications are sent to the GPS, so

MIM module pad TXD is left unconnected.

-- If desired, connect 0-5V analog signals to pads A0

through A4.

-- If desired, connect 0-5V logic signals to pads D0

through D7.

-- PTT on radio to MIM module pad PTT (unless HT

operation is chosen).

-- TXA on radio to MIM module pad TXA.

-- "Hold off", if implemented, to MIM module pad HOLD

OFF.  Figure 1 illustrates a suitable hold off circuit. 

RXA is the received audio.  The transistor is a 2N3904

or similar.  Other component values are:  R1=100 ,

C1=10microF, R2=100K , and C2=0.68microF.



V.  "MIC104.EXE" Configuration Software.  The program

"MIC104.EXE" allows the user to customize  the MIM

device using conventional TNC commands and store the

configuration information in an on-board EEPROM.  This

EEPROM has an erase/write life in excess of 1,000,000

operations.  A three-wire communications system is

employed -- RS232 send, RS232 receive, and signal

ground.  From a PC with a DB9 connector, pin #2 is RS232

RXD, pin #3 is TXD, and pin #5 is ground.  These should

be connected to MIM module pads TXD, RXD, and GND,

respectively.  (Note:  PCs use male connectors for

serial ports, so a female connector is necessary when

making a cable for configuration.)

    The software communicates with the MIM module using

either COM1 or COM2 (4800 baud, N81, since that is also

the standard used for GPS communications).  The program

is executed from the DOS prompt with the command "mic104

[1|2]", where either 1 or 2 is used depending upon the

serial port connected.  If no argument is given, the

program checks both COM ports (first COM1, then COM2)

for an active MIM module.  Upon running "MIC104.EXE",

the software first queries the MIM module to find its

current configuration.  If the MIM module is not

attached or is powered down, a default screen will

appear and the message "Comm Link FAILURE" flashes (or

displayed in gray text) in the upper right corner of the

screen..  Simply attach the communications cable, power

the module up, and either execute the "com" command

which will search for the active device.  If the COM

port is known (e.g. COM1), issue the command "com 1". 

Or, if the correct COM port is already selected, the

"disp" command will read and display the current

configuration.  The message "Comm Link OK" is shown once

communications are established.  At this point, the user

may wish to change one or more of the parameters and

download the new configuration to the MIM module using

the command "perm" (short for PERMANENT).  The

configuration, once stored, remains valid until changed

by the user.

   Parameter changes which have not yet been downloaded

to the MIM module are shown in blinking or grayed text

(select this option with the "blink" command).  If

communications fail while performing a "perm" command,

the parameters will no longer flash on the screen, but

neither can one be sure that they were successfully

received and stored by the MIM.  Try executing the

"perm" command again to re-establish communications and

store parameters.  It is a good idea to execute the

"disp" command after sending new parameters to ensure

they were correctly downloaded.

   Figure 2 shows the primary run-time screen of

"MIC104.EXE".  Only the capital letters beginning each

command may be used and the program accepts both upper

and lower case characters.

   When operating in MIM mode, any NMEA sentence may be

selected and will be sent "as received" from the GPS

receiver.  For example, to select the GPGLL sentence,

issue the command "str GPGLL".  If no argument is given,

the default GPRMC is selected.  Wildcard characters (?)

may also be used to select several messages.  For

example, "str GP???" (or simply "str GP", since the

string is padded with "?" characters) will cause the MIM

module to send ALL NMEA strings.  There is, of course, a

practical limitation to this feature.  Since the GPS

receiver sends its strings at 4800 baud but the MIM

transmits AX.25 packets at 1200 baud, overrun will occur

and the actual sentences which get transmitted cannot be

determined exactly.  This feature is primarily intended

for future migration to 9600 baud packet radio which

would allow all sentences to be transmitted reliably.



VI.  Example Configuration Session.  Suppose we wish to

configure the MIM module using COM1 as follows:

   --   Call sign "N3XLM-1".

