1756-DNB/ABPLC模塊
RS-422與RS-485標(biāo)準(zhǔn)都規(guī)定了接收器門限為±200mV。這樣規(guī)定能夠提供比較高的噪聲抑制能力,如前文所述,當(dāng)接收器A電平比B電平高+200mV以上時(shí),輸出為正邏輯,反之,則輸出為負(fù)邏輯。但由于第三態(tài)的存在,即在主機(jī)在發(fā)端發(fā)完一個(gè)信息數(shù)據(jù)后,將總線置于第三態(tài),即總線空閑時(shí)沒有任何信號(hào)驅(qū)動(dòng)總線,使AB之間的電壓在-200~+200mV直至趨于0V,這帶來了一個(gè)問題:接收器輸出狀態(tài)不確定。如果接收機(jī)的輸出為0V,網(wǎng)絡(luò)中從機(jī)將把其解釋為一個(gè)新的啟動(dòng)位,并試圖讀取后續(xù)字節(jié),由于永遠(yuǎn)不會(huì)有停止位,產(chǎn)生一個(gè)幀錯(cuò)誤結(jié)果,不再有設(shè)備請(qǐng)求總線,網(wǎng)絡(luò)陷于癱瘓狀態(tài)。除上述所述的總線空閑會(huì)造成兩線電壓差低于200mV的情況外,開路或短路時(shí)也會(huì)出現(xiàn)這種情況。故應(yīng)采取一定的措施避免接收器處于不確定狀態(tài)。
1756-DNB/ABPLC模塊
1756
MFR. PART NUMBER DESCRIPTION
1756-A10 10 SLOT CONTROLLOGIX CHASSIS
1756-A13 13 SLOT CONTROLLOGIC CHASSIS
1756-A17 CHASSIS 17 SLOT CONTROLLOGIX
1756-A4 4 SLOT CONTROLLOGIX CHASIS
1756-A7 7 SLOT CONTROLLOGIX CHASSIS
1756-BA1 LOGIX5000 BATTERY ASSEMBLY
1756-CFM 2 CHANNEL /2 OUT CONFIGURABLE FLOWMETER
1756-CNB CONTROLNET BRIDGE MODULE
1756-CNBR CONTROLNET REDUNDANT BRIDGE MODULE
1756-CP3 LOGIX5000 RS232 PROGRAMMER CABLE
1756-DHRIO 2 CHANNEL DH+ AND/OR RIO SCANNER
1756-DNB DEVICE NET BRIDGE/SCANNER MODULE
1756-DPALEN CONTROLLOGIC SYSTEM DOCUMENTATION SET PRINTED
1756-ENET ETHERNET/IP BRIDGE MODULE
1756-GTWY CONTROLLOGIC GATEWAY CONFIGURATION SOFTWARE
1756-GTWYR SUPT REINSTATEMENT CONTRACT FOR 1756GTWY
1756-GTWYS SUPT RENEWAL CONTRACT FOR 1756GTWY
1756-HSC 2 CHANNEL /4 OUTPUT HIGH SPEED COUNTER
1756-IA16 79-132 VAC INPUT 16PTS
1756-IA16I 79-132 VAC ISOLATED INPUT 16PTS
1756-IA8D VAC DIAGNOSTIC INPUT 8PTS
1756-IB16 VDC INPUT 16PTS
1756-IB16D VDC DIAGNOSTIC INPUT
1756-IB16I VDC ISLOTED INPUT 16PTS