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ZBasic System Library 165 ZX Microcontroller Family Hardware USART Channel Assignment and I/O Pin Usage ZX Model USART Serial Chan Tx Pin Rx Pin ZX-24, ZX-24a USART0 Com1¹ 1, D.3 2, D.2 ZX-24p, ZX-24n, ZX-24r, ZX-24s USART0 Com1¹ 1, D.1 2, D.0 USART1 Com2 11, D.3 6, D.2 ZX-24x USARTD0 Com1¹ 1, D.3 2, D.2 USARTD1 Com2 D.7 D.6 USARTC0 Com7 9, C.3 10, C.2 USARTC1 Com8 13, C.7 14, C.6 USARTE0 Com9 19, E.3 18, E.2 ZX-32a4 USARTD0 Com1 23, D.3 22, D.2 USARTD1 Com2 27, D.7 26, D.6 USARTC0 Com7 13, C.3 12, C.2 USARTC1 Com8 17, C.7 16, C.6 USARTE0 Com9 33, E.3 32, E.2 ZX-128a1 USARTD0 Com1 28, D.3 27, D.2 USARTD1 Com2 32, D.7 31, D.6 USARTC0 Com7 18, C.3 17, C.2 USARTC1 Com8 22, C.7 21, C.6 USARTE0 Com9 38, E.3 37, E.2 USARTE1 Com10 42, E.7 41, E.6 USARTF0 Com11 48, F.3 47, F.2 USARTF1 Com12 52, F.7 51, F.6 ZX-40, ZX-40a USART0 Com1 15, D.1 14, D.0 ZX-40p, ZX-40n, ZX-40r, ZX-40s USART0 Com1 15, D.1 14, D.0 USART1 Com2 17, D.3 16, D.2 ZX-44, ZX-44a USART0 Com1 10, D.1 9, D.0 ZX-44p, ZX-44n, ZX-44r, ZX-44s USART0 Com1 10, D.1 9, D.0 USART1 Com2 12, D.3 11, D.2 ZX-328n, ZX-328l USART0 Com1 3, D.1 2, D.0 ZX-32n, ZX-32l USART0 Com1 31, D.1 30, D.0 ZX-1281 USART1 Com1 28, D.3 27, D.2 USART0 Com2 3, E.1 2, E.0 ZX-1280 USART0 Com1 3, E.1 2, E.0 USART1 Com2 46, D.3 45, D.2 USART2 Com7 13, H.1 12, H.0 USART3 Com8 64, J.1 63, J.0 ZX-24e, ZX-24ae USART0 Com1¹ 1, D.1 2, D.0 ZX-24ne, ZX-24pe, ZX-24nu, ZX-24pu USART0 Com1¹ 1, D.1 2, D.0 USART1 Com2 17, D.3 18, D.2 ZX-328nu USART0 Com1¹ 19, D.1 20, D.0 ZX-128e, ZX-128ne, ZX-1281e, ZX-1281ne USART0 Com1¹ 19, E.1 20, E.0 USART1 Com2 9, D.3 10, D.2 ¹For these devices, the Com1 signals are logically inverted, the Com2 signals are not, however. It is important to note that on the ZX-24p, ZX-24n, ZX-24r and ZX-24s, the Com2 serial channel cannot be used at the same time as the hardware I2C channel because the pin 11 is shared between the TxD pin of Com2 and the SDA signal. The software-based serial channels are implemented using the Serial Timer (see table below) to regulate the bit timing. When one or more of the channels 3-6 are open the corresponding timer busy flag will be True indicating that Serial Timer is in use. When all of the channels 3-6 are closed, corresponding timer busy flag will again be False indicating that the Serial Timer is available for other uses.