   --   Via "WIDE, WB4APR-11, N3MIM-5, WIDE".

   --   PTT active low (default)

   --   Display character "A" (ASCII value)

   --   Default transmission delay.

   --   Telemetry sent every 60 seconds (= 01:00 mm:ss).

   --   GPS position report sent every 02:30 mm:ss.

   --   GPS string to use: "GPRMC" (default)

   --   Beacon Text sent every 05:00 mm:ss.

   --   Beacon Text:  "Wx Balloon, 9/30/96, N3XLM"

   --   CW ID sent every 20:00 mm:ss.

   --   CW ID Text:  "DE N3XLM"



Solution:

   Since the greatest common factor in the four

transmission periods is 30 seconds (i.e., all four time

periods are multiples of 30 seconds), we will set the

base transmission period to 30 seconds.  Note that the

10 minute interval of the CWID will not permit a base

transmission period of less than 20*60/254 = 4.72   5

seconds since CWID can be at most 254 times the base

transmission period.



(1)    Run "MIC104.EXE" from the DOS prompt (C:>) with

the command:   "mic104 1 <cr>".  The symbol <cr> means a

carriage return (or enter key) and the quotes ("") are

not typed.

(2)    From the "cmd:" prompt, enter the following

commands:

   cmd: "myc n3xlm-1 <cr>"

   cmd: "via wide, wb4apr-11, n3mim-5, wide <cr>"

   cmd: "ptt 0 <cr>"

   cmd: "mys A <cr>"

   cmd: "txd <cr>"

   cmd: "per 30 <cr>"

   cmd: "tele 2 <cr>"

   cmd: "pos 5 <cr>"

   cmd: "str GPRMC <cr>"

   cmd: "b 10 <cr>"

   cmd: "bt Wx Balloon, 9/30/96, N3XLM <cr>"

   cmd: "cw 40 <cr>"

   cmd: "cwi DE N3XLM <cr>"

   cmd: "perm <cr>"

   cmd: "disp <cr>"

   cmd: "reset <cr>"

   cmd: "q <cr>"



   Note that the CW ID text is not case-sensitive but

will be converted to upper-case for display.  Beacon

text can be upper or lower case, as desired, and will be

transmitted as such.  The software reset is not

necessary if the unit is going to be powered down and up

before use.  The first timeout period for each enabled

message type (and hence, the first transmission) occurs

one second after reset (software or hardware).  The

telemetry message looks as follows:

   T#000,aaa,aaa,aaa,aaa,aaa,dddddddd, MIM v1.06



The three digits following the "T#" are the message

sequence number (000-255).  This number increments to

255 and then rolls over to 000 and continues.  The five

groups denoted "aaa" are the five analog data channels

displayed in the range 000-255 (ASCII BCD).  Channel A0

is the first group and A4 the last.  Similarly, the

symbol "dddddddd" represents the eight bits of digital

data, with D7 sent first.

   As an example of the analog data sent back, if A/D

channel 1 read 3 VDC (of a maximum 5 VDC), the

transmitted value would be (3/5)*255 = 153.

   The message stream would look as follows (with the

time of arrival after reset shown):



TIME 00:00 mm:ss

   N3XLM/1>APRS, WIDE, WB4APR-11, N3MIM-5, WIDE:

   @072914h3858.88N/07629.17WA270/031/GPS FIX

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#000,127,031,056,198,227,11010111, MIM v1.06

   N3XLM/1>APRS, WIDE, WB4APR-11, N3MIM-5, WIDE:

   Wx Balloon, 9/30/96, N3XLM

TIME 01:00 mm:ss

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#001,129,031,045,108,226,11010111, MIM v1.06

TIME 02:00 mm:ss

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#002,133,032,044,105,227,00010111, MIM v1.06

TIME 02:30 mm:ss

   N3XLM/1>APRS, WIDE, WB4APR-11, N3MIM-5, WIDE:

   @073144h3858.88N/07630.83WA270/031/GPS FIX

TIME 03:00 mm:ss

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#003,132,032,041,106,226,00110101, MIM v1.06

TIME 04:00 mm:ss

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#004,132,032,041,106,226,00110101, MIM v1.06

TIME 05:00 mm:ss

   N3XLM/1>APRS, WIDE, WB4APR-11, N3MIM-5, WIDE:

   @073414h3858.88N/07632.49WA270/031/GPS FIX

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#005,133,031,040,108,222,00110111, MIM v1.06

   N3XLM/1>APRS, WIDE, WB4APR-11, N3MIM-5, WIDE:

   Wx Balloon, 9/30/96, N3XLM

TIME 06:00 mm:ss

   N3XLM/1>APRTLM, WIDE, WB4APR-11, N3MIM-5, WIDE:

   T#006,133,031,040,108,222,00110111, MIM v1.06



If a GPS unit had not been attached, the GPS messages

would not have been sent.  The characters in the GPS

message have the following meaning (looking at the 02:30

mm:ss transmission).  The characters "073144h" are the

time stamp (hhmmss) in GMT.  "3858.88N" is the latitude

(ddmm.mm N/S) and "07630.83W" is longitude (dddmm.mm

E/W).  The display character is the "A" and the course

and speed are given by "270/031" (degrees/knots). 

Finally, the fact that this is a current GPS fix is

indicated by the words "GPS FIX".  Had this been an old

fix, the text "LAST FIX" would have been sent.



VII.  Message Formats.  Depending on the position

reporting interval chosen and the NMEA sentence

selected, the position report may have various formats.



RAW mode (Position reporting interval 2 seconds, MIM

mode selected, GPRMC or GPGGA sentence):



GPGGA sentence:



   213714Z3916.26N07644.20W,NUL,VTG,36M,T#071,002,094,

       ... 226,035,112,11111110

   <time>Z<latitude><longitude>,NUL,VTG,

   <altitude, meters>M,<telemetry message>

or



   NO GPS FIX,T#071,002,094,226,035,112,11111110

     (if GPS indicates no fix)



   The fields "NUL,VTG" are place holders for later

implementations when the GPVTG sentence will be parsed

for speed and course.



GPRMC sentence:



   213713Z3916.26N07644.20W,000,360,T#017,002,094,226,

       ... 035,112,11111110

   <time>Z<latitude><longitude>,<speed, kts>,

   <course, true>,<telemetry message>

or

   NO GPS FIX,T#017,002,094,226,035,112,11111110



Note that for RAW mode, these combined position and

telemetry messages are only sent if a GPS receiver is

attached and sending NMEA data (even if no fix has yet

been obtained).  If a powered GPS receiver is not

attached while in RAW mode, the regular telemetry,

beacon, and CW ID transmissions will occur at their

scheduled intervals.



NORMAL mode (Position reporting interval>2 seconds OR a

message other than GPRMC or GPGGA is selected):



GPRMC sentence (interval>2 seconds):

   @073414h3858.88N/07632.49WA270/031/GPS FIX

   @<time>h<latitude>/<longitude>

    <display character><course>/<speed>

or

   @073414h3858.88N/07632.49WA270/031/LAST FIX



Other sentence (e.g., GPGLL):

   $GPGLL,3916.265,N,07644.206,W,142040,A/GPS FIX

or

   $GPGLL,3916.265,N,07644.206,W,142040,A/LAST FIX



   Note that the sentence is sent "as received" from the

GPS receiver, but without the asterisk (*) and check-sum

which normally appear at the end.  Instead, the text

"/GPS FIX" or "/LAST FIX" is appended to indicate the

currency of the information.



VIII.  Context-Sensitive Help.  Help for "MIC104.EXE"

commands is obtained by using the command "h <cmd>"

where <cmd> is the command identifier (the capital

letters for that command).  The program will respond

with a help screen corresponding to the command in

question.  For example, to inquire about the "perm"

command, type "h perm <cr>".

