diff --git a/src/dom.ts b/src/dom.ts
index f132a19..fa7bc12 100644
--- a/src/dom.ts
+++ b/src/dom.ts
@@ -1,5 +1,6 @@
 import { Converters } from "./converters";
 import { Frame } from "./frame";
+import { FunctionCodes } from "./function-codes";
 
 export class TableDataColumn {
     readonly td: HTMLElement = document.createElement('td');
@@ -118,10 +119,10 @@ export class Dom {
     static readonly sendForm = new DomForm<SendFormData>(document.querySelector('form[name=send]')!);
 
     private static readonly functionCodeList = document.getElementById('functionCodeList')!;
-    static addFunctionCodeListOption(code: string, description: string): void {
+    static addFunctionCodeListOption(code: number): void {
         const option = document.createElement('option');
-        option.value = code;
-        option.appendChild(document.createTextNode(`${Converters.byteToHex(+code)} ${description}`));
+        option.value = code.toString();
+        option.appendChild(document.createTextNode(FunctionCodes.getDescription(code)));
         Dom.functionCodeList.appendChild(option);
     }
 
diff --git a/src/frame.ts b/src/frame.ts
index ed78677..e4fb946 100644
--- a/src/frame.ts
+++ b/src/frame.ts
@@ -1,7 +1,7 @@
 import { Converters } from "./converters";
 import { dataFieldStrategies } from "./data-field";
 import { Dom, TableColumnButton, TableDataColumn } from "./dom";
-import { getFunctionCodeDescription } from "./function-codes";
+import { FunctionCodes } from "./function-codes";
 
 export class Frame {
     readonly slaveAddress: number;
@@ -46,7 +46,7 @@ export class Frame {
         return [
             new TableDataColumn(Frame.getDateTime(), this.type),
             new TableDataColumn(`${this.slaveAddress}=0x${Converters.byteToHex(this.slaveAddress!)}`, this.type),
-            new TableDataColumn(`${this.functionCode}=${this.functionCode === undefined ? '' : getFunctionCodeDescription(this.functionCode)}`, this.type),
+            new TableDataColumn(FunctionCodes.getDescription(this.functionCode), this.type),
             new TableDataColumn(this.getDataLength().toString(), this.type),
             new TableDataColumn(this.getDataAsText(), this.isNoValidDataFormat() ? 'error' : this.type),
             this.type === 'error' ? new TableDataColumn('', this.type) : new TableColumnButton('To send form', () => Dom.sendForm.setFormData(this)),
@@ -57,7 +57,7 @@ export class Frame {
         if (this.type === 'error') {
             return `Invalid frame: 0x${this.hexData}`;
         } else if (this.isUnknownFrame()) {
-            return `Data field format is unknown, data field: 0x${this.hexData}`;
+            return `No strategies to format data field. Raw data field is: 0x${this.hexData}`;
         } else if (this.isNoValidDataFormat()) {
             return `This frame format does not fit to the known function code: fromMasterToSlaveError=${this.fromMasterToSlaveError}; fromSlaveToMasterError=${this.fromSlaveToMasterError}; for: 0x${this.hexData}`;
         } else {
diff --git a/src/function-codes.ts b/src/function-codes.ts
index bf4b37a..f6371fa 100644
--- a/src/function-codes.ts
+++ b/src/function-codes.ts
@@ -1,37 +1,56 @@
 import { Converters } from "./converters";
+import { DataField } from "./data-field";
 
-export const functionCodes: { [code: number]: string } = {
-    0x01: 'Read Coils',
-    0x02: 'Read Discrete Inputs',
-    0x03: 'Read Holding Registers',
-    0x04: 'Read Input Registers',
-    0x05: 'Write Single Coil',
-    0x06: 'Write Single Register',
-    0x07: 'Read Exceptions Status',
-    0x08: 'Diagnostic Test',
-    0x0f: 'Write Multiple Coils',
-    0x10: 'Write Multiple Registers',
-    0x11: 'Identify Device Server',
-};
+interface FunctionCodeDetails {
+    readonly code: number;
+    readonly description: string;
+    readonly masterRequestStrategy?: DataField;
+    readonly slaveResponseStrategy?: DataField;
+}
 
-export const errorCodes: { [code: number]: string } = {
-    0x81: 'Illegal Function',
-    0x82: 'Illegal Data Address',
-    0x83: 'Illegal Data Value',
-    0x84: 'Server Device Failure',
-    0x85: 'Acknowledge',
-    0x86: 'Server Device Busy',
-    0x87: 'Negative Acknowledge',
-    0x88: 'Memory Parity Error',
-    0x90: 'Gateway Path Unavailable',
-    0x91: 'Gateway Target Device Failed to Respond',
-};
+const allFunctionCodeDetails: ReadonlySet<FunctionCodeDetails> = new Set<FunctionCodeDetails>([
+    { code: 0x01, description: 'Read Coils' },
+    { code: 0x02, description: 'Read Discrete Inputs' },
+    { code: 0x03, description: 'Read Holding Registers' },
+    { code: 0x04, description: 'Read Input Registers' },
+    { code: 0x05, description: 'Write Single Coil' },
+    { code: 0x06, description: 'Write Single Register' },
+    { code: 0x07, description: 'Read Exceptions Status' },
+    { code: 0x08, description: 'Diagnostic Test' },
+    { code: 0x0f, description: 'Write Multiple Coils' },
+    { code: 0x10, description: 'Write Multiple Registers' },
+    { code: 0x11, description: 'Identify Device Server' },
+    { code: 0x81, description: 'Illegal Function' },
+    { code: 0x82, description: 'Illegal Data Address' },
+    { code: 0x83, description: 'Illegal Data Value' },
+    { code: 0x84, description: 'Server Device Failure' },
+    { code: 0x85, description: 'Acknowledge' },
+    { code: 0x86, description: 'Server Device Busy' },
+    { code: 0x87, description: 'Negative Acknowledge' },
+    { code: 0x88, description: 'Memory Parity Error' },
+    { code: 0x90, description: 'Gateway Path Unavailable' },
+    { code: 0x91, description: 'Gateway Target Device Failed to Respond' },
+]);
 
+export class FunctionCodes {
+    static descriptions: ReadonlyMap<number, string> = new Map<number, string>([...allFunctionCodeDetails].map((it) => [it.code, it.description]));
 
-export const getFunctionCodeDescription = (code: number): string => {
-    const description = functionCodes[code] ?? errorCodes[code];
-    if (description) {
-        return `0x${Converters.byteToHex(code)} ${description}`;
+    static getDescription(code: number | undefined): string {
+        if (code === undefined) {
+            return '';
+        }
+        const description = this.descriptions.get(code);
+        if (description) {
+            return this._getDescription(code, description);
+        }
+        return this._getDescription(code, '<UNKNOWN>');
     }
-    return `0x${Converters.byteToHex(code)} UNKNOWN`;
-}
+
+    private static _getDescription(code: number, description: string): string {
+        return `${code} = 0x${Converters.byteToHex(code)} => ${description} ${this.isError(code) ? '(ERROR)' : ''}`;
+    }
+
+    static isError(code: number): boolean {
+        return !!(code & 0x80);
+    }
+}
\ No newline at end of file
diff --git a/src/main.ts b/src/main.ts
index db496d4..ddaa415 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -3,8 +3,8 @@
 import { AsciiModeStrategy, ModeStrategy, RtuModeStrategy } from "./mode";
 import { clearSniffingTable, Dom, downloadAllSniffedEntries, insertFrameRow } from "./dom";
 import { Frame } from "./frame";
-import { errorCodes, functionCodes } from "./function-codes";
 import { intTest } from "./int.spec";
+import { FunctionCodes } from "./function-codes";
 
 const serial: Serial = navigator.serial;
 if (!serial) {
@@ -68,7 +68,7 @@ const start = (serialOptions: SerialOptions, mode: ModeStrategy) => {
     }, console.warn);
 };
 
-[...Object.entries(functionCodes), ...Object.entries(errorCodes)].forEach(([code, description]) => Dom.addFunctionCodeListOption(code, description));
+[...FunctionCodes.descriptions.keys()].forEach((code: number) => Dom.addFunctionCodeListOption(code));
 
 Dom.sendForm.submit = (formData) => {
     formData.slaveAddress = +formData.slaveAddress